When we declare something dynamically like this
int *ptr = new int [100];
and then change the pointer address (i.e point it to something else)
int pointer[5] = {1,2,1,3,1,};
ptr = pointer ;
now what happens to that memory which contained 100 integers?
is there anyway to regain control of that memory ? and if not, does it lead to a memory leak?
now what happens to that memory which contained 100 integers?
It is "leaked" because you lost the only handle you had to it.
is there anyway to regain control of that memory ?
No, unless you take care to create another handle before re-assigning ptr.
int* ptr = new int[100];
int* ptr2 = ptr;
...
ptr = pointer;
delete [] ptr2;
In real code you would not rely on raw pointers to allocated memory, but rather use whichever RAII type works best for your problem (e.g. std::vector, std::unique_ptr etc.)
In the sample you give, ptr is the only pointer that references the hundred int created by new int [100] (i.e. points at the first element).
Changing the value of ptr does not affect the hundred int. It means that ptr no longer contains the address of the first element.
Practically, after the assignment ptr = pointer, the allocated memory is said to be leaked - it has been allocated, there is no way in standard C++ to access it, and no way in standard C++ to release it (e.g. make the memory it consumes available for reallocation in another new expression).
The only way to keep control of the memory in your case is to store the value of ptr in another variable (or a member of a structure, or an element of an array of pointers, ....) BEFORE reassigning it. If the value of ptr is stored before reassigning ptr, and the place it is stored still exists, the value can be retrieved.
Practically, it would be better to use a standard container (e.g. a std::vector<int>) and not use a new expression in your code. That way, the contents of the container are accessible as long as the container exists (and cease to exist, as far as your program is concerned, when the container does).
Any block of memory allocated with new is unavailable until it is released, either by a corresponding delete or when the process in which the memory was allocated terminates.
The memory allocator actually has no clue about what references you maintain on allocated objects.It just allows you to release the corresponding memory blocks if you manage to provide valid addresses.
How you keep track of these addresses is entirely up to you.
So if for some reason you loose track of a chunk of memory (by overwriting a data pointer as in your example, or simply forgetting to use it at some point to release the memory), you will deprive your system of this bit of memory until the underlying process terminates.
This becomes a memory leak if the "memory lapse" is unintentional, even more so if this occurs repeatedly.
For instance, if you need some data that has the same life span as the process, you can do a one-time allocation and never release the memory (since it will be released automatically upon termination). That could horrify the good practices zealots, but would nevertheless not qualify as a memory leak.
On the other hand, there are other ways of messing up you memory allocator : passing an invalid pointer to delete (including a reference to an already released object, i.e. deleting the same instance of an object more than once, or a pointer to something that was not allocated by new, like a local or global variable) will bring you the wonderful realm of undefined behaviours, which could cause random memory losses, data corruption or nuclear meltdowns.
Related
# include <iostream>
int main()
{
using std::cout;
int *p= new int;
*p = 10;
cout<<*p<<"\t"<<p<<"\n";
delete p;
cout<<*p<<"\t"<<p<<"\n";
return 0;
}
Output:
10 0x237c010
0 0x237c010
Here after deleting p, why the pointer p retains its value? Doesn't delete frees the pointer p?
What exactly is meant by 'freeing the pointer'?
Does 'delete p' simply mean '*p = 0'?(which seems from the output)
Here after deleting p, why the pointer p retains its value?
It's how the language is designed. If you want the pointer you hold to be zeroed, you'll need to assign it zero yourself. The pointer p is another piece of memory, separate from the allocation/object it points to.
Doesn't delete frees the pointer p?
It calls the destructor the object and returns the memory to the system (like free). If it is an array (delete[]), destructors for all elements will be called, then the memory will be returned.
What exactly is meant by 'freeing the pointer'?
When you want a piece of memory from the system, you allocate it (e.g. using new). When you are finished using it, you return it using the corresponding free/delete call. It's a resource, which you must return. If you do not, your program will leak (and nobody wants that).
