c++: when to use pointers?

c++: when to use pointers? - c++

After reading some tutorials I came to the conclusion that one should always use pointers for objects. But I have also seen a few exceptions while reading some QT tutorials (http://zetcode.com/gui/qt4/painting/) where QPaint object is created on the stack. So now I am confused. When should I use pointers?

If you don't know when you should use pointers just don't use them.
It will become apparent when you need to use them, every situation is different. It is not easy to sum up concisely when they should be used. Do not get into the habit of 'always using pointers for objects', that is certainly bad advice.

Main reasons for using pointers:
control object lifetime;
can't use references (e.g. you want to store something non-copyable in vector);
you should pass pointer to some third party function;
maybe some optimization reasons, but I'm not sure.

It's not clear to me if your question is ptr-to-obj vs stack-based-obj or ptr-to-obj vs reference-to-obj. There are also uses that don't fall into either category.
Regarding vs stack, that seems to already be covered above. Several reasons, most obvious is lifetime of object.
Regarding vs references, always strive to use references, but there are things you can do only with ptrs, for example (there are many uses):
walking through elements in an array (e.g., marching over a standard array[])
when a called function allocates something & returns it via a ptr
Most importantly, pointers (and references, as opposed to automatic/stack-based & static objects) support polymorphism. A pointer to a base class may actually point to a derived class. This is fundamental to the OO behavior supported in C++.

First off, the question is wrong: the dilemma is not between pointers and stack, but between heap and stack. You can have an object on the stack and pass the pointer to that object. I assume what you are really asking is whether you should declare a pointer to class or an instance of class.
The answer is that it depends on what you want to do with the object. If the object has to exist after the control leaves the function, then you have to use a pointer and create the object on heap. You will do this, for example, when your function has to return the pointer to the created object or add the object to a list that was created before calling your function.
On the other hand, if the objects is local to the function, then it is better to use it on stack. This enables the compiler to call the destructor when the control leaves the function.

Which tutorials would those be? Actually, the rule is that you should use pointers only when you absolutely have to, which is quite rarely. You need to read a good book on C++, like Accelerated C++ by Koenig & Moo.
Edit: To clarify a bit - two instances where you would not use a pointer (string is being used here as an exemplar - same would go for any other type):
class Person {
public:
string name; // NOT string * name;
...
};
void f() {
string value; // NOT string * value
// use vvalue
}

You usually have to use pointers in the following scenarios:
You need a collection of objects that belong to different classes (in most cases they will have a common base).
You need a stack-allocated collection of objects so large that it'll likely cause stack overflow.
You need a data structure that can rearrange objects quickly - like a linked list, tree ar similar.
You need some complex logic of lifetime management for your object.
You need a data structure that allows for direct navigation from object to object - like a linked list, tree or any other graph.

In addition to points others make (esp. w.r.t. controlling the object lifetime), if you need to handle NULL objects, you should use pointers, not references. It's possible to create a NULL reference through typecasting, but it's generally a bad idea.

Generally use pointers / references to objects when:
passing them to other methods
creating a large array (I'm not sure what the normal stack size is)
Use the stack when:
You are creating an object that lives and dies within the method
The object is the size of a CPU register or smaller

I actually use pointers in this situation:
class Foo
{
Bar* bar;
Foo(Bar& bar) : bar(&bar) { }
Bar& Bar() const { return *bar; }
};
Before that, I used reference members, initialized from the constructor, but the compiler has a problem creating copy constructors, assignment operators, and the lot.
Dave

using pointers is connected with two orthogonal things:
Dynamic allocation. In general, you should allocate dynamically, when the object is intended to live longer that the scope in which it's created. Such an object is a resource which owner have to be clearly specified (most commonly some sort of smart pointer).
Accessing by address (regardless of how the object was created). In this context pointer doesn't mean ownership. Such accessing could be needed when:
some already existing interface requires that.
association which could be null should be modeled.
copying of large objects should be avoided or copying is impossible at all, but the reference can't be used (e.g., stl collections).
The #1 and #2 can occur in different configurations, for example you can imagine dynamically allocated object accessed by pointer, but such the object could also by passed by reference to some function. You also can get pointer to some object which is created on the stack, etc.

Pass by value with well behaved copyable objects is the way to go for a large amount of your code.
If speed really matters, use pass by reference where you can, and finally use pointers.

If possible never use pointers. Rely on pass by reference or if you are going to return a structure or class, assume that your compiler has return value optimization. (You have to avoid conditional construction of the returned class however).
There is a reason why Java doesn't have pointers. C++ doesn't need them either. If you avoid their use you will get the added benefit of automatic object destruction when the object leaves scope. Otherwise your code will be generating memory errors of various types. Memory leaks can be very tricky to find and often occur in C++ due to unhandled exceptions.
If you must use pointers, consider some of the smart pointer classes like auto_ptr. Auto destruction of objects is more than just releasing the underlying memory. There is a concept called RAII. Some objects require additionally handing on destruction. e.g. mutexes and closing files etc.

Use pointers when you don't want your object to be destroyed when the stack frame is emptied.
Use references for passing parameters where possible.

Speaking about C++, objects created on the stack cannot be used when the program has left the scope it was created in. So generally, when you know you don't need a variable past a function or past a close brace, you can create it on the stack.
Speaking about Qt specifically, Qt helps the programmer by handling a lot of the memory management of heap objects. For objects that are derived from QObject (almost all classes prefixed by "Q" are), constructors take an optional parameter parent. The parent then owns the object, and when the parent is deleted, all owned objects are deleted as well. In essence, the responsibility of the children's destruction is passed to the parent object. When using this mechanism, child QObjects must be created on the heap.
In short, in Qt you can easily create objects on the heap, and as long as you set a proper parent, you'll only have to worry about destroying the parent. In general C++, however, you'll need to remember to destroy heap objects, or use smart pointers.

Related

Difference between Object object = new Object() and Object object [duplicate]

I'm coming from a Java background and have started working with objects in C++. But one thing that occurred to me is that people often use pointers to objects rather than the objects themselves, for example this declaration:
Object *myObject = new Object;
rather than:
Object myObject;
Or instead of using a function, let's say testFunc(), like this:
myObject.testFunc();
we have to write:
myObject->testFunc();
But I can't figure out why should we do it this way. I would assume it has to do with efficiency and speed since we get direct access to the memory address. Am I right?

It's very unfortunate that you see dynamic allocation so often. That just shows how many bad C++ programmers there are.
In a sense, you have two questions bundled up into one. The first is when should we use dynamic allocation (using new)? The second is when should we use pointers?
The important take-home message is that you should always use the appropriate tool for the job. In almost all situations, there is something more appropriate and safer than performing manual dynamic allocation and/or using raw pointers.
Dynamic allocation
In your question, you've demonstrated two ways of creating an object. The main difference is the storage duration of the object. When doing Object myObject; within a block, the object is created with automatic storage duration, which means it will be destroyed automatically when it goes out of scope. When you do new Object(), the object has dynamic storage duration, which means it stays alive until you explicitly delete it. You should only use dynamic storage duration when you need it.
That is, you should always prefer creating objects with automatic storage duration when you can.
The main two situations in which you might require dynamic allocation:
You need the object to outlive the current scope - that specific object at that specific memory location, not a copy of it. If you're okay with copying/moving the object (most of the time you should be), you should prefer an automatic object.
You need to allocate a lot of memory, which may easily fill up the stack. It would be nice if we didn't have to concern ourselves with this (most of the time you shouldn't have to), as it's really outside the purview of C++, but unfortunately, we have to deal with the reality of the systems we're developing for.
When you do absolutely require dynamic allocation, you should encapsulate it in a smart pointer or some other type that performs RAII (like the standard containers). Smart pointers provide ownership semantics of dynamically allocated objects. Take a look at std::unique_ptr and std::shared_ptr, for example. If you use them appropriately, you can almost entirely avoid performing your own memory management (see the Rule of Zero).
Pointers
However, there are other more general uses for raw pointers beyond dynamic allocation, but most have alternatives that you should prefer. As before, always prefer the alternatives unless you really need pointers.
You need reference semantics. Sometimes you want to pass an object using a pointer (regardless of how it was allocated) because you want the function to which you're passing it to have access that that specific object (not a copy of it). However, in most situations, you should prefer reference types to pointers, because this is specifically what they're designed for. Note this is not necessarily about extending the lifetime of the object beyond the current scope, as in situation 1 above. As before, if you're okay with passing a copy of the object, you don't need reference semantics.
You need polymorphism. You can only call functions polymorphically (that is, according to the dynamic type of an object) through a pointer or reference to the object. If that's the behavior you need, then you need to use pointers or references. Again, references should be preferred.
You want to represent that an object is optional by allowing a nullptr to be passed when the object is being omitted. If it's an argument, you should prefer to use default arguments or function overloads. Otherwise, you should preferably use a type that encapsulates this behavior, such as std::optional (introduced in C++17 - with earlier C++ standards, use boost::optional).
You want to decouple compilation units to improve compilation time. The useful property of a pointer is that you only require a forward declaration of the pointed-to type (to actually use the object, you'll need a definition). This allows you to decouple parts of your compilation process, which may significantly improve compilation time. See the Pimpl idiom.
You need to interface with a C library or a C-style library. At this point, you're forced to use raw pointers. The best thing you can do is make sure you only let your raw pointers loose at the last possible moment. You can get a raw pointer from a smart pointer, for example, by using its get member function. If a library performs some allocation for you which it expects you to deallocate via a handle, you can often wrap the handle up in a smart pointer with a custom deleter that will deallocate the object appropriately.

