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Closed 9 years ago.
I know this is a really basic question, but I've just started with some basic C++ programming after coding a few projects with high-level languages.
Basically I have three questions:
Why use pointers over normal variables?
When and where should I use pointers?
How do you use pointers with arrays?
Why use pointers over normal variables?
Short answer is: Don't. ;-) Pointers are to be used where you can't use anything else. It is either because the lack of appropriate functionality, missing data types or for pure perfomance. More below...
When and where should I use pointers?
Short answer here is: Where you cannot use anything else. In C you don't have any support for complex datatypes such as a string. There are also no way of passing a variable "by reference" to a function. That's where you have to use pointers. Also you can have them to point at virtually anything, linked lists, members of structs and so on. But let's not go into that here.
How do you use pointers with arrays?
With little effort and much confusion. ;-) If we talk about simple data types such as int and char there is little difference between an array and a pointer.
These declarations are very similar (but not the same - e.g., sizeof will return different values):
char* a = "Hello";
char a[] = "Hello";
You can reach any element in the array like this
printf("Second char is: %c", a[1]);
Index 1 since the array starts with element 0. :-)
Or you could equally do this
printf("Second char is: %c", *(a+1));
The pointer operator (the *) is needed since we are telling printf that we want to print a character. Without the *, the character representation of the memory address itself would be printed. Now we are using the character itself instead. If we had used %s instead of %c, we would have asked printf to print the content of the memory address pointed to by 'a' plus one (in this example above), and we wouldn't have had to put the * in front:
printf("Second char is: %s", (a+1)); /* WRONG */
But this would not have just printed the second character, but instead all characters in the next memory addresses, until a null character (\0) were found. And this is where things start to get dangerous. What if you accidentally try and print a variable of the type integer instead of a char pointer with the %s formatter?
char* a = "Hello";
int b = 120;
printf("Second char is: %s", b);
This would print whatever is found on memory address 120 and go on printing until a null character was found. It is wrong and illegal to perform this printf statement, but it would probably work anyway, since a pointer actually is of the type int in many environments. Imagine the problems you might cause if you were to use sprintf() instead and assign this way too long "char array" to another variable, that only got a certain limited space allocated. You would most likely end up writing over something else in the memory and cause your program to crash (if you are lucky).
Oh, and if you don't assign a string value to the char array / pointer when you declare it, you MUST allocate sufficient amount of memory to it before giving it a value. Using malloc, calloc or similar. This since you only declared one element in your array / one single memory address to point at. So here's a few examples:
char* x;
/* Allocate 6 bytes of memory for me and point x to the first of them. */
x = (char*) malloc(6);
x[0] = 'H';
x[1] = 'e';
x[2] = 'l';
x[3] = 'l';
x[4] = 'o';
x[5] = '\0';
printf("String \"%s\" at address: %d\n", x, x);
/* Delete the allocation (reservation) of the memory. */
/* The char pointer x is still pointing to this address in memory though! */
free(x);
/* Same as malloc but here the allocated space is filled with null characters!*/
x = (char *) calloc(6, sizeof(x));
x[0] = 'H';
x[1] = 'e';
x[2] = 'l';
x[3] = 'l';
x[4] = 'o';
x[5] = '\0';
printf("String \"%s\" at address: %d\n", x, x);
/* And delete the allocation again... */
free(x);
/* We can set the size at declaration time as well */
char xx[6];
xx[0] = 'H';
xx[1] = 'e';
xx[2] = 'l';
xx[3] = 'l';
xx[4] = 'o';
xx[5] = '\0';
printf("String \"%s\" at address: %d\n", xx, xx);
Do note that you can still use the variable x after you have performed a free() of the allocated memory, but you do not know what is in there. Also do notice that the two printf() might give you different addresses, since there is no guarantee that the second allocation of memory is performed in the same space as the first one.
One reason to use pointers is so that a variable or an object can be modified in a called function.
In C++ it is a better practice to use references than pointers. Though references are essentially pointers, C++ to some extent hides the fact and makes it seem as if you are passing by value. This makes it easy to change the way the calling function receives the value without having to modify the semantics of passing it.
Consider the following examples:
Using references:
public void doSomething()
{
int i = 10;
doSomethingElse(i); // passes i by references since doSomethingElse() receives it
// by reference, but the syntax makes it appear as if i is passed
// by value
}
public void doSomethingElse(int& i) // receives i as a reference
{
cout << i << endl;
}
Using pointers:
public void doSomething()
{
int i = 10;
doSomethingElse(&i);
}
public void doSomethingElse(int* i)
{
cout << *i << endl;
}
Pointers allow you to refer to the same space in memory from multiple locations. This means that you can update memory in one location and the change can be seen from another location in your program. You will also save space by being able to share components in your data structures.
You should use pointers any place where you need to obtain and pass around the address to a specific spot in memory. You can also use pointers to navigate arrays:
An array is a block of contiguous memory that has been allocated with a specific type. The name of the array contains the value of the starting spot of the array. When you add 1, that takes you to the second spot. This allows you to write loops that increment a pointer that slides down the array without having an explicit counter for use in accessing the array.