In order to understand what freeing memory means, you must first understand what allocating memory means. What follows is a simplified explanation.
There exists memory. Memory is a large blob of stuff that you could access. But since it's global, you need some way to portion it out. Some way to govern who can access which pieces of memory. One of the systems that governs the apportionment of memory is called the "heap".
The heap owns some quantity of memory (some is owned by the stack and some is owned by static data, but nevermind that now). At the beginning of your program, the heap says that you have access to no heap-owned memory.
What new int does is two fold. First, it goes to the heap system and says, "I want a piece of memory suitable to store an int into." It gets back a pointer to exactly that: a piece of the heap, into which you can safely store and retrieve exactly one value of type int.
You are now the proud owner of one int's worth of memory. The heap guarantees that as long as its rules are followed, whatever you put there will be preserved until you explicitly change it. This is the covenant between you and the almighty heap.
The other thing new int does is initialize that piece of the heap with an int value. In this case, it is default initialized, because no value was passed (new int(5) would initialize it with the value 5).
From this point forward, you are legally allowed to store exactly one int in this piece of memory. You are allowed to retrieve the int stored there. And you're allowed to do one other thing: tell the heap that you are finished using that memory.
When you call delete p, two things happen. First, p is deinitialized. Again, because it is an int, nothing happens. If this were a class, its destructor would be called.
But after that, delete goes out to the heap and says, "Hey heap: remember this pointer to an int you gave me? I'm done with it now." The heap system can do whatever it wants. Maybe it will clear the memory, as some heaps do in debug-builds. In release builds however, the memory may not be cleared.
Of course, the reason why the heap can do whatever it wants is because, the moment you delete that pointer, you enter into a new agreement with the heap. Previously, you asked for a piece of memory for an int, and the heap obliged. You owned that memory, and the heap guaranteed that it was yours for as long as you wanted. Stuff you put there would remain there.
After you had your fun, you returned it to the heap. And here's where the contract comes in. When you say delete p, for any object p, you are saying the following:
I solemnly swear not to touch this memory address again!
Now, the heap might give that memory address back to you if you call new int again. It might give you a different one. But you only have access to memory allocated by the heap during the time between new and delete.
Given this, what does this mean?
delete p;
cout << *p << "\t" << p << "\n";
In C++ parlance, this is called "undefined behavior". The C++ specification has a lot of things that are stated to be "undefined". When you trigger undefined behavior anything can happen! *p could be 0. *p could be the value it used to be. Doing *p could crash your program.
The C++ specification is a contract between you and your compiler/computer. It says what you can do, and it says how the system responds. "Undefined behavior" is what happens when you break the contract, when you do something the C++ specification says you aren't supposed to. At that point, anything can happen.
When you called delete p, you told the system that you were finished using p. By using it again, you were lying to the system. And therefore, the system no longer has to abide by any rules, like storing the values you want to store. Or continuing to run. Or not spawning demons from your nose. Or whatever.
You broke the rules. And you must suffer the consequences.
So no, delete p is not the equivalent of *p = 0. The latter simply means "set 0 into the memory pointed to by p." The former means "I'm finished using the memory pointed to by p, and I won't use it again until you tell me I can."
Here after deleting p, why the pointer p retains its value? Doesn't delete frees the pointer p?
It frees the memory the pointer points to (after calling any appropriate destructor). The value of the pointer itself is unchanged.
What exactly is meant by 'freeing the pointer'?
As above - it means freeing the memory the pointer points to.
Does 'delete p' simply mean '*p = 0'?(which seems from the output)
No. The system doesn't have to write anything to the memory that's freed, and if it does write something it doesn't have to write 0. However, the system does generally have to manage that memory in some way, and that might actually write to the area of memory that the pointer was pointing to. Also, the just-freed memory can be allocated to something else (and in a multi-threaded application, that could happen before the delete operation even returns). The new owner of that memory block can of course write whatever they want to that memory.