There are many use cases for pointers.
Polymorphic behavior. For polymorphic types, pointers (or references) are used to avoid slicing:
class Base { ... };
class Derived : public Base { ... };
void fun(Base b) { ... }
void gun(Base* b) { ... }
void hun(Base& b) { ... }
Derived d;
fun(d); // oops, all Derived parts silently "sliced" off
gun(&d); // OK, a Derived object IS-A Base object
hun(d); // also OK, reference also doesn't slice
Reference semantics and avoiding copying. For non-polymorphic types, a pointer (or a reference) will avoid copying a potentially expensive object
Base b;
fun(b); // copies b, potentially expensive
gun(&b); // takes a pointer to b, no copying
hun(b); // regular syntax, behaves as a pointer
Note that C++11 has move semantics that can avoid many copies of expensive objects into function argument and as return values. But using a pointer will definitely avoid those and will allow multiple pointers on the same object (whereas an object can only be moved from once).
Resource acquisition. Creating a pointer to a resource using the new operator is an anti-pattern in modern C++. Use a special resource class (one of the Standard containers) or a smart pointer (std::unique_ptr<> or std::shared_ptr<>). Consider:
{
auto b = new Base;
... // oops, if an exception is thrown, destructor not called!
delete b;
}
vs.
{
auto b = std::make_unique<Base>();
... // OK, now exception safe
}
A raw pointer should only be used as a "view" and not in any way involved in ownership, be it through direct creation or implicitly through return values. See also this Q&A from the C++ FAQ.
More fine-grained life-time control Every time a shared pointer is being copied (e.g. as a function argument) the resource it points to is being kept alive. Regular objects (not created by new, either directly by you or inside a resource class) are destroyed when going out of scope.

There are many excellent answers to this question, including the important use cases of forward declarations, polymorphism etc. but I feel a part of the "soul" of your question is not answered - namely what the different syntaxes mean across Java and C++.
Let's examine the situation comparing the two languages:
Java:
Object object1 = new Object(); //A new object is allocated by Java
Object object2 = new Object(); //Another new object is allocated by Java
object1 = object2;
//object1 now points to the object originally allocated for object2
//The object originally allocated for object1 is now "dead" - nothing points to it, so it
//will be reclaimed by the Garbage Collector.
//If either object1 or object2 is changed, the change will be reflected to the other
The closest equivalent to this, is:
C++:
Object * object1 = new Object(); //A new object is allocated on the heap
Object * object2 = new Object(); //Another new object is allocated on the heap
delete object1;
//Since C++ does not have a garbage collector, if we don't do that, the next line would
//cause a "memory leak", i.e. a piece of claimed memory that the app cannot use
//and that we have no way to reclaim...
object1 = object2; //Same as Java, object1 points to object2.
Let's see the alternative C++ way:
Object object1; //A new object is allocated on the STACK
Object object2; //Another new object is allocated on the STACK
object1 = object2;//!!!! This is different! The CONTENTS of object2 are COPIED onto object1,
//using the "copy assignment operator", the definition of operator =.
//But, the two objects are still different. Change one, the other remains unchanged.
//Also, the objects get automatically destroyed once the function returns...
The best way to think of it is that -- more or less -- Java (implicitly) handles pointers to objects, while C++ may handle either pointers to objects, or the objects themselves.
There are exceptions to this -- for example, if you declare Java "primitive" types, they are actual values that are copied, and not pointers.
So,
Java:
int object1; //An integer is allocated on the stack.
int object2; //Another integer is allocated on the stack.
object1 = object2; //The value of object2 is copied to object1.
That said, using pointers is NOT necessarily either the correct or the wrong way to handle things; however other answers have covered that satisfactorily. The general idea though is that in C++ you have much more control on the lifetime of the objects, and on where they will live.
Take home point -- the Object * object = new Object() construct is actually what is closest to typical Java (or C# for that matter) semantics.

Preface
Java is nothing like C++, contrary to hype. The Java hype machine would like you to believe that because Java has C++ like syntax, that the languages are similar. Nothing can be further from the truth. This misinformation is part of the reason why Java programmers go to C++ and use Java-like syntax without understanding the implications of their code.
Onwards we go
But I can't figure out why should we do it this way. I would assume it
has to do with efficiency and speed since we get direct access to the
memory address. Am I right?
To the contrary, actually. The heap is much slower than the stack, because the stack is very simple compared to the heap. Automatic storage variables (aka stack variables) have their destructors called once they go out of scope. For example:
{
std::string s;
}
// s is destroyed here
On the other hand, if you use a pointer dynamically allocated, its destructor must be called manually. delete calls this destructor for you.
{
std::string* s = new std::string;
delete s; // destructor called
}
This has nothing to do with the new syntax prevalent in C# and Java. They are used for completely different purposes.
Benefits of dynamic allocation
1. You don't have to know the size of the array in advance
One of the first problems many C++ programmers run into is that when they are accepting arbitrary input from users, you can only allocate a fixed size for a stack variable. You cannot change the size of arrays either. For example:
char buffer[100];
std::cin >> buffer;
// bad input = buffer overflow
Of course, if you used an std::string instead, std::string internally resizes itself so that shouldn't be a problem. But essentially the solution to this problem is dynamic allocation. You can allocate dynamic memory based on the input of the user, for example:
int * pointer;
std::cout << "How many items do you need?";
std::cin >> n;
pointer = new int[n];
Side note: One mistake many beginners make is the usage of
variable length arrays. This is a GNU extension and also one in Clang
because they mirror many of GCC's extensions. So the following
int arr[n] should not be relied on.
Because the heap is much bigger than the stack, one can arbitrarily allocate/reallocate as much memory as he/she needs, whereas the stack has a limitation.
2. Arrays are not pointers
How is this a benefit you ask? The answer will become clear once you understand the confusion/myth behind arrays and pointers. It is commonly assumed that they are the same, but they are not. This myth comes from the fact that pointers can be subscripted just like arrays and because of arrays decay to pointers at the top level in a function declaration. However, once an array decays to a pointer, the pointer loses its sizeof information. So sizeof(pointer) will give the size of the pointer in bytes, which is usually 8 bytes on a 64-bit system.
You cannot assign to arrays, only initialize them. For example:
int arr[5] = {1, 2, 3, 4, 5}; // initialization
int arr[] = {1, 2, 3, 4, 5}; // The standard dictates that the size of the array
// be given by the amount of members in the initializer
arr = { 1, 2, 3, 4, 5 }; // ERROR
On the other hand, you can do whatever you want with pointers. Unfortunately, because the distinction between pointers and arrays are hand-waved in Java and C#, beginners don't understand the difference.
3. Polymorphism
Java and C# have facilities that allow you to treat objects as another, for example using the as keyword. So if somebody wanted to treat an Entity object as a Player object, one could do Player player = Entity as Player; This is very useful if you intend to call functions on a homogeneous container that should only apply to a specific type. The functionality can be achieved in a similar fashion below:
std::vector<Base*> vector;
vector.push_back(&square);
vector.push_back(&triangle);
for (auto& e : vector)
{
auto test = dynamic_cast<Triangle*>(e); // I only care about triangles
if (!test) // not a triangle
e.GenericFunction();
else
e.TriangleOnlyMagic();
}
So say if only Triangles had a Rotate function, it would be a compiler error if you tried to call it on all objects of the class. Using dynamic_cast, you can simulate the as keyword. To be clear, if a cast fails, it returns an invalid pointer. So !test is essentially a shorthand for checking if test is NULL or an invalid pointer, which means the cast failed.
Benefits of automatic variables
After seeing all the great things dynamic allocation can do, you're probably wondering why wouldn't anyone NOT use dynamic allocation all the time? I already told you one reason, the heap is slow. And if you don't need all that memory, you shouldn't abuse it. So here are some disadvantages in no particular order:
It is error-prone. Manual memory allocation is dangerous and you are prone to leaks. If you are not proficient at using the debugger or valgrind (a memory leak tool), you may pull your hair out of your head. Luckily RAII idioms and smart pointers alleviate this a bit, but you must be familiar with practices such as The Rule Of Three and The Rule Of Five. It is a lot of information to take in, and beginners who either don't know or don't care will fall into this trap.
It is not necessary. Unlike Java and C# where it is idiomatic to use the new keyword everywhere, in C++, you should only use it if you need to. The common phrase goes, everything looks like a nail if you have a hammer. Whereas beginners who start with C++ are scared of pointers and learn to use stack variables by habit, Java and C# programmers start by using pointers without understanding it! That is literally stepping off on the wrong foot. You must abandon everything you know because the syntax is one thing, learning the language is another.
1. (N)RVO - Aka, (Named) Return Value Optimization
One optimization many compilers make are things called elision and return value optimization. These things can obviate unnecessary copys which is useful for objects that are very large, such as a vector containing many elements. Normally the common practice is to use pointers to transfer ownership rather than copying the large objects to move them around. This has lead to the inception of move semantics and smart pointers.
If you are using pointers, (N)RVO does NOT occur. It is more beneficial and less error-prone to take advantage of (N)RVO rather than returning or passing pointers if you are worried about optimization. Error leaks can happen if the caller of a function is responsible for deleteing a dynamically allocated object and such. It can be difficult to track the ownership of an object if pointers are being passed around like a hot potato. Just use stack variables because it is simpler and better.