Here is an example in C:
char hello[] = "hello";
char *p = hello;
while (*p)
{
*p += 1; // increase the character by one
p += 1; // move to the next spot
}
printf(hello);
prints
ifmmp
because it takes the value for each character and increments it by one.
Pointers are one way of getting an indirect reference to another variable. Instead of holding the value of a variable, they tell you its address. This is particularly useful when dealing with arrays, since using a pointer to the first element in an array (its address) you can quickly find the next element by incrementing the pointer (to the next address location).
The best explanation of pointers and pointer arithmetic that I've read is in K & R's The C Programming Language. A good book for beginning learning C++ is C++ Primer.
Let me try and answer this too.
Pointers are similar to references. In other words, they're not copies, but rather a way to refer to the original value.
Before anything else, one place where you will typically have to use pointers a lot is when you're dealing with embedded hardware. Maybe you need to toggle the state of a digital IO pin. Maybe you're processing an interrupt and need to store a value at a specific location. You get the picture. However, if you're not dealing with hardware directly and are just wondering about which types to use, read on.
Why use pointers as opposed to normal variables? The answer becomes clearer when you're dealing with complex types, like classes, structures and arrays. If you were to use a normal variable, you might end up making a copy (compilers are smart enough to prevent this in some situations and C++11 helps too, but we'll stay away from that discussion for now).
Now what happens if you want to modify the original value? You could use something like this:
MyType a; //let's ignore what MyType actually is right now.
a = modify(a);
That will work just fine and if you don't know exactly why you're using pointers, you shouldn't use them. Beware of the "they're probably faster" reason. Run your own tests and if they actually are faster, then use them.
However, let's say you're solving a problem where you need to allocate memory. When you allocate memory, you need to deallocate it. The memory allocation may or may not be successful. This is where pointers come in useful - they allow you to test for the existence of the object you've allocated and they allow you to access the object the memory was allocated for by de-referencing the pointer.
MyType *p = NULL; //empty pointer
if(p)
{
//we never reach here, because the pointer points to nothing
}
//now, let's allocate some memory
p = new MyType[50000];
if(p) //if the memory was allocated, this test will pass
{
//we can do something with our allocated array
for(size_t i=0; i!=50000; i++)
{
MyType &v = *(p+i); //get a reference to the ith object
//do something with it
//...
}
delete[] p; //we're done. de-allocate the memory
}
This is the key to why you would use pointers - references assume the element you're referencing exists already. A pointer does not.
The other reason why you would use pointers (or at least end up having to deal with them) is because they're a data type that existed before references. Therefore, if you end up using libraries to do the things that you know they're better at, you will find that a lot of these libraries use pointers all over the place, simply because of how long they've been around (a lot of them were written before C++).
If you didn't use any libraries, you could design your code in such a way that you could stay away from pointers, but given that pointers are one of the basic types of the language, the faster you get comfortable using them, the more portable your C++ skills would be.
From a maintainability point of view, I should also mention that when you do use pointers, you either have to test for their validity and handle the case when they're not valid, or, just assume they are valid and accept the fact that your program will crash or worse WHEN that assumption is broken. Put another way, your choice with pointers is to either introduce code complexity or more maintenance effort when something breaks and you're trying to track down a bug that belongs to a whole class of errors that pointers introduce, like memory corruption.
So if you control all of your code, stay away from pointers and instead use references, keeping them const when you can. This will force you to think about the life times of your objects and will end up keeping your code easier to understand.
Just remember this difference: A reference is essentially a valid pointer. A pointer is not always valid.
So am I saying that its impossible to create an invalid reference? No. Its totally possible, because C++ lets you do almost anything. It's just harder to do unintentionally and you will be amazed at how many bugs are unintentional :)
Here's a slightly different, but insightful take on why many features of C make sense: http://steve.yegge.googlepages.com/tour-de-babel#C
Basically, the standard CPU architecture is a Von Neumann architecture, and it's tremendously useful to be able to refer to the location of a data item in memory, and do arithmetic with it, on such a machine. If you know any variant of assembly language, you will quickly see how crucial this is at the low level.
C++ makes pointers a bit confusing, since it sometimes manages them for you and hides their effect in the form of "references." If you use straight C, the need for pointers is much more obvious: there's no other way to do call-by-reference, it's the best way to store a string, it's the best way to iterate through an array, etc.
One use of pointers (I won't mention things already covered in other people's posts) is to access memory that you haven't allocated. This isn't useful much for PC programming, but it's used in embedded programming to access memory mapped hardware devices.
Back in the old days of DOS, you used to be able to access the video card's video memory directly by declaring a pointer to:
unsigned char *pVideoMemory = (unsigned char *)0xA0000000;
Many embedded devices still use this technique.
In large part, pointers are arrays (in C/C++) - they are addresses in memory, and can be accessed like an array if desired (in "normal" cases).
Since they're the address of an item, they're small: they take up only the space of an address. Since they're small, sending them to a function is cheap. And then they allow that function to work on the actual item rather than a copy.