A pointer that is pointing to a freed block of memory is often known as a 'dangling' pointer. It is an error to dereference a dangling pointer (for read or write). You will sometimes see code immediately assign a NULL or 0 to a pointer immediately after deleting the pointer, sometimes using a macro or function template that both deletes and clears the pointer. Note that this won't fix all bugs with dangling pointers, since other pointers may have been set to point to the memory block.
The modern method of dealing with these kinds of problems is to avoid using raw pointers altogether in favor of using smart pointers such as shared_ptr or unique_ptr.
delete p simply frees the memory allocated during a call to the new operator. It does not change the value of the pointer or the content of the deallocated memory.
(Note the following isn't how it actually works so take it with a grain of salt.)
Inside the implementation of new it keeps a list of all available memory when you said "int *p= new int;" it cut a int sized block off of its list of available memory and gave it to you. When you run "delete p;" it gets put back in the list of available memory. If your program called new 30 times without calling delete at all you would get 30 different int sized chunks from new. If you called new then delete 30 times in a row you might (but not necessarily) get the same int sized block. This is because you said you weren't using it any more when you called delete and so new was free to reuse it.
TLDR; Delete notifies new that this section of memory is available again it doesn't touch your variable.
I don't quite get the point of dynamically allocated memory and I am hoping you guys can make things clearer for me.
First of all, every time we allocate memory we simply get a pointer to that memory.
int * dynInt = new int;
So what is the difference between doing what I did above and:
int someInt;
int* dynInt = &someInt;
As I understand, in both cases memory is allocated for an int, and we get a pointer to that memory.
So what's the difference between the two. When is one method preferred to the other.
Further more why do I need to free up memory with
delete dynInt;
in the first case, but not in the second case.
My guesses are:
When dynamically allocating memory for an object, the object doesn't get initialized while if you do something like in the second case, the object get's initialized. If this is the only difference, is there a any motivation behind this apart from the fact that dynamically allocating memory is faster.
The reason we don't need to use delete for the second case is because the fact that the object was initialized creates some kind of an automatic destruction routine.
Those are just guesses would love it if someone corrected me and clarified things for me.
The difference is in storage duration.
Objects with automatic storage duration are your "normal" objects that automatically go out of scope at the end of the block in which they're defined.
Create them like int someInt;
You may have heard of them as "stack objects", though I object to this terminology.
Objects with dynamic storage duration have something of a "manual" lifetime; you have to destroy them yourself with delete, and create them with the keyword new.
You may have heard of them as "heap objects", though I object to this, too.
The use of pointers is actually not strictly relevant to either of them. You can have a pointer to an object of automatic storage duration (your second example), and you can have a pointer to an object of dynamic storage duration (your first example).
But it's rare that you'll want a pointer to an automatic object, because:
you don't have one "by default";
the object isn't going to last very long, so there's not a lot you can do with such a pointer.
By contrast, dynamic objects are often accessed through pointers, simply because the syntax comes close to enforcing it. new returns a pointer for you to use, you have to pass a pointer to delete, and (aside from using references) there's actually no other way to access the object. It lives "out there" in a cloud of dynamicness that's not sitting in the local scope.
Because of this, the usage of pointers is sometimes confused with the usage of dynamic storage, but in fact the former is not causally related to the latter.
An object created like this:
int foo;
has automatic storage duration - the object lives until the variable foo goes out of scope. This means that in your first example, dynInt will be an invalid pointer once someInt goes out of scope (for example, at the end of a function).
An object created like this:
int foo* = new int;
Has dynamic storage duration - the object lives until you explicitly call delete on it.
Initialization of the objects is an orthogonal concept; it is not directly related to which type of storage-duration you use. See here for more information on initialization.
Your program gets an initial chunk of memory at startup. This memory is called the stack. The amount is usually around 2MB these days.
Your program can ask the OS for additional memory. This is called dynamic memory allocation. This allocates memory on the free store (C++ terminology) or the heap (C terminology). You can ask for as much memory as the system is willing to give (multiple gigabytes).