Another good reason to use pointers would be for forward declarations. In a large enough project they can really speed up compile time.

In C++, objects allocated on the stack (using Object object; statement within a block) will only live within the scope they are declared in. When the block of code finishes execution, the object declared are destroyed.
Whereas if you allocate memory on heap, using Object* obj = new Object(), they continue to live in heap until you call delete obj.
I would create an object on heap when I like to use the object not only in the block of code which declared/allocated it.

C++ gives you three ways to pass an object: by pointer, by reference, and by value. Java limits you with the latter one (the only exception is primitive types like int, boolean etc). If you want to use C++ not just like a weird toy, then you'd better get to know the difference between these three ways.
Java pretends that there is no such problem as 'who and when should destroy this?'. The answer is: The Garbage Collector, Great and Awful. Nevertheless, it can't provide 100% protection against memory leaks (yes, java can leak memory). Actually, GC gives you a false sense of safety. The bigger your SUV, the longer your way to the evacuator.
C++ leaves you face-to-face with object's lifecycle management. Well, there are means to deal with that (smart pointers family, QObject in Qt and so on), but none of them can be used in 'fire and forget' manner like GC: you should always keep in mind memory handling. Not only should you care about destroying an object, you also have to avoid destroying the same object more than once.
Not scared yet? Ok: cyclic references - handle them yourself, human. And remember: kill each object precisely once, we C++ runtimes don't like those who mess with corpses, leave dead ones alone.
So, back to your question.
When you pass your object around by value, not by pointer or by reference, you copy the object (the whole object, whether it's a couple of bytes or a huge database dump - you're smart enough to care to avoid latter, aren't you?) every time you do '='. And to access the object's members, you use '.' (dot).
When you pass your object by pointer, you copy just a few bytes (4 on 32-bit systems, 8 on 64-bit ones), namely - the address of this object. And to show this to everyone, you use this fancy '->' operator when you access the members. Or you can use the combination of '*' and '.'.
When you use references, then you get the pointer that pretends to be a value. It's a pointer, but you access the members through '.'.
And, to blow your mind one more time: when you declare several variables separated by commas, then (watch the hands):
Type is given to everyone
Value/pointer/reference modifier is individual
Example:
struct MyStruct
{
int* someIntPointer, someInt; //here comes the surprise
MyStruct *somePointer;
MyStruct &someReference;
};
MyStruct s1; //we allocated an object on stack, not in heap
s1.someInt = 1; //someInt is of type 'int', not 'int*' - value/pointer modifier is individual
s1.someIntPointer = &s1.someInt;
*s1.someIntPointer = 2; //now s1.someInt has value '2'
s1.somePointer = &s1;
s1.someReference = s1; //note there is no '&' operator: reference tries to look like value
s1.somePointer->someInt = 3; //now s1.someInt has value '3'
*(s1.somePointer).someInt = 3; //same as above line
*s1.somePointer->someIntPointer = 4; //now s1.someInt has value '4'
s1.someReference.someInt = 5; //now s1.someInt has value '5'
//although someReference is not value, it's members are accessed through '.'
MyStruct s2 = s1; //'NO WAY' the compiler will say. Go define your '=' operator and come back.
//OK, assume we have '=' defined in MyStruct
s2.someInt = 0; //s2.someInt == 0, but s1.someInt is still 5 - it's two completely different objects, not the references to the same one

But I can't figure out why should we use it like this?
I will compare how it works inside the function body if you use:
Object myObject;
Inside the function, your myObject will get destroyed once this function returns. So this is useful if you don't need your object outside your function. This object will be put on current thread stack.
If you write inside function body:
Object *myObject = new Object;
then Object class instance pointed by myObject will not get destroyed once the function ends, and allocation is on the heap.
Now if you are Java programmer, then the second example is closer to how object allocation works under java. This line: Object *myObject = new Object; is equivalent to java: Object myObject = new Object();. The difference is that under java myObject will get garbage collected, while under c++ it will not get freed, you must somewhere explicitly call `delete myObject;' otherwise you will introduce memory leaks.
Since c++11 you can use safe ways of dynamic allocations: new Object, by storing values in shared_ptr/unique_ptr.
std::shared_ptr<std::string> safe_str = make_shared<std::string>("make_shared");
// since c++14
std::unique_ptr<std::string> safe_str = make_unique<std::string>("make_shared");
also, objects are very often stored in containers, like map-s or vector-s, they will automatically manage a lifetime of your objects.

Technically it is a memory allocation issue, however here are two more practical aspects of this.
It has to do with two things:
1) Scope, when you define an object without a pointer you will no longer be able to access it after the code block it is defined in, whereas if you define a pointer with "new" then you can access it from anywhere you have a pointer to this memory until you call "delete" on the same pointer.
2) If you want to pass arguments to a function you want to pass a pointer or a reference in order to be more efficient. When you pass an Object then the object is copied, if this is an object that uses a lot of memory this might be CPU consuming (e.g. you copy a vector full of data). When you pass a pointer all you pass is one int (depending of implementation but most of them are one int).
Other than that you need to understand that "new" allocates memory on the heap that needs to be freed at some point. When you don't have to use "new" I suggest you use a regular object definition "on the stack".

Well the main question is Why should I use a pointer rather than the object itself? And my answer, you should (almost) never use pointer instead of object, because C++ has references, it is safer then pointers and guarantees the same performance as pointers.
Another thing you mentioned in your question:
Object *myObject = new Object;
How does it work? It creates pointer of Object type, allocates memory to fit one object and calls default constructor, sounds good, right? But actually it isn't so good, if you dynamically allocated memory (used keyword new), you also have to free memory manually, that means in code you should have:
delete myObject;
This calls destructor and frees memory, looks easy, however in big projects may be difficult to detect if one thread freed memory or not, but for that purpose you can try shared pointers, these slightly decreases performance, but it is much easier to work with them.
And now some introduction is over and go back to question.
You can use pointers instead of objects to get better performance while transferring data between function.
Take a look, you have std::string (it is also object) and it contains really much data, for example big XML, now you need to parse it, but for that you have function void foo(...) which can be declarated in different ways:
void foo(std::string xml);
In this case you will copy all data from your variable to function stack, it takes some time, so your performance will be low.
void foo(std::string* xml);
In this case you will pass pointer to object, same speed as passing size_t variable, however this declaration has error prone, because you can pass NULL pointer or invalid pointer. Pointers usually used in C because it doesn't have references.
void foo(std::string& xml);
Here you pass reference, basically it is the same as passing pointer, but compiler does some stuff and you cannot pass invalid reference (actually it is possible to create situation with invalid reference, but it is tricking compiler).
void foo(const std::string* xml);
Here is the same as second, just pointer value cannot be changed.
void foo(const std::string& xml);
Here is the same as third, but object value cannot be changed.
What more I want to mention, you can use these 5 ways to pass data no matter which allocation way you have chosen (with new or regular).
Another thing to mention, when you create object in regular way, you allocate memory in stack, but while you create it with new you allocate heap. It is much faster to allocate stack, but it is kind a small for really big arrays of data, so if you need big object you should use heap, because you may get stack overflow, but usually this issue is solved using STL containers and remember std::string is also container, some guys forgot it :)