If you want to do dynamic storage allocation (such as for a linked-list), you must use pointers, because they're the only way to grab memory from the heap.
Pointers are important in many data structures whose design requires the ability to link or chain one "node" to another efficiently. You would not "choose" a pointer over say a normal data type like float, they simply have different purposes.
Pointers are useful where you require high performance and/or compact memory footprint.
The address of the first element in your array can be assigned to a pointer. This then allows you to access the underlying allocated bytes directly. The whole point of an array is to avoid you needing to do this though.
One way to use pointers over variables is to eliminate duplicate memory required. For example, if you have some large complex object, you can use a pointer to point to that variable for each reference you make. With a variable, you need to duplicate the memory for each copy.
In C++, if you want to use subtype polymorphism, you have to use pointers. See this post: C++ Polymorphism without pointers.
Really, when you think about it, this makes sense. When you use subtype polymorphism, ultimately, you don't know ahead of time which class's or subclass's implementation of the method will be invoked because you don't know what the actual class is.
This idea of having a variable that holds an object of an unknown class is incompatible with C++'s default (non-pointer) mode of storing objects on the stack, where the amount of space allocated directly corresponds to the class. Note: if a class has 5 instance fields versus 3, more space will need to be allocated.
Note that if you are using '&' to pass arguments by reference, indirection (i.e., pointers) is still involved behind the scenes. The '&' is just syntactic sugar that (1) saves you the trouble of using pointer syntax and (2) allows the compiler to be more strict (such as prohibiting null pointers).
Because copying big objects all over the places wastes time and memory.
Here's my anwser, and I won't promse to be an expert, but I've found pointers to be great in one of my libraries I'm trying to write. In this library (It's a graphics API with OpenGL:-)) you can create a triangle with vertex objects passed into them. The draw method takes these triangle objects, and well.. draws them based on the vertex objects i created. Well, its ok.
But, what if i change a vertex coordinate? Move it or something with moveX() in the vertex class? Well, ok, now i have to update the triangle, adding more methods and performance is being wasted because i have to update the triangle every time a vertex moves. Still not a big deal, but it's not that great.
Now, what if i have a mesh with tons of vertices and tons of triangles, and the mesh is rotateing, and moveing, and such. I'll have to update every triangle that uses these vertices, and probably every triangle in the scene because i wouldn't know which ones use which vertices. That's hugely computer intensive, and if I have several meshes ontop of a landscape, oh god! I'm in trouble, because im updateing every triangle almost every frame because these vertices are changing al the time!
With pointers, you don't have to update the triangles.
If I had three *Vertex objects per triangle class, not only am i saving room because a zillion triangles don't have three vertex objects which are large themselves, but also these pointers will always point to the Vertices they are meant to, no matter how often the vertices change. Since the pointers still point to the same vertex, the triangles don't change, and the update process is easier to handle. If I confused you, I wouldn't doubt it, I don't pretend to be an expert, just throwing my two cents into the discussion.
The need for pointers in C language is described here
The basic idea is that many limitations in the language (like using arrays, strings and modifying multiple variables in functions) could be removed by manipulating with the memory location of the data. To overcome these limitations, pointers were introduced in C.
Further, it is also seen that using pointers, you can run your code faster and save memory in cases where you are passing big data types (like a structure with many fields) to a function. Making a copy of such data types before passing would take time and would consume memory. This is another reason why programmers prefer pointers for big data types.
PS: Please refer the link provided for detailed explanation with sample code.
In java and C# all the object references are pointers, the thing with c++ is that you have more control on where you pointer points. Remember With great power comes grand responsibility.
Regarding your second question, generally you don't need to use pointers while programming, however there is one exception to this and that is when you make a public API.
The problem with C++ constructs that people generally use to replace pointers are very dependent on the toolset that you use which is fine when you have all the control you need over the source code, however if you compile a static library with visual studio 2008 for instance and try to use it in a visual studio 2010 you will get a ton of linker errors because the new project is linked with a newer version of STL which is not backwards compatible. Things get even nastier if you compile a DLL and give an import library that people use in a different toolset because in that case your program will crash sooner or later for no apparent reason.
So for the purpose of moving large data sets from one library to another you could consider giving a pointer to an array to the function that is supposed to copy the data if you don't want to force others to use the same tools that you use. The good part about this is that it doesn't even have to be a C-style array, you can use a std::vector and give the pointer by giving the address of the first element &vector[0] for instance, and use the std::vector to manage the array internally.
Another good reason to use pointers in C++ again relates to libraries, consider having a dll that cannot be loaded when your program runs, so if you use an import library then the dependency isn't satisfied and the program crashes. This is the case for instance when you give a public api in a dll alongside your application and you want to access it from other applications. In this case in order to use the API you need to load the dll from its' location (usually it's in a registry key) and then you need to use a function pointer to be able to call functions inside the DLL. Sometimes the people that make the API are nice enough to give you a .h file that contain helper functions to automate this process and give you all the function pointers that you need, but if not you can use LoadLibrary and GetProcAddress on windows and dlopen and dlsym on unix to get them (considering that you know the entire signature of the function).