The syntax for allocating a variable on the stack looks like this:
{
int a; // allocate on the stack
} // automatic cleanup on scope exit
The syntax for allocating a variable using memory from the free store looks like this:
int * a = new int; // ask OS memory for storing an int
delete a; // user is responsible for deleting the object
To answer your questions:
When is one method preferred to the other.
Generally stack allocation is preferred.
Dynamic allocation required when you need to store a polymorphic object using its base type.
Always use smart pointer to automate deletion:
C++03: boost::scoped_ptr, boost::shared_ptr or std::auto_ptr.
C++11: std::unique_ptr or std::shared_ptr.
For example:
// stack allocation (safe)
Circle c;
// heap allocation (unsafe)
Shape * shape = new Circle;
delete shape;
// heap allocation with smart pointers (safe)
std::unique_ptr<Shape> shape(new Circle);
Further more why do I need to free up memory in the first case, but not in the second case.
As I mentioned above stack allocated variables are automatically deallocated on scope exit.
Note that you are not allowed to delete stack memory. Doing so would inevitably crash your application.
For a single integer it only makes sense if you need the keep the value after for example, returning from a function. Had you declared someInt as you said, it would have been invalidated as soon as it went out of scope.
However, in general there is a greater use for dynamic allocation. There are many things that your program doesn't know before allocation and depends on input. For example, your program needs to read an image file. How big is that image file? We could say we store it in an array like this:
unsigned char data[1000000];
But that would only work if the image size was less than or equal to 1000000 bytes, and would also be wasteful for smaller images. Instead, we can dynamically allocate the memory:
unsigned char* data = new unsigned char[file_size];
Here, file_size is determined at runtime. You couldn't possibly tell this value at the time of compilation.
Read more about dynamic memory allocation and also garbage collection
You really need to read a good C or C++ programming book.
Explaining in detail would take a lot of time.
The heap is the memory inside which dynamic allocation (with new in C++ or malloc in C) happens. There are system calls involved with growing and shrinking the heap. On Linux, they are mmap & munmap (used to implement malloc and new etc...).
You can call a lot of times the allocation primitive. So you could put int *p = new int; inside a loop, and get a fresh location every time you loop!
Don't forget to release memory (with delete in C++ or free in C). Otherwise, you'll get a memory leak -a naughty kind of bug-. On Linux, valgrind helps to catch them.
Whenever you are using new in C++ memory is allocated through malloc which calls the sbrk system call (or similar) itself. Therefore no one, except the OS, has knowledge about the requested size. So you'll have to use delete (which calls free which goes to sbrk again) for giving memory back to the system. Otherwise you'll get a memory leak.
Now, when it comes to your second case, the compiler has knowledge about the size of the allocated memory. That is, in your case, the size of one int. Setting a pointer to the address of this int does not change anything in the knowledge of the needed memory. Or with other words: The compiler is able to take care about freeing of the memory. In the first case with new this is not possible.
In addition to that: new respectively malloc do not need to allocate exactly the requsted size, which makes things a bit more complicated.
Edit
Two more common phrases: The first case is also known as static memory allocation (done by the compiler), the second case refers to dynamic memory allocation (done by the runtime system).
What happens if your program is supposed to let the user store any number of integers? Then you'll need to decide during run-time, based on the user's input, how many ints to allocate, so this must be done dynamically.
In a nutshell, dynamically allocated object's lifetime is controlled by you and not by the language. This allows you to let it live as long as it is required (as opposed to end of the scope), possibly determined by a condition that can only be calculated at run-rime.
Also, dynamic memory is typically much more "scalable" - i.e. you can allocate more and/or larger objects compared to stack-based allocation.
The allocation essentially "marks" a piece of memory so no other object can be allocated in the same space. De-allocation "unmarks" that piece of memory so it can be reused for later allocations. If you fail to deallocate memory after it is no longer needed, you get a condition known as "memory leak" - your program is occupying a memory it no longer needs, leading to possible failure to allocate new memory (due to the lack of free memory), and just generally putting an unnecessary strain on the system.