Let's say that you have class A that contain class B When you want to call some function of class B outside class A you will simply obtain a pointer to this class and you can do whatever you want and it will also change context of class B in your class A
But be careful with dynamic object

There are many benefits of using pointers to object -
Efficiency (as you already pointed out). Passing objects to
functions mean creating new copies of object.
Working with objects from third party libraries. If your object
belongs to a third party code and the authors intend the usage of their objects through pointers only (no copy constructors etc) the only way you can pass around this
object is using pointers. Passing by value may cause issues. (Deep
copy / shallow copy issues).
if the object owns a resource and you want that the ownership should not be sahred with other objects.

This is has been discussed at length, but in Java everything is a pointer. It makes no distinction between stack and heap allocations (all objects are allocated on the heap), so you don't realize you're using pointers. In C++, you can mix the two, depending on your memory requirements. Performance and memory usage is more deterministic in C++ (duh).

Object *myObject = new Object;
Doing this will create a reference to an Object (on the heap) which has to be deleted explicitly to avoid memory leak.
Object myObject;
Doing this will create an object(myObject) of the automatic type (on the stack) that will be automatically deleted when the object(myObject) goes out of scope.

A pointer directly references the memory location of an object. Java has nothing like this. Java has references that reference the location of object through hash tables. You cannot do anything like pointer arithmetic in Java with these references.
To answer your question, it's just your preference. I prefer using the Java-like syntax.

The key strength of object pointers in C++ is allowing for polymorphic arrays and maps of pointers of the same superclass. It allows, for example, to put parakeets, chickens, robins, ostriches, etc. in an array of Bird.
Additionally, dynamically allocated objects are more flexible, and can use HEAP memory whereas a locally allocated object will use the STACK memory unless it is static. Having large objects on the stack, especially when using recursion, will undoubtedly lead to stack overflow.

One reason for using pointers is to interface with C functions. Another reason is to save memory; for example: instead of passing an object which contains a lot of data and has a processor-intensive copy-constructor to a function, just pass a pointer to the object, saving memory and speed especially if you're in a loop, however a reference would be better in that case, unless you're using an C-style array.

In areas where memory utilization is at its premium , pointers comes handy. For example consider a minimax algorithm, where thousands of nodes will be generated using recursive routine, and later use them to evaluate the next best move in game, ability to deallocate or reset (as in smart pointers) significantly reduces memory consumption. Whereas the non-pointer variable continues to occupy space till it's recursive call returns a value.

I will include one important use case of pointer. When you are storing some object in the base class, but it could be polymorphic.
Class Base1 {
};
Class Derived1 : public Base1 {
};
Class Base2 {
Base *bObj;
virtual void createMemerObects() = 0;
};
Class Derived2 {
virtual void createMemerObects() {
bObj = new Derived1();
}
};
So in this case you can't declare bObj as an direct object, you have to have pointer.

tl;dr: Don't "use a pointer rather than the object itself" (usually)
You asked why you should prefer a pointer rather than the object itself. Well, you shouldn't, as a general rule.
Now, there are indeed multiple exceptions to this rule, and other answers have spelled them out. The thing is, these days, many of these exceptions are no longer valid! Let us consider the exceptions listed in the accepted answer:
You need reference semantics.
If you need reference semantics, use references, not pointers; see #ST3's answer's answer. In fact, one could argue that, in Java, what you pass around are usually references.
You need polymorphism.
If you know the set of classes you'll be working with, very often you can just use an std::variant<ClassA, ClassB, ClassC> (see description here) and operate on them using a visitor pattern. Now, granted, C++'s variant implementation is not the prettiest sight; but I'd usually prefer it over getting down-and-dirty with pointers.
You want to represent that an object is optional
Absolutely don't use pointers for that. You have std::optional, and unlike std::variant, it's quite convenient. Use that instead. nullopt is an empty (or "null") optional. And - it's not a pointer.
You want to decouple compilation units to improve compilation time.
You can use references rather than pointers to achieve this as well. To use Object& in a piece of code, it's sufficient to say class Object;, i.e. to use a forward-declaration.
You need to interface with a C library or a C-style library.
Yeah, well, if you work with code that already uses pointers, then - you have to use pointers yourself, can't get around that :-( and C doesn't have references.
Also, some people may tell you to use pointers to avoid making copies of objects. Well this is not really a problem for return values, due to the return-value and named-return-value optimizations (RVO and NRVO). And in other cases - references avoid copying just fine.
The bottom-line rule is still the same as the accepted answer, though: Only use a pointer when you have a good reason to need one.
PS - If you do need a pointer, you should still avoid using new and delete directly. You will probably be better served by a smart pointer - which is automagically freed (not like in Java, but still).

With pointers ,
can directly talk to the memory.
can prevent lot of memory leaks of a program by manipulating pointers.

"Necessity is the mother of invention."
The most of important difference that I would like to point out is the outcome of my own experience of coding.
Sometimes you need to pass objects to functions. In that case, if your object is of a very big class then passing it as an object will copy its state (which you might not want ..AND CAN BE BIG OVERHEAD) thus resulting in an overhead of copying object .while pointer is fixed 4-byte size (assuming 32 bit). Other reasons are already mentioned above...

There are many excellent answers already, but let me give you one example:
I have an simple Item class:
class Item
{
public:
std::string name;
int weight;
int price;
};
I make a vector to hold a bunch of them.
std::vector<Item> inventory;
I create one million Item objects, and push them back onto the vector. I sort the vector by name, and then do a simple iterative binary search for a particular item name. I test the program, and it takes over 8 minutes to finish executing. Then I change my inventory vector like so:
std::vector<Item *> inventory;
...and create my million Item objects via new. The ONLY changes I make to my code are to use the pointers to Items, excepting a loop I add for memory cleanup at the end. That program runs in under 40 seconds, or better than a 10x speed increase.
EDIT: The code is at http://pastebin.com/DK24SPeW
With compiler optimizations it shows only a 3.4x increase on the machine I just tested it on, which is still considerable.

I don't understand the point of pointers to classes [duplicate]

I'm coming from a Java background and have started working with objects in C++. But one thing that occurred to me is that people often use pointers to objects rather than the objects themselves, for example this declaration:
Object *myObject = new Object;
rather than:
Object myObject;
Or instead of using a function, let's say testFunc(), like this:
myObject.testFunc();
we have to write:
myObject->testFunc();
But I can't figure out why should we do it this way. I would assume it has to do with efficiency and speed since we get direct access to the memory address. Am I right?

There are many use cases for pointers.
Polymorphic behavior. For polymorphic types, pointers (or references) are used to avoid slicing:
class Base { ... };
class Derived : public Base { ... };
void fun(Base b) { ... }
void gun(Base* b) { ... }
void hun(Base& b) { ... }
Derived d;
fun(d); // oops, all Derived parts silently "sliced" off
gun(&d); // OK, a Derived object IS-A Base object
hun(d); // also OK, reference also doesn't slice
Reference semantics and avoiding copying. For non-polymorphic types, a pointer (or a reference) will avoid copying a potentially expensive object
Base b;
fun(b); // copies b, potentially expensive
gun(&b); // takes a pointer to b, no copying
hun(b); // regular syntax, behaves as a pointer
Note that C++11 has move semantics that can avoid many copies of expensive objects into function argument and as return values. But using a pointer will definitely avoid those and will allow multiple pointers on the same object (whereas an object can only be moved from once).
Resource acquisition. Creating a pointer to a resource using the new operator is an anti-pattern in modern C++. Use a special resource class (one of the Standard containers) or a smart pointer (std::unique_ptr<> or std::shared_ptr<>). Consider:
{
auto b = new Base;
... // oops, if an exception is thrown, destructor not called!
delete b;
}
vs.
{
auto b = std::make_unique<Base>();
... // OK, now exception safe
}
A raw pointer should only be used as a "view" and not in any way involved in ownership, be it through direct creation or implicitly through return values. See also this Q&A from the C++ FAQ.
More fine-grained life-time control Every time a shared pointer is being copied (e.g. as a function argument) the resource it points to is being kept alive. Regular objects (not created by new, either directly by you or inside a resource class) are destroyed when going out of scope.