In some cases, function pointers are required to use functions that are in a shared library (.DLL or .so). This includes performing stuff across languages, where oftentimes a DLL interface is provided.
Making compilers
Making scientific calculators, where you have an array or vector or string map of function pointers?
Trying to modify video memory directly - making your own graphics package
Making an API!
Data structures - node link pointers for special trees you are making
There are Lots of reasons for pointers. Having C name mangling especially is important in DLLs if you want to maintain cross-language compatibility.
As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance.
Closed 9 years ago.
I know this is a really basic question, but I've just started with some basic C++ programming after coding a few projects with high-level languages.
Basically I have three questions:
Why use pointers over normal variables?
When and where should I use pointers?
How do you use pointers with arrays?
Why use pointers over normal variables?
Short answer is: Don't. ;-) Pointers are to be used where you can't use anything else. It is either because the lack of appropriate functionality, missing data types or for pure perfomance. More below...
When and where should I use pointers?
Short answer here is: Where you cannot use anything else. In C you don't have any support for complex datatypes such as a string. There are also no way of passing a variable "by reference" to a function. That's where you have to use pointers. Also you can have them to point at virtually anything, linked lists, members of structs and so on. But let's not go into that here.
How do you use pointers with arrays?
With little effort and much confusion. ;-) If we talk about simple data types such as int and char there is little difference between an array and a pointer.
These declarations are very similar (but not the same - e.g., sizeof will return different values):
char* a = "Hello";
char a[] = "Hello";
You can reach any element in the array like this
printf("Second char is: %c", a[1]);
Index 1 since the array starts with element 0. :-)
Or you could equally do this
printf("Second char is: %c", *(a+1));
The pointer operator (the *) is needed since we are telling printf that we want to print a character. Without the *, the character representation of the memory address itself would be printed. Now we are using the character itself instead. If we had used %s instead of %c, we would have asked printf to print the content of the memory address pointed to by 'a' plus one (in this example above), and we wouldn't have had to put the * in front:
printf("Second char is: %s", (a+1)); /* WRONG */
But this would not have just printed the second character, but instead all characters in the next memory addresses, until a null character (\0) were found. And this is where things start to get dangerous. What if you accidentally try and print a variable of the type integer instead of a char pointer with the %s formatter?
char* a = "Hello";
int b = 120;
printf("Second char is: %s", b);
This would print whatever is found on memory address 120 and go on printing until a null character was found. It is wrong and illegal to perform this printf statement, but it would probably work anyway, since a pointer actually is of the type int in many environments. Imagine the problems you might cause if you were to use sprintf() instead and assign this way too long "char array" to another variable, that only got a certain limited space allocated. You would most likely end up writing over something else in the memory and cause your program to crash (if you are lucky).
Oh, and if you don't assign a string value to the char array / pointer when you declare it, you MUST allocate sufficient amount of memory to it before giving it a value. Using malloc, calloc or similar. This since you only declared one element in your array / one single memory address to point at. So here's a few examples:
char* x;
/* Allocate 6 bytes of memory for me and point x to the first of them. */
x = (char*) malloc(6);
x[0] = 'H';
x[1] = 'e';
x[2] = 'l';
x[3] = 'l';
x[4] = 'o';
x[5] = '\0';
printf("String \"%s\" at address: %d\n", x, x);
/* Delete the allocation (reservation) of the memory. */
/* The char pointer x is still pointing to this address in memory though! */
free(x);
/* Same as malloc but here the allocated space is filled with null characters!*/
x = (char *) calloc(6, sizeof(x));
x[0] = 'H';
x[1] = 'e';
x[2] = 'l';
x[3] = 'l';
x[4] = 'o';
x[5] = '\0';
printf("String \"%s\" at address: %d\n", x, x);
/* And delete the allocation again... */
free(x);
/* We can set the size at declaration time as well */
char xx[6];
xx[0] = 'H';
xx[1] = 'e';
xx[2] = 'l';
xx[3] = 'l';
xx[4] = 'o';
xx[5] = '\0';
printf("String \"%s\" at address: %d\n", xx, xx);
Do note that you can still use the variable x after you have performed a free() of the allocated memory, but you do not know what is in there. Also do notice that the two printf() might give you different addresses, since there is no guarantee that the second allocation of memory is performed in the same space as the first one.
One reason to use pointers is so that a variable or an object can be modified in a called function.
In C++ it is a better practice to use references than pointers. Though references are essentially pointers, C++ to some extent hides the fact and makes it seem as if you are passing by value. This makes it easy to change the way the calling function receives the value without having to modify the semantics of passing it.
Consider the following examples:
Using references:
public void doSomething()
{
int i = 10;
doSomethingElse(i); // passes i by references since doSomethingElse() receives it
// by reference, but the syntax makes it appear as if i is passed
// by value
}
public void doSomethingElse(int& i) // receives i as a reference
{
cout << i << endl;
}
Using pointers:
public void doSomething()
{
int i = 10;
doSomethingElse(&i);
}
public void doSomethingElse(int* i)
{
cout << *i << endl;
}
Pointers allow you to refer to the same space in memory from multiple locations. This means that you can update memory in one location and the change can be seen from another location in your program. You will also save space by being able to share components in your data structures.