If I have a pointer pointing to a specific memory address at the heap. I would like this same pointer to point to another memory address, should I first delete the pointer? But, in this case am I actually deleting the pointer or just breaking the reference (memory address) the pointer is pointing at?
So, in other words, if I delete a pointer, does this mean it doesn't exist any more? Or, it is there, but not pointing to where it was?
The syntax of delete is a bit misleading. When you write
T* ptr = /* ... */
delete ptr;
You are not deleting the variable ptr. Instead, you are deleting the object that ptr points at. The value of ptr is unchanged and it still points where it used to, so you should be sure not to dereference it without first reassigning it.
There is no requirement that you delete a pointer before you reassign it. However, you should ensure that if you are about to reassign a pointer in a way that causes you to lose your last reference to the object being pointed at (for example, if this pointer is the only pointer in the program to its pointee), then you should delete it to ensure that you don't leak memory.
One technique many C++ programmers use to simplify the logic for when to free memory is to use smart pointers, objects that overload the operators necessary to mimic a pointer and that have custom code that executes automatically to help keep track of resources. The new C++0x standard, for example, will provide a shared_ptr and unique_ptr type for this purpose. shared_ptr acts like a regular pointer, except that it keeps track of how many shared_ptrs there are to a resource. When the last shared_ptr to a resource changes where it's pointing (either by being reassigned or by being destroyed), it then frees the resource. For example:
{
shared_ptr<int> myPtr(new int);
*myPtr = 137;
{
shared_ptr<int> myOtherPtr = myPtr;
*myPtr = 42;
}
}
Notice that nowhere in this code is there a call to delete to match the call to new! This is because the shared_ptr is smart enough to notice when the last pointer stops pointing to the resource.
There are a few idiosyncrasies to be aware of when using smart pointers, but they're well worth the time investment to learn about. You can write much cleaner code once you understand how they work.
When you delete a pointer you release the memory allocated to the pointed to object. So if you just want your pointer to point to a new memory location you should not delete the pointer. But if you want to destroy the object currently it is pointing to and then then point to a different object then you should delete the pointer.
xtofl, while being funny, is a bit correct.
If YOU 'new' a memory address, then you should delete it, otherwise leave it alone. Maybe you are thinking too much about it, but you think about it like this. Yea, the memory is always there, but if you put a fence around it, you need to take the fence down, or no one else can you it.
When you call delete you mark the memory pointed to by the pointer as free - the heap takes ownership of it and can reuse it, that's all, the pointer itself is usually unchanged.
What to do with the pointer depends on what you want. If you no longer need that memory block - use delete to free the block. If you need it later - store the address somewhere where you can retrieve it later.
To answer your question directly, this has been asked before. delete will delete what your pointer points to, but there was a suggestion by Bjarne Stroustrup that the value of the pointer itself can no longer be relied upon, particularly if it is an l-value. However that does not affect the ability to reassign it so this would be valid:
for( p = first; p != last; ++p )
{
delete p;
}
if you are iterating over an array of pointers, all of which had been allocated with new.
Memory management in C++ is best done with a technique called RAII, "resource acquisition is initialization".
What that actually means is that at the time you allocate the resource you immediately take care of its lifetime, i.e. you "manage" it by putting it inside some object that will delete it for you when it's no longer required.
shared_ptr is a technique commonly used where the resource will be used in many places and you do not know for certain which will be the last one to "release" it, i.e. no longer require it.
shared_ptr is often used in other places simply for its semantics, i.e. you can copy and assign them easily enough.
There are other memory management smart pointers, in particular std::auto_ptr which will be superceded by unique_ptr, and there is also scoped_ptr. weak_ptr is a methodology to be able to obtain a shared_ptr if one exists somewhere, but not holding a reference yourself. You call "lock()" which gives you a shared_ptr to memory or a NULL one if all the current shared pointers have gone.
For arrays, you would not normally use a smart pointer but simply use vector.
For strings you would normally use the string class rather than think of it as a vector of char.
In short, you do not "delete a pointer", you delete whatever the pointer points to.