Another good reason to use pointers would be for forward declarations. In a large enough project they can really speed up compile time.

In C++, objects allocated on the stack (using Object object; statement within a block) will only live within the scope they are declared in. When the block of code finishes execution, the object declared are destroyed.
Whereas if you allocate memory on heap, using Object* obj = new Object(), they continue to live in heap until you call delete obj.
I would create an object on heap when I like to use the object not only in the block of code which declared/allocated it.

C++ gives you three ways to pass an object: by pointer, by reference, and by value. Java limits you with the latter one (the only exception is primitive types like int, boolean etc). If you want to use C++ not just like a weird toy, then you'd better get to know the difference between these three ways.
Java pretends that there is no such problem as 'who and when should destroy this?'. The answer is: The Garbage Collector, Great and Awful. Nevertheless, it can't provide 100% protection against memory leaks (yes, java can leak memory). Actually, GC gives you a false sense of safety. The bigger your SUV, the longer your way to the evacuator.
C++ leaves you face-to-face with object's lifecycle management. Well, there are means to deal with that (smart pointers family, QObject in Qt and so on), but none of them can be used in 'fire and forget' manner like GC: you should always keep in mind memory handling. Not only should you care about destroying an object, you also have to avoid destroying the same object more than once.
Not scared yet? Ok: cyclic references - handle them yourself, human. And remember: kill each object precisely once, we C++ runtimes don't like those who mess with corpses, leave dead ones alone.
So, back to your question.
When you pass your object around by value, not by pointer or by reference, you copy the object (the whole object, whether it's a couple of bytes or a huge database dump - you're smart enough to care to avoid latter, aren't you?) every time you do '='. And to access the object's members, you use '.' (dot).
When you pass your object by pointer, you copy just a few bytes (4 on 32-bit systems, 8 on 64-bit ones), namely - the address of this object. And to show this to everyone, you use this fancy '->' operator when you access the members. Or you can use the combination of '*' and '.'.
When you use references, then you get the pointer that pretends to be a value. It's a pointer, but you access the members through '.'.
And, to blow your mind one more time: when you declare several variables separated by commas, then (watch the hands):
Type is given to everyone
Value/pointer/reference modifier is individual
Example:
struct MyStruct
{
int* someIntPointer, someInt; //here comes the surprise
MyStruct *somePointer;
MyStruct &someReference;
};
MyStruct s1; //we allocated an object on stack, not in heap
s1.someInt = 1; //someInt is of type 'int', not 'int*' - value/pointer modifier is individual
s1.someIntPointer = &s1.someInt;
*s1.someIntPointer = 2; //now s1.someInt has value '2'
s1.somePointer = &s1;
s1.someReference = s1; //note there is no '&' operator: reference tries to look like value
s1.somePointer->someInt = 3; //now s1.someInt has value '3'
*(s1.somePointer).someInt = 3; //same as above line
*s1.somePointer->someIntPointer = 4; //now s1.someInt has value '4'
s1.someReference.someInt = 5; //now s1.someInt has value '5'
//although someReference is not value, it's members are accessed through '.'
MyStruct s2 = s1; //'NO WAY' the compiler will say. Go define your '=' operator and come back.
//OK, assume we have '=' defined in MyStruct
s2.someInt = 0; //s2.someInt == 0, but s1.someInt is still 5 - it's two completely different objects, not the references to the same one

But I can't figure out why should we use it like this?
I will compare how it works inside the function body if you use:
Object myObject;
Inside the function, your myObject will get destroyed once this function returns. So this is useful if you don't need your object outside your function. This object will be put on current thread stack.
If you write inside function body:
Object *myObject = new Object;
then Object class instance pointed by myObject will not get destroyed once the function ends, and allocation is on the heap.
Now if you are Java programmer, then the second example is closer to how object allocation works under java. This line: Object *myObject = new Object; is equivalent to java: Object myObject = new Object();. The difference is that under java myObject will get garbage collected, while under c++ it will not get freed, you must somewhere explicitly call `delete myObject;' otherwise you will introduce memory leaks.
Since c++11 you can use safe ways of dynamic allocations: new Object, by storing values in shared_ptr/unique_ptr.
std::shared_ptr<std::string> safe_str = make_shared<std::string>("make_shared");
// since c++14
std::unique_ptr<std::string> safe_str = make_unique<std::string>("make_shared");
also, objects are very often stored in containers, like map-s or vector-s, they will automatically manage a lifetime of your objects.

Technically it is a memory allocation issue, however here are two more practical aspects of this.
It has to do with two things:
1) Scope, when you define an object without a pointer you will no longer be able to access it after the code block it is defined in, whereas if you define a pointer with "new" then you can access it from anywhere you have a pointer to this memory until you call "delete" on the same pointer.
2) If you want to pass arguments to a function you want to pass a pointer or a reference in order to be more efficient. When you pass an Object then the object is copied, if this is an object that uses a lot of memory this might be CPU consuming (e.g. you copy a vector full of data). When you pass a pointer all you pass is one int (depending of implementation but most of them are one int).
Other than that you need to understand that "new" allocates memory on the heap that needs to be freed at some point. When you don't have to use "new" I suggest you use a regular object definition "on the stack".

Well the main question is Why should I use a pointer rather than the object itself? And my answer, you should (almost) never use pointer instead of object, because C++ has references, it is safer then pointers and guarantees the same performance as pointers.
Another thing you mentioned in your question:
Object *myObject = new Object;
How does it work? It creates pointer of Object type, allocates memory to fit one object and calls default constructor, sounds good, right? But actually it isn't so good, if you dynamically allocated memory (used keyword new), you also have to free memory manually, that means in code you should have:
delete myObject;
This calls destructor and frees memory, looks easy, however in big projects may be difficult to detect if one thread freed memory or not, but for that purpose you can try shared pointers, these slightly decreases performance, but it is much easier to work with them.
And now some introduction is over and go back to question.
You can use pointers instead of objects to get better performance while transferring data between function.
Take a look, you have std::string (it is also object) and it contains really much data, for example big XML, now you need to parse it, but for that you have function void foo(...) which can be declarated in different ways:
void foo(std::string xml);
In this case you will copy all data from your variable to function stack, it takes some time, so your performance will be low.
void foo(std::string* xml);
In this case you will pass pointer to object, same speed as passing size_t variable, however this declaration has error prone, because you can pass NULL pointer or invalid pointer. Pointers usually used in C because it doesn't have references.
void foo(std::string& xml);
Here you pass reference, basically it is the same as passing pointer, but compiler does some stuff and you cannot pass invalid reference (actually it is possible to create situation with invalid reference, but it is tricking compiler).
void foo(const std::string* xml);
Here is the same as second, just pointer value cannot be changed.
void foo(const std::string& xml);
Here is the same as third, but object value cannot be changed.
What more I want to mention, you can use these 5 ways to pass data no matter which allocation way you have chosen (with new or regular).
Another thing to mention, when you create object in regular way, you allocate memory in stack, but while you create it with new you allocate heap. It is much faster to allocate stack, but it is kind a small for really big arrays of data, so if you need big object you should use heap, because you may get stack overflow, but usually this issue is solved using STL containers and remember std::string is also container, some guys forgot it :)

Let's say that you have class A that contain class B When you want to call some function of class B outside class A you will simply obtain a pointer to this class and you can do whatever you want and it will also change context of class B in your class A
But be careful with dynamic object

There are many benefits of using pointers to object -
Efficiency (as you already pointed out). Passing objects to
functions mean creating new copies of object.
Working with objects from third party libraries. If your object
belongs to a third party code and the authors intend the usage of their objects through pointers only (no copy constructors etc) the only way you can pass around this
object is using pointers. Passing by value may cause issues. (Deep
copy / shallow copy issues).
if the object owns a resource and you want that the ownership should not be sahred with other objects.

This is has been discussed at length, but in Java everything is a pointer. It makes no distinction between stack and heap allocations (all objects are allocated on the heap), so you don't realize you're using pointers. In C++, you can mix the two, depending on your memory requirements. Performance and memory usage is more deterministic in C++ (duh).