You should use pointers any place where you need to obtain and pass around the address to a specific spot in memory. You can also use pointers to navigate arrays:
An array is a block of contiguous memory that has been allocated with a specific type. The name of the array contains the value of the starting spot of the array. When you add 1, that takes you to the second spot. This allows you to write loops that increment a pointer that slides down the array without having an explicit counter for use in accessing the array.
Here is an example in C:
char hello[] = "hello";
char *p = hello;
while (*p)
{
*p += 1; // increase the character by one
p += 1; // move to the next spot
}
printf(hello);
prints
ifmmp
because it takes the value for each character and increments it by one.
Pointers are one way of getting an indirect reference to another variable. Instead of holding the value of a variable, they tell you its address. This is particularly useful when dealing with arrays, since using a pointer to the first element in an array (its address) you can quickly find the next element by incrementing the pointer (to the next address location).
The best explanation of pointers and pointer arithmetic that I've read is in K & R's The C Programming Language. A good book for beginning learning C++ is C++ Primer.
Let me try and answer this too.
Pointers are similar to references. In other words, they're not copies, but rather a way to refer to the original value.
Before anything else, one place where you will typically have to use pointers a lot is when you're dealing with embedded hardware. Maybe you need to toggle the state of a digital IO pin. Maybe you're processing an interrupt and need to store a value at a specific location. You get the picture. However, if you're not dealing with hardware directly and are just wondering about which types to use, read on.
Why use pointers as opposed to normal variables? The answer becomes clearer when you're dealing with complex types, like classes, structures and arrays. If you were to use a normal variable, you might end up making a copy (compilers are smart enough to prevent this in some situations and C++11 helps too, but we'll stay away from that discussion for now).
Now what happens if you want to modify the original value? You could use something like this:
MyType a; //let's ignore what MyType actually is right now.
a = modify(a);
That will work just fine and if you don't know exactly why you're using pointers, you shouldn't use them. Beware of the "they're probably faster" reason. Run your own tests and if they actually are faster, then use them.
However, let's say you're solving a problem where you need to allocate memory. When you allocate memory, you need to deallocate it. The memory allocation may or may not be successful. This is where pointers come in useful - they allow you to test for the existence of the object you've allocated and they allow you to access the object the memory was allocated for by de-referencing the pointer.
MyType *p = NULL; //empty pointer
if(p)
{
//we never reach here, because the pointer points to nothing
}
//now, let's allocate some memory
p = new MyType[50000];
if(p) //if the memory was allocated, this test will pass
{
//we can do something with our allocated array
for(size_t i=0; i!=50000; i++)
{
MyType &v = *(p+i); //get a reference to the ith object
//do something with it
//...
}
delete[] p; //we're done. de-allocate the memory
}
This is the key to why you would use pointers - references assume the element you're referencing exists already. A pointer does not.
The other reason why you would use pointers (or at least end up having to deal with them) is because they're a data type that existed before references. Therefore, if you end up using libraries to do the things that you know they're better at, you will find that a lot of these libraries use pointers all over the place, simply because of how long they've been around (a lot of them were written before C++).
If you didn't use any libraries, you could design your code in such a way that you could stay away from pointers, but given that pointers are one of the basic types of the language, the faster you get comfortable using them, the more portable your C++ skills would be.
From a maintainability point of view, I should also mention that when you do use pointers, you either have to test for their validity and handle the case when they're not valid, or, just assume they are valid and accept the fact that your program will crash or worse WHEN that assumption is broken. Put another way, your choice with pointers is to either introduce code complexity or more maintenance effort when something breaks and you're trying to track down a bug that belongs to a whole class of errors that pointers introduce, like memory corruption.
So if you control all of your code, stay away from pointers and instead use references, keeping them const when you can. This will force you to think about the life times of your objects and will end up keeping your code easier to understand.
Just remember this difference: A reference is essentially a valid pointer. A pointer is not always valid.
So am I saying that its impossible to create an invalid reference? No. Its totally possible, because C++ lets you do almost anything. It's just harder to do unintentionally and you will be amazed at how many bugs are unintentional :)
Here's a slightly different, but insightful take on why many features of C make sense: http://steve.yegge.googlepages.com/tour-de-babel#C
Basically, the standard CPU architecture is a Von Neumann architecture, and it's tremendously useful to be able to refer to the location of a data item in memory, and do arithmetic with it, on such a machine. If you know any variant of assembly language, you will quickly see how crucial this is at the low level.
C++ makes pointers a bit confusing, since it sometimes manages them for you and hides their effect in the form of "references." If you use straight C, the need for pointers is much more obvious: there's no other way to do call-by-reference, it's the best way to store a string, it's the best way to iterate through an array, etc.
One use of pointers (I won't mention things already covered in other people's posts) is to access memory that you haven't allocated. This isn't useful much for PC programming, but it's used in embedded programming to access memory mapped hardware devices.
Back in the old days of DOS, you used to be able to access the video card's video memory directly by declaring a pointer to:
unsigned char *pVideoMemory = (unsigned char *)0xA0000000;
Many embedded devices still use this technique.