This is a classical problem: If you delete it, and someone else is pointing to it, they will read garbage (and most likely crash your application). On the other hand, if you do not and this was the last pointer, your application will leak memory.
In addition, a pointer may point to thing that were not originally allocated by "new", e.g. a static variable, an object on the stack, or into the middle of another object. In all those cases you're not allowed to delete whatever the pointer points to.
Typically, when designing an application, you (yes, you) have to decide which part of the application owns a specific object. It, and only it, should delete the objects when it is done with it.
what is the difference between deleting a pointer, setting it to null, and freeing it.
delete ptr;
vs.
ptr=NULL;
vs.
free(ptr);
Your question suggests that you come from a language that has garbage collection. C++ does not have garbage collection.
If you set a pointer to NULL, this does not cause the memory to return to the pool of available memory. If no other pointers point to this block of memory, you now simply have an "orphaned" block of memory that remains allocated but is now unreachable -- a leak. Leaks only cause a program to crash if they build up to a point where no memory is left to allocate.
There's also the converse situation, where you delete a block of memory using a pointer, and later try to access that memory as though it was still allocated. This is possible because calling delete on a pointer does not set the pointer to NULL -- it still points to the address of memory that previously was allocated. A pointer to memory that is no longer allocated is called a dangling pointer and accessing it will usually cause strange program behaviour and crashes, since its contents are probably not what you expect -- that piece of memory may have since been reallocated for some other purpose.
[EDIT] As stinky472 mentions, another difference between delete and free() is that only the former calls the object's destructor. (Remember that you must call delete on an object allocated with new, and free() for memory allocated with malloc() -- they can't be mixed.) In C++, it's always best to use static allocation if possible, but if not, then prefer new to malloc().
delete will give allocated memory back to the C++ runtime library. You always need a matching new, which allocated the memory on the heap before. NULL is something completely different. A "placeholder" to signify that it points to no address. In C++, NULLis a MACRO defined as 0. So if don't like MACROS, using 0 directly is also possible. In C++0x nullptr is introduced and preferable.
Example:
int* a; //declare pointer
a = NULL; //point 'a' to NULL to show that pointer is not yet initialized
a = new int; //reserve memory on the heap for int
//... do more stuff with 'a' and the memory it points to
delete a; //release memory
a = NULL; //(depending on your program), you now want to set the pointer back to
// 'NULL' to show, that a is not pointing to anything anymore
well, if you created object dynamically(with 'new'), setting pointer to any value does not delete object from memory - and you get a memory leak.
As with any indirection, there are two objects involved when using pointers: the referrer (the pointer, in your example ptr) and the referenced object (what it points to, *ptr). You need to learn to differentiate between them.
When you assign NULL to a pointer, only the pointer is affected, the object it refers to is left alone. If the pointer was the last one pointing to that object, you have lost the last referring pointer pointing to it and thus cannot use it anymore. In C++, however, that does not mean the object will be deleted. C++ does not have a garbage collection. So the object is leaked.
In order for objects to be deleted, you will have to delete them manually by passing their address (stored in a pointer) to the delete operator. If you do this, only the object referred to will be affected, the pointer is left alone. It might still point to the address where the object resided in memory, even though that isn't usable anymore. That is called a dangling pointer.
When you create an object using new, you need to use delete to release its memory back to the system. The memory will then be available for others to reuse.
int* a = new int;
delete a;
NULL is just a pre-defined macro that can be assigned a pointer to mean that it does not point to anything.
int* a = new int;
a = NULL;
In the above case, after allocating storage for a, you are assigning NULL to it. However, the memory previously allocated for a was not released and cannot be reused by the system. This is what we call memory leak.
int * ptr = null;
Calling free(ptr) will do nothing. According to standard if given pointer is null then no action happens.
Using delete p will also do nothing. C++ standard says that no action will happen because it points to nothing and no type is available.
I was working on a piece of code and I was attacked by a doubt: What happens to the memory allocated to a pointer if I assign NULL to that pointer?