Object *myObject = new Object;
Doing this will create a reference to an Object (on the heap) which has to be deleted explicitly to avoid memory leak.
Object myObject;
Doing this will create an object(myObject) of the automatic type (on the stack) that will be automatically deleted when the object(myObject) goes out of scope.

A pointer directly references the memory location of an object. Java has nothing like this. Java has references that reference the location of object through hash tables. You cannot do anything like pointer arithmetic in Java with these references.
To answer your question, it's just your preference. I prefer using the Java-like syntax.

The key strength of object pointers in C++ is allowing for polymorphic arrays and maps of pointers of the same superclass. It allows, for example, to put parakeets, chickens, robins, ostriches, etc. in an array of Bird.
Additionally, dynamically allocated objects are more flexible, and can use HEAP memory whereas a locally allocated object will use the STACK memory unless it is static. Having large objects on the stack, especially when using recursion, will undoubtedly lead to stack overflow.

One reason for using pointers is to interface with C functions. Another reason is to save memory; for example: instead of passing an object which contains a lot of data and has a processor-intensive copy-constructor to a function, just pass a pointer to the object, saving memory and speed especially if you're in a loop, however a reference would be better in that case, unless you're using an C-style array.

In areas where memory utilization is at its premium , pointers comes handy. For example consider a minimax algorithm, where thousands of nodes will be generated using recursive routine, and later use them to evaluate the next best move in game, ability to deallocate or reset (as in smart pointers) significantly reduces memory consumption. Whereas the non-pointer variable continues to occupy space till it's recursive call returns a value.

I will include one important use case of pointer. When you are storing some object in the base class, but it could be polymorphic.
Class Base1 {
};
Class Derived1 : public Base1 {
};
Class Base2 {
Base *bObj;
virtual void createMemerObects() = 0;
};
Class Derived2 {
virtual void createMemerObects() {
bObj = new Derived1();
}
};
So in this case you can't declare bObj as an direct object, you have to have pointer.

With pointers ,
can directly talk to the memory.
can prevent lot of memory leaks of a program by manipulating pointers.

"Necessity is the mother of invention."
The most of important difference that I would like to point out is the outcome of my own experience of coding.
Sometimes you need to pass objects to functions. In that case, if your object is of a very big class then passing it as an object will copy its state (which you might not want ..AND CAN BE BIG OVERHEAD) thus resulting in an overhead of copying object .while pointer is fixed 4-byte size (assuming 32 bit). Other reasons are already mentioned above...

There are many excellent answers already, but let me give you one example:
I have an simple Item class:
class Item
{
public:
std::string name;
int weight;
int price;
};
I make a vector to hold a bunch of them.
std::vector<Item> inventory;
I create one million Item objects, and push them back onto the vector. I sort the vector by name, and then do a simple iterative binary search for a particular item name. I test the program, and it takes over 8 minutes to finish executing. Then I change my inventory vector like so:
std::vector<Item *> inventory;
...and create my million Item objects via new. The ONLY changes I make to my code are to use the pointers to Items, excepting a loop I add for memory cleanup at the end. That program runs in under 40 seconds, or better than a 10x speed increase.
EDIT: The code is at http://pastebin.com/DK24SPeW
With compiler optimizations it shows only a 3.4x increase on the machine I just tested it on, which is still considerable.

Why should I use a pointer rather than the object itself?

I'm coming from a Java background and have started working with objects in C++. But one thing that occurred to me is that people often use pointers to objects rather than the objects themselves, for example this declaration:
Object *myObject = new Object;
rather than:
Object myObject;
Or instead of using a function, let's say testFunc(), like this:
myObject.testFunc();
we have to write:
myObject->testFunc();
But I can't figure out why should we do it this way. I would assume it has to do with efficiency and speed since we get direct access to the memory address. Am I right?

There are many use cases for pointers.
Polymorphic behavior. For polymorphic types, pointers (or references) are used to avoid slicing:
class Base { ... };
class Derived : public Base { ... };
void fun(Base b) { ... }
void gun(Base* b) { ... }
void hun(Base& b) { ... }
Derived d;
fun(d); // oops, all Derived parts silently "sliced" off
gun(&d); // OK, a Derived object IS-A Base object
hun(d); // also OK, reference also doesn't slice
Reference semantics and avoiding copying. For non-polymorphic types, a pointer (or a reference) will avoid copying a potentially expensive object
Base b;
fun(b); // copies b, potentially expensive
gun(&b); // takes a pointer to b, no copying
hun(b); // regular syntax, behaves as a pointer
Note that C++11 has move semantics that can avoid many copies of expensive objects into function argument and as return values. But using a pointer will definitely avoid those and will allow multiple pointers on the same object (whereas an object can only be moved from once).
Resource acquisition. Creating a pointer to a resource using the new operator is an anti-pattern in modern C++. Use a special resource class (one of the Standard containers) or a smart pointer (std::unique_ptr<> or std::shared_ptr<>). Consider:
{
auto b = new Base;
... // oops, if an exception is thrown, destructor not called!
delete b;
}
vs.
{
auto b = std::make_unique<Base>();
... // OK, now exception safe
}
A raw pointer should only be used as a "view" and not in any way involved in ownership, be it through direct creation or implicitly through return values. See also this Q&A from the C++ FAQ.
More fine-grained life-time control Every time a shared pointer is being copied (e.g. as a function argument) the resource it points to is being kept alive. Regular objects (not created by new, either directly by you or inside a resource class) are destroyed when going out of scope.

Another good reason to use pointers would be for forward declarations. In a large enough project they can really speed up compile time.

In C++, objects allocated on the stack (using Object object; statement within a block) will only live within the scope they are declared in. When the block of code finishes execution, the object declared are destroyed.
Whereas if you allocate memory on heap, using Object* obj = new Object(), they continue to live in heap until you call delete obj.
I would create an object on heap when I like to use the object not only in the block of code which declared/allocated it.

C++ gives you three ways to pass an object: by pointer, by reference, and by value. Java limits you with the latter one (the only exception is primitive types like int, boolean etc). If you want to use C++ not just like a weird toy, then you'd better get to know the difference between these three ways.
Java pretends that there is no such problem as 'who and when should destroy this?'. The answer is: The Garbage Collector, Great and Awful. Nevertheless, it can't provide 100% protection against memory leaks (yes, java can leak memory). Actually, GC gives you a false sense of safety. The bigger your SUV, the longer your way to the evacuator.
C++ leaves you face-to-face with object's lifecycle management. Well, there are means to deal with that (smart pointers family, QObject in Qt and so on), but none of them can be used in 'fire and forget' manner like GC: you should always keep in mind memory handling. Not only should you care about destroying an object, you also have to avoid destroying the same object more than once.
Not scared yet? Ok: cyclic references - handle them yourself, human. And remember: kill each object precisely once, we C++ runtimes don't like those who mess with corpses, leave dead ones alone.
So, back to your question.
When you pass your object around by value, not by pointer or by reference, you copy the object (the whole object, whether it's a couple of bytes or a huge database dump - you're smart enough to care to avoid latter, aren't you?) every time you do '='. And to access the object's members, you use '.' (dot).
When you pass your object by pointer, you copy just a few bytes (4 on 32-bit systems, 8 on 64-bit ones), namely - the address of this object. And to show this to everyone, you use this fancy '->' operator when you access the members. Or you can use the combination of '*' and '.'.
When you use references, then you get the pointer that pretends to be a value. It's a pointer, but you access the members through '.'.
And, to blow your mind one more time: when you declare several variables separated by commas, then (watch the hands):
Type is given to everyone
Value/pointer/reference modifier is individual
Example:
struct MyStruct
{
int* someIntPointer, someInt; //here comes the surprise
MyStruct *somePointer;
MyStruct &someReference;
};
MyStruct s1; //we allocated an object on stack, not in heap
s1.someInt = 1; //someInt is of type 'int', not 'int*' - value/pointer modifier is individual
s1.someIntPointer = &s1.someInt;
*s1.someIntPointer = 2; //now s1.someInt has value '2'
s1.somePointer = &s1;
s1.someReference = s1; //note there is no '&' operator: reference tries to look like value
s1.somePointer->someInt = 3; //now s1.someInt has value '3'
*(s1.somePointer).someInt = 3; //same as above line
*s1.somePointer->someIntPointer = 4; //now s1.someInt has value '4'
s1.someReference.someInt = 5; //now s1.someInt has value '5'
//although someReference is not value, it's members are accessed through '.'
MyStruct s2 = s1; //'NO WAY' the compiler will say. Go define your '=' operator and come back.
//OK, assume we have '=' defined in MyStruct
s2.someInt = 0; //s2.someInt == 0, but s1.someInt is still 5 - it's two completely different objects, not the references to the same one

But I can't figure out why should we use it like this?
I will compare how it works inside the function body if you use:
Object myObject;
Inside the function, your myObject will get destroyed once this function returns. So this is useful if you don't need your object outside your function. This object will be put on current thread stack.
If you write inside function body:
Object *myObject = new Object;
then Object class instance pointed by myObject will not get destroyed once the function ends, and allocation is on the heap.
Now if you are Java programmer, then the second example is closer to how object allocation works under java. This line: Object *myObject = new Object; is equivalent to java: Object myObject = new Object();. The difference is that under java myObject will get garbage collected, while under c++ it will not get freed, you must somewhere explicitly call `delete myObject;' otherwise you will introduce memory leaks.
Since c++11 you can use safe ways of dynamic allocations: new Object, by storing values in shared_ptr/unique_ptr.
std::shared_ptr<std::string> safe_str = make_shared<std::string>("make_shared");
// since c++14
std::unique_ptr<std::string> safe_str = make_unique<std::string>("make_shared");
also, objects are very often stored in containers, like map-s or vector-s, they will automatically manage a lifetime of your objects.