In large part, pointers are arrays (in C/C++) - they are addresses in memory, and can be accessed like an array if desired (in "normal" cases).
Since they're the address of an item, they're small: they take up only the space of an address. Since they're small, sending them to a function is cheap. And then they allow that function to work on the actual item rather than a copy.
If you want to do dynamic storage allocation (such as for a linked-list), you must use pointers, because they're the only way to grab memory from the heap.
Pointers are important in many data structures whose design requires the ability to link or chain one "node" to another efficiently. You would not "choose" a pointer over say a normal data type like float, they simply have different purposes.
Pointers are useful where you require high performance and/or compact memory footprint.
The address of the first element in your array can be assigned to a pointer. This then allows you to access the underlying allocated bytes directly. The whole point of an array is to avoid you needing to do this though.
One way to use pointers over variables is to eliminate duplicate memory required. For example, if you have some large complex object, you can use a pointer to point to that variable for each reference you make. With a variable, you need to duplicate the memory for each copy.
In C++, if you want to use subtype polymorphism, you have to use pointers. See this post: C++ Polymorphism without pointers.
Really, when you think about it, this makes sense. When you use subtype polymorphism, ultimately, you don't know ahead of time which class's or subclass's implementation of the method will be invoked because you don't know what the actual class is.
This idea of having a variable that holds an object of an unknown class is incompatible with C++'s default (non-pointer) mode of storing objects on the stack, where the amount of space allocated directly corresponds to the class. Note: if a class has 5 instance fields versus 3, more space will need to be allocated.
Note that if you are using '&' to pass arguments by reference, indirection (i.e., pointers) is still involved behind the scenes. The '&' is just syntactic sugar that (1) saves you the trouble of using pointer syntax and (2) allows the compiler to be more strict (such as prohibiting null pointers).
Because copying big objects all over the places wastes time and memory.
Here's my anwser, and I won't promse to be an expert, but I've found pointers to be great in one of my libraries I'm trying to write. In this library (It's a graphics API with OpenGL:-)) you can create a triangle with vertex objects passed into them. The draw method takes these triangle objects, and well.. draws them based on the vertex objects i created. Well, its ok.
But, what if i change a vertex coordinate? Move it or something with moveX() in the vertex class? Well, ok, now i have to update the triangle, adding more methods and performance is being wasted because i have to update the triangle every time a vertex moves. Still not a big deal, but it's not that great.
Now, what if i have a mesh with tons of vertices and tons of triangles, and the mesh is rotateing, and moveing, and such. I'll have to update every triangle that uses these vertices, and probably every triangle in the scene because i wouldn't know which ones use which vertices. That's hugely computer intensive, and if I have several meshes ontop of a landscape, oh god! I'm in trouble, because im updateing every triangle almost every frame because these vertices are changing al the time!
With pointers, you don't have to update the triangles.
If I had three *Vertex objects per triangle class, not only am i saving room because a zillion triangles don't have three vertex objects which are large themselves, but also these pointers will always point to the Vertices they are meant to, no matter how often the vertices change. Since the pointers still point to the same vertex, the triangles don't change, and the update process is easier to handle. If I confused you, I wouldn't doubt it, I don't pretend to be an expert, just throwing my two cents into the discussion.
The need for pointers in C language is described here
The basic idea is that many limitations in the language (like using arrays, strings and modifying multiple variables in functions) could be removed by manipulating with the memory location of the data. To overcome these limitations, pointers were introduced in C.
Further, it is also seen that using pointers, you can run your code faster and save memory in cases where you are passing big data types (like a structure with many fields) to a function. Making a copy of such data types before passing would take time and would consume memory. This is another reason why programmers prefer pointers for big data types.
PS: Please refer the link provided for detailed explanation with sample code.
In java and C# all the object references are pointers, the thing with c++ is that you have more control on where you pointer points. Remember With great power comes grand responsibility.
Regarding your second question, generally you don't need to use pointers while programming, however there is one exception to this and that is when you make a public API.
The problem with C++ constructs that people generally use to replace pointers are very dependent on the toolset that you use which is fine when you have all the control you need over the source code, however if you compile a static library with visual studio 2008 for instance and try to use it in a visual studio 2010 you will get a ton of linker errors because the new project is linked with a newer version of STL which is not backwards compatible. Things get even nastier if you compile a DLL and give an import library that people use in a different toolset because in that case your program will crash sooner or later for no apparent reason.
So for the purpose of moving large data sets from one library to another you could consider giving a pointer to an array to the function that is supposed to copy the data if you don't want to force others to use the same tools that you use. The good part about this is that it doesn't even have to be a C-style array, you can use a std::vector and give the pointer by giving the address of the first element &vector[0] for instance, and use the std::vector to manage the array internally.