For instance:
A = new MyClass();
{...do something in the meantime...}
A = NULL;
The space is still allocated, but there is no reference to it. Will that space be freed later on, will it be reused, will it remain on stack, or what?
This is a classic leak.
As you say, the memory remains allocated but nothing is referencing it, so it can never be reclaimed - until the process exits.
The memory should be deallocated with delete - but using a smart pointer (e.g. std::auto_ptr or boost::shared_ptr (or tr1::shared_ptr) to wrap the pointer is a much safer way of working with pointers.
Here's how you might rewrite your example using std::auto_ptr:
std::auto_ptr a( new MyClass() );
/*...do something in the meantime...*/
a.reset();
(Instead of the call to reset() you could just let the auto_ptr instance go out of scope)
Under most circumstances, that will cause a memory leak in your process. You have several options for managing memory in C++.
Use a delete to manually free memory when you're done with it. This can be hard to get right, especially in the context of exception handling.
Use a smart pointer to manage memory for you (auto_ptr, shared_ptr, unique_ptr, etc.)
C++ does not come with a garbage collector, but nothing prevents you from using one (such as the Boehm GC) if you want to go down that route.
That is a memory leak. You have to delete memory you allocate manually.
A = new MyClass();
{...do something in the meantime...}
A = NULL;
The way I keep track of it is that there are two separate objects. Somewhere on the heap, a MyClass instance is allocated by new. And on the stack, there is a pointer named A.
A is just a pointer, there is nothing magical about out, and it doesn't have some special connection to the heap-allocated MyClass object. It just happens to point to that right now, but that can change.
And on the last line, that is exactly what happens. You change the pointer to point to something else. That doesn't affect other objects. It doesn't affect the object it used to point to, and it doesn't affect the object (if any) that it is set to point to now. Again, A is just a dumb raw pointer like any other. It might be NULL, or it might point to an object on the stack, or it might point to an object on the heap, or it might be uninitialized and point to random garbage. But that's all it does. It points, it doesn't in any way take ownership of, or modify, the object it points to.
You need to delete A;
For regular objects setting the pointer to NULL does nothing but invalidating the pointer, the object is still around in memory, this is particularly true if you notice that you may have more than one pointer to the same object, changing one shouldn't affect the others.
On most modern OSs, the application's memory will be reclaimed at exiting the application. Meanwhile, you have a memory leak.
As per Phil Nash's comment, for every new, there is a corresponding delete, likewise, for every malloc, there is a corresponding free. If the corresponding delete/free is not there, you have a leak.
Hope this helps,
Best regards,
Tom.
C++ does't have garbage collector, like some other languages has (Java, C#, ...) so you must delete allocaled objects yourself.
No, it will be lost to the process forever. You will have a memory leak. If you keep doing this, your program will eventually run out of memory!! To avoid this, delete the object when you no longer need it.
Often people will set the pointer to NULL after deleting it, so that other parts of the program can verify that the object has been deleted, and thereby avoid accessing or deleting it again.
Variables stored on the stack, are the local variables of each function, e.g. int big[10];
Variables stored on the heap, are the variables which you initiated using explicit memory allocation routines such as malloc(), calloc(), new(), etc.
Variables stored on the stack have a lifetime that is equal to the lifetime of the current stack frame. In English, when the function returns, you can no longer assume that the variables hold what you expect them to hold. That's why its a classic mistake to return a variable that was declared local in a function.
By assigning NULL to the pointer you will not free allocated memory. You should call deallocation function to free allocated memory. According to C++ Standard 5.3.4/8: "If the allocated type is a non-array type, the allocation function’s name is operator new and the deallocation function’s name is operator delete". I could suggest the following function to safely delete pointers (with assigning NULL to them):
template<typename T>
inline void SafeDelete( T*& p )
{
// Check whether type is complete.
// Deleting incomplete type will lead to undefined behavior
// according to C++ Standard 5.3.5/5.
typedef char type_must_be_complete[ sizeof(T)? 1: -1 ];
(void) sizeof(type_must_be_complete);
delete p;
p = NULL;
}