Technically it is a memory allocation issue, however here are two more practical aspects of this.
It has to do with two things:
1) Scope, when you define an object without a pointer you will no longer be able to access it after the code block it is defined in, whereas if you define a pointer with "new" then you can access it from anywhere you have a pointer to this memory until you call "delete" on the same pointer.
2) If you want to pass arguments to a function you want to pass a pointer or a reference in order to be more efficient. When you pass an Object then the object is copied, if this is an object that uses a lot of memory this might be CPU consuming (e.g. you copy a vector full of data). When you pass a pointer all you pass is one int (depending of implementation but most of them are one int).
Other than that you need to understand that "new" allocates memory on the heap that needs to be freed at some point. When you don't have to use "new" I suggest you use a regular object definition "on the stack".

Well the main question is Why should I use a pointer rather than the object itself? And my answer, you should (almost) never use pointer instead of object, because C++ has references, it is safer then pointers and guarantees the same performance as pointers.
Another thing you mentioned in your question:
Object *myObject = new Object;
How does it work? It creates pointer of Object type, allocates memory to fit one object and calls default constructor, sounds good, right? But actually it isn't so good, if you dynamically allocated memory (used keyword new), you also have to free memory manually, that means in code you should have:
delete myObject;
This calls destructor and frees memory, looks easy, however in big projects may be difficult to detect if one thread freed memory or not, but for that purpose you can try shared pointers, these slightly decreases performance, but it is much easier to work with them.
And now some introduction is over and go back to question.
You can use pointers instead of objects to get better performance while transferring data between function.
Take a look, you have std::string (it is also object) and it contains really much data, for example big XML, now you need to parse it, but for that you have function void foo(...) which can be declarated in different ways:
void foo(std::string xml);
In this case you will copy all data from your variable to function stack, it takes some time, so your performance will be low.
void foo(std::string* xml);
In this case you will pass pointer to object, same speed as passing size_t variable, however this declaration has error prone, because you can pass NULL pointer or invalid pointer. Pointers usually used in C because it doesn't have references.
void foo(std::string& xml);
Here you pass reference, basically it is the same as passing pointer, but compiler does some stuff and you cannot pass invalid reference (actually it is possible to create situation with invalid reference, but it is tricking compiler).
void foo(const std::string* xml);
Here is the same as second, just pointer value cannot be changed.
void foo(const std::string& xml);
Here is the same as third, but object value cannot be changed.
What more I want to mention, you can use these 5 ways to pass data no matter which allocation way you have chosen (with new or regular).
Another thing to mention, when you create object in regular way, you allocate memory in stack, but while you create it with new you allocate heap. It is much faster to allocate stack, but it is kind a small for really big arrays of data, so if you need big object you should use heap, because you may get stack overflow, but usually this issue is solved using STL containers and remember std::string is also container, some guys forgot it :)

Let's say that you have class A that contain class B When you want to call some function of class B outside class A you will simply obtain a pointer to this class and you can do whatever you want and it will also change context of class B in your class A
But be careful with dynamic object

There are many benefits of using pointers to object -
Efficiency (as you already pointed out). Passing objects to
functions mean creating new copies of object.
Working with objects from third party libraries. If your object
belongs to a third party code and the authors intend the usage of their objects through pointers only (no copy constructors etc) the only way you can pass around this
object is using pointers. Passing by value may cause issues. (Deep
copy / shallow copy issues).
if the object owns a resource and you want that the ownership should not be sahred with other objects.

This is has been discussed at length, but in Java everything is a pointer. It makes no distinction between stack and heap allocations (all objects are allocated on the heap), so you don't realize you're using pointers. In C++, you can mix the two, depending on your memory requirements. Performance and memory usage is more deterministic in C++ (duh).

Object *myObject = new Object;
Doing this will create a reference to an Object (on the heap) which has to be deleted explicitly to avoid memory leak.
Object myObject;
Doing this will create an object(myObject) of the automatic type (on the stack) that will be automatically deleted when the object(myObject) goes out of scope.

A pointer directly references the memory location of an object. Java has nothing like this. Java has references that reference the location of object through hash tables. You cannot do anything like pointer arithmetic in Java with these references.
To answer your question, it's just your preference. I prefer using the Java-like syntax.

The key strength of object pointers in C++ is allowing for polymorphic arrays and maps of pointers of the same superclass. It allows, for example, to put parakeets, chickens, robins, ostriches, etc. in an array of Bird.
Additionally, dynamically allocated objects are more flexible, and can use HEAP memory whereas a locally allocated object will use the STACK memory unless it is static. Having large objects on the stack, especially when using recursion, will undoubtedly lead to stack overflow.

One reason for using pointers is to interface with C functions. Another reason is to save memory; for example: instead of passing an object which contains a lot of data and has a processor-intensive copy-constructor to a function, just pass a pointer to the object, saving memory and speed especially if you're in a loop, however a reference would be better in that case, unless you're using an C-style array.

In areas where memory utilization is at its premium , pointers comes handy. For example consider a minimax algorithm, where thousands of nodes will be generated using recursive routine, and later use them to evaluate the next best move in game, ability to deallocate or reset (as in smart pointers) significantly reduces memory consumption. Whereas the non-pointer variable continues to occupy space till it's recursive call returns a value.

I will include one important use case of pointer. When you are storing some object in the base class, but it could be polymorphic.
Class Base1 {
};
Class Derived1 : public Base1 {
};
Class Base2 {
Base *bObj;
virtual void createMemerObects() = 0;
};
Class Derived2 {
virtual void createMemerObects() {
bObj = new Derived1();
}
};
So in this case you can't declare bObj as an direct object, you have to have pointer.

With pointers ,
can directly talk to the memory.
can prevent lot of memory leaks of a program by manipulating pointers.

"Necessity is the mother of invention."
The most of important difference that I would like to point out is the outcome of my own experience of coding.
Sometimes you need to pass objects to functions. In that case, if your object is of a very big class then passing it as an object will copy its state (which you might not want ..AND CAN BE BIG OVERHEAD) thus resulting in an overhead of copying object .while pointer is fixed 4-byte size (assuming 32 bit). Other reasons are already mentioned above...

There are many excellent answers already, but let me give you one example:
I have an simple Item class:
class Item
{
public:
std::string name;
int weight;
int price;
};
I make a vector to hold a bunch of them.
std::vector<Item> inventory;
I create one million Item objects, and push them back onto the vector. I sort the vector by name, and then do a simple iterative binary search for a particular item name. I test the program, and it takes over 8 minutes to finish executing. Then I change my inventory vector like so:
std::vector<Item *> inventory;
...and create my million Item objects via new. The ONLY changes I make to my code are to use the pointers to Items, excepting a loop I add for memory cleanup at the end. That program runs in under 40 seconds, or better than a 10x speed increase.
EDIT: The code is at http://pastebin.com/DK24SPeW
With compiler optimizations it shows only a 3.4x increase on the machine I just tested it on, which is still considerable.