Another good reason to use pointers in C++ again relates to libraries, consider having a dll that cannot be loaded when your program runs, so if you use an import library then the dependency isn't satisfied and the program crashes. This is the case for instance when you give a public api in a dll alongside your application and you want to access it from other applications. In this case in order to use the API you need to load the dll from its' location (usually it's in a registry key) and then you need to use a function pointer to be able to call functions inside the DLL. Sometimes the people that make the API are nice enough to give you a .h file that contain helper functions to automate this process and give you all the function pointers that you need, but if not you can use LoadLibrary and GetProcAddress on windows and dlopen and dlsym on unix to get them (considering that you know the entire signature of the function).
In some cases, function pointers are required to use functions that are in a shared library (.DLL or .so). This includes performing stuff across languages, where oftentimes a DLL interface is provided.
Making compilers
Making scientific calculators, where you have an array or vector or string map of function pointers?
Trying to modify video memory directly - making your own graphics package
Making an API!
Data structures - node link pointers for special trees you are making
There are Lots of reasons for pointers. Having C name mangling especially is important in DLLs if you want to maintain cross-language compatibility.
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Closed 9 years ago.
I know this is a really basic question, but I've just started with some basic C++ programming after coding a few projects with high-level languages.
Basically I have three questions:
Why use pointers over normal variables?
When and where should I use pointers?
How do you use pointers with arrays?
Why use pointers over normal variables?
Short answer is: Don't. ;-) Pointers are to be used where you can't use anything else. It is either because the lack of appropriate functionality, missing data types or for pure perfomance. More below...
When and where should I use pointers?
Short answer here is: Where you cannot use anything else. In C you don't have any support for complex datatypes such as a string. There are also no way of passing a variable "by reference" to a function. That's where you have to use pointers. Also you can have them to point at virtually anything, linked lists, members of structs and so on. But let's not go into that here.
How do you use pointers with arrays?
With little effort and much confusion. ;-) If we talk about simple data types such as int and char there is little difference between an array and a pointer.
These declarations are very similar (but not the same - e.g., sizeof will return different values):
char* a = "Hello";
char a[] = "Hello";
You can reach any element in the array like this
printf("Second char is: %c", a[1]);
Index 1 since the array starts with element 0. :-)
Or you could equally do this
printf("Second char is: %c", *(a+1));
The pointer operator (the *) is needed since we are telling printf that we want to print a character. Without the *, the character representation of the memory address itself would be printed. Now we are using the character itself instead. If we had used %s instead of %c, we would have asked printf to print the content of the memory address pointed to by 'a' plus one (in this example above), and we wouldn't have had to put the * in front:
printf("Second char is: %s", (a+1)); /* WRONG */
But this would not have just printed the second character, but instead all characters in the next memory addresses, until a null character (\0) were found. And this is where things start to get dangerous. What if you accidentally try and print a variable of the type integer instead of a char pointer with the %s formatter?
char* a = "Hello";
int b = 120;
printf("Second char is: %s", b);
This would print whatever is found on memory address 120 and go on printing until a null character was found. It is wrong and illegal to perform this printf statement, but it would probably work anyway, since a pointer actually is of the type int in many environments. Imagine the problems you might cause if you were to use sprintf() instead and assign this way too long "char array" to another variable, that only got a certain limited space allocated. You would most likely end up writing over something else in the memory and cause your program to crash (if you are lucky).
Oh, and if you don't assign a string value to the char array / pointer when you declare it, you MUST allocate sufficient amount of memory to it before giving it a value. Using malloc, calloc or similar. This since you only declared one element in your array / one single memory address to point at. So here's a few examples:
char* x;
/* Allocate 6 bytes of memory for me and point x to the first of them. */
x = (char*) malloc(6);
x[0] = 'H';
x[1] = 'e';
x[2] = 'l';
x[3] = 'l';
x[4] = 'o';
x[5] = '\0';
printf("String \"%s\" at address: %d\n", x, x);
/* Delete the allocation (reservation) of the memory. */
/* The char pointer x is still pointing to this address in memory though! */
free(x);
/* Same as malloc but here the allocated space is filled with null characters!*/
x = (char *) calloc(6, sizeof(x));
x[0] = 'H';
x[1] = 'e';
x[2] = 'l';
x[3] = 'l';
x[4] = 'o';
x[5] = '\0';
printf("String \"%s\" at address: %d\n", x, x);
/* And delete the allocation again... */
free(x);
/* We can set the size at declaration time as well */
char xx[6];
xx[0] = 'H';
xx[1] = 'e';
xx[2] = 'l';
xx[3] = 'l';
xx[4] = 'o';
xx[5] = '\0';
printf("String \"%s\" at address: %d\n", xx, xx);
Do note that you can still use the variable x after you have performed a free() of the allocated memory, but you do not know what is in there. Also do notice that the two printf() might give you different addresses, since there is no guarantee that the second allocation of memory is performed in the same space as the first one.
One reason to use pointers is so that a variable or an object can be modified in a called function.
In C++ it is a better practice to use references than pointers. Though references are essentially pointers, C++ to some extent hides the fact and makes it seem as if you are passing by value. This makes it easy to change the way the calling function receives the value without having to modify the semantics of passing it.
Consider the following examples:
Using references:
public void doSomething()
{
int i = 10;
doSomethingElse(i); // passes i by references since doSomethingElse() receives it
// by reference, but the syntax makes it appear as if i is passed
// by value
}
public void doSomethingElse(int& i) // receives i as a reference
{
cout << i << endl;
}
Using pointers:
public void doSomething()
{
int i = 10;
doSomethingElse(&i);
}
public void doSomethingElse(int* i)
{
cout << *i << endl;
}
Pointers allow you to refer to the same space in memory from multiple locations. This means that you can update memory in one location and the change can be seen from another location in your program. You will also save space by being able to share components in your data structures.
You should use pointers any place where you need to obtain and pass around the address to a specific spot in memory. You can also use pointers to navigate arrays:
An array is a block of contiguous memory that has been allocated with a specific type. The name of the array contains the value of the starting spot of the array. When you add 1, that takes you to the second spot. This allows you to write loops that increment a pointer that slides down the array without having an explicit counter for use in accessing the array.
Here is an example in C:
char hello[] = "hello";
char *p = hello;
while (*p)
{
*p += 1; // increase the character by one
p += 1; // move to the next spot
}
printf(hello);
prints
ifmmp
because it takes the value for each character and increments it by one.
Pointers are one way of getting an indirect reference to another variable. Instead of holding the value of a variable, they tell you its address. This is particularly useful when dealing with arrays, since using a pointer to the first element in an array (its address) you can quickly find the next element by incrementing the pointer (to the next address location).
The best explanation of pointers and pointer arithmetic that I've read is in K & R's The C Programming Language. A good book for beginning learning C++ is C++ Primer.
Let me try and answer this too.
Pointers are similar to references. In other words, they're not copies, but rather a way to refer to the original value.
Before anything else, one place where you will typically have to use pointers a lot is when you're dealing with embedded hardware. Maybe you need to toggle the state of a digital IO pin. Maybe you're processing an interrupt and need to store a value at a specific location. You get the picture. However, if you're not dealing with hardware directly and are just wondering about which types to use, read on.
Why use pointers as opposed to normal variables? The answer becomes clearer when you're dealing with complex types, like classes, structures and arrays. If you were to use a normal variable, you might end up making a copy (compilers are smart enough to prevent this in some situations and C++11 helps too, but we'll stay away from that discussion for now).
Now what happens if you want to modify the original value? You could use something like this:
MyType a; //let's ignore what MyType actually is right now.
a = modify(a);
That will work just fine and if you don't know exactly why you're using pointers, you shouldn't use them. Beware of the "they're probably faster" reason. Run your own tests and if they actually are faster, then use them.
However, let's say you're solving a problem where you need to allocate memory. When you allocate memory, you need to deallocate it. The memory allocation may or may not be successful. This is where pointers come in useful - they allow you to test for the existence of the object you've allocated and they allow you to access the object the memory was allocated for by de-referencing the pointer.
MyType *p = NULL; //empty pointer
if(p)
{
//we never reach here, because the pointer points to nothing
}
//now, let's allocate some memory
p = new MyType[50000];
if(p) //if the memory was allocated, this test will pass
{
//we can do something with our allocated array
for(size_t i=0; i!=50000; i++)
{
MyType &v = *(p+i); //get a reference to the ith object
//do something with it
//...
}
delete[] p; //we're done. de-allocate the memory
}
This is the key to why you would use pointers - references assume the element you're referencing exists already. A pointer does not.
The other reason why you would use pointers (or at least end up having to deal with them) is because they're a data type that existed before references. Therefore, if you end up using libraries to do the things that you know they're better at, you will find that a lot of these libraries use pointers all over the place, simply because of how long they've been around (a lot of them were written before C++).
If you didn't use any libraries, you could design your code in such a way that you could stay away from pointers, but given that pointers are one of the basic types of the language, the faster you get comfortable using them, the more portable your C++ skills would be.
From a maintainability point of view, I should also mention that when you do use pointers, you either have to test for their validity and handle the case when they're not valid, or, just assume they are valid and accept the fact that your program will crash or worse WHEN that assumption is broken. Put another way, your choice with pointers is to either introduce code complexity or more maintenance effort when something breaks and you're trying to track down a bug that belongs to a whole class of errors that pointers introduce, like memory corruption.
So if you control all of your code, stay away from pointers and instead use references, keeping them const when you can. This will force you to think about the life times of your objects and will end up keeping your code easier to understand.
Just remember this difference: A reference is essentially a valid pointer. A pointer is not always valid.
So am I saying that its impossible to create an invalid reference? No. Its totally possible, because C++ lets you do almost anything. It's just harder to do unintentionally and you will be amazed at how many bugs are unintentional :)
Here's a slightly different, but insightful take on why many features of C make sense: http://steve.yegge.googlepages.com/tour-de-babel#C
Basically, the standard CPU architecture is a Von Neumann architecture, and it's tremendously useful to be able to refer to the location of a data item in memory, and do arithmetic with it, on such a machine. If you know any variant of assembly language, you will quickly see how crucial this is at the low level.
C++ makes pointers a bit confusing, since it sometimes manages them for you and hides their effect in the form of "references." If you use straight C, the need