Pointers and Object Oriented Programming

How do pointers work with the concepts of Object oriented programming?
As I understand it (and please recognize, I'm classified as an ID-10T), the main tenet of OOP is containment and keeping management responsibility (memory/implementation/etc.) contained within the class; but when an object's method returns a pointers it seems like we are 'popping' the object. Now, somebody might need to worry about:
Are they supposed to delete the pointer's associated object?
But what if the class still needs the object?
Can they change the object? If so, how? (I recognize const might solve this issue)
and so forth...
It seems the user of the object now needs to know much more about how the class works and what the class expects of the user. It feels like a "cat's out of the bag" scenario which seems to slap in the face of OOP.
NOTE: I notice this is a language independent question; however, I was prompted to ask the question while working in a C++ environment.

What you describe are ownership issues. These are orthogonal (i.e. independent, you can have either without the other or even both) to object orientation. You have the same issues if you do not use OOP and juggle pointers to POD structs. You don't have the issue if you use OOP but solve it somehow. You can (try to) solve it using more OOP or in another way.
They are also orthogonal to the use of pointers (unless you nit pick and extend the definition of pointer). For example, the same issues arise if two separate places hold indices into an array and mutate, resize and ultimately delete the array.
In C++, the usual solution is to select the right smart pointer type (e.g. return a shared pointer when you wish to share the object, or a unique pointer to signify exclusive ownership), along with extensive documentation. Actually, the latter is a key ingredient in any language.
One OOP-related thing you can do to help this is encapsulation (of course, you can have encaptulation just fine without OOP). For instance, don't expose the object at all, only expose methods which query the object under the hood. Or don't expose raw pointers, only expose smart pointers.

For starters... You can't have polymorphism without pointers or
references. In C++, traditionally, objects are copied, and have (for
the most part) automatic storage duration. But copy doesn't work with
polymorphic objects—they tend to get sliced. And OO also often
means identity, which in turn means you don't want copy. So the
solution is for the object to be dynamically allocated, and to pass
around pointers. What you do with them is part of the design:
If the object is logically part of another object, then that object is
responsible for its lifetime, and objects which receive the pointer
should take steps to ensure that they don't use it after the owning
object disappears. (Note that this is true even in languages with
garbage collection. The object won't disappear as long as you've got a
pointer to it, but once the owning object is invalid, the owned object
may become invalid as well. The fact that the garbage collector won't
recycle the memory won't guarantee that the object you point to is
usable.)
If the object is a first class entity itself, rather than being
logically part of another object, then it should probably take care of
itself. Again, other objects which may hold a pointer to it must be
informed if it ceases to exist (or becomes invalid). The use of the
Observer pattern is the usual solution. Back when I started C++, there
was a fashion for "relationship management", with some sort of
management classes where you registered relationships, and which
supposedly ensured that everything worked out OK. In practice, they
either didn't work, or didn't do any more than the simple observer
pattern, and you don't hear any more of them today.
For the most part, your precise questions are part of the contract that
each class has to establish for each of its functions. For true OO
classes (entity objects), you should probably never delete them: that's
there business, not yours. But there are exceptions: if you're dealing
with transactions, for example, a deleted object cannot be rolled back,
so when an object decides to delete itself, it will usually register
this fact with the transaction manager, who will delete it as part of
the commit, once it's established that roll back won't be necessary. As
for changing the object, that's a question of the contract: in a lot of
applications, there are mapping objects, which are used to map an
external identifier of some sort to the object. With the goal, often,
of being able to modify the object.

From my understanding and experience, it generally revolves around what it is that you are trying to do as well as the language using pointers (e.g. C++ vs Objective-C).
Usually, though, in C++ terms, I've found that it's best to return either a reference to a smart pointer (such as std::shared_ptr) by reference (perhaps even const reference, depending on the situation), or simply hide the pointer in the class, and if it NEEDS to be accessed or used by something outside of it, use a getter method which either copies the pointer and returns that, or returns a reference to a pointer (granted, AFAIK ref-to-ptr is only possible in C++). If someone doesn't know that you shouldn't delete a ref-to-ptr in most situations (of course, if its deallocation is handled by the class internally), you should really think twice about whether or not they're ready to be doing C++ stuff on your team.
It's fairly common to just use public references for class members if they can be stack allocated (i.e., if they won't take up too much memory), while managing heap allocated objects internally. If you need to set the class member outside of the class, it's possible to just use a set method which takes the required value, rather than access it directly.

When do you want to use pointers vs values in C++?

I'm coming from Java and attempting to learn C++.
As far as I can tell, using Pointers is very similar to how reference variables work in Java, in that you pass a memory address to the value. So I feel like I have gotten a pretty good understanding of them. I also understand that these variables are stored on the heap.
However, I see that there is another way in which you can declare variables in C++, without the new operator/pointers simply doing something like:
Employee boss("Frank");
Which will create a value of Employee with "Frank" as the parameter. These variables are stored on the stack.
So, you have these 2 very different ways of creating variables and both with their own unique behavior (with memory management as well?).
My question is, when is it appropriate to use pointers VS values? What is the best practice? How should I know in what way I want to declare my variables most of the time?

C++ pointers operate exactly like Java objects, in that they may be invalid (NULL) and are cheap to pass to procedures.
C++ references are a thin wrapper around pointers. They're not supposed to be invalid, but in some cases may be. For example, a reference might be created from a NULL pointer, or the memory it references might be deleted.
In the code example you give:
Employee boss("Frank");
you are using something called a "value". Unlike pointers and references, values are the object they represent, not an indirection.
My question is, when is it appropriate
to use Pointers VS [values]?
What is the best practice? How should
I know in what way I want to declare
my variables most of the time?
C++ doesn't have garbage collection, so values are used when the scope of a variable is limited. For example, instead of allocating one with new and deallocating with delete for every variable in a procedure, you can allocate them on the stack and not worry about the memory.

There are two different issues at play here: Creating objects, and referring to them.
Creating
There are two places that objects are created: the stack and the heap. If you use the syntax you described:
Employee boss("Frank");
Will create it on the stack. If you write this:
Employee* boss = new Employee("Frank");
It will create it on the heap. If you're not familiar with the concepts of stack and heap, it is vitally important to being a good C++ coder, so learn about it!
Referring
Referring to objects is somewhat different. Regardless of how an object is created, it can be referred to using a pointer or a reference (or just a standard variable). Using references and pointers in C/C++ is actually very much the same thing, though there are important differences.
When using pointers or references, a copy of the object is not made.
// No copies made:
Employee& frank = boss; // Using References
Employee* frank = &boss; // Using a Pointer
A copy is made when you use neither.
// Copy is made:
Employee frank = boss;
So when would you use pointers, and when would you use references? I find that a good practise is to only use pointers when it is meaningful for it to be null. If something should not be null, make it a reference.

Generally you want to stick with references as much as possible. The reason is because of RAII. Basically, this guarantees no memory leaks if you are using RAII.
However, IMO you should use pointers when the objects you are using would be very expensive to copy. Passing around container types will cause duplicates of the containers to occur... not a great idea.
The best way is to use smart pointers... basically references which hold a pointer and keep track of the number of references to the pointer. This is really the best of both worlds... Cheap initialization/copies and reference counting to practically eliminate memory leaks.

Note: I will use "object" to refer to objects and to primitive types such as int, float... these are not the same in C++ (but usually you can ignore this).
Use values when you are creating an object that you control from that scope, and it should die when that scope ends. Also, use value when you want to use a copy of an external object, but you only want to handle the copy not the real object. Example:
int myFunction(int external_value1, Object external_value2){
---
}
Use pointers/references when you are creating an Object that should not die when it's scope of creation ends (ensure to pass a pointer to it to some other scope!), or when using an external value that is expensive to copy (like a container), or when you want to operate directly on that external object and not on a copy of it. That's why I/O parameters of functions are usually pointers or references, as you want to act on an external object (defined outside of the scope of the function) directly and not on a local copy. Example:
int myOtherFunction(int *external_value1, Object *external_value2{
---
}
In this example, if you operate on the value pointed by the parameters, you are affecting exactly that: the value those pointers point at, thus changing a variable outside of the function scope. In fact is a pass-by-value, but you are copying only the pointers, and using them to "attack" the external values.
References are, as stated by other post, just syntactic sugar for pointers. Once you understand pointers, you understand references ;).

The C++ FAQ has a good answer to this specific question:
http://www.parashift.com/c++-faq-lite/references.html#faq-8.6

When to use pointers and when not to?
C++ References and Java References
Switching from Java to C++ - What's the Easy Way?
many more

Two different beasts: if you assign by pointer, you end-up with n-to-1 relationships which you must handle through proper resource management. This is normally taken care of in Java.
In what you call "Reference Objects", you get different objects (which need to be tracked etc.).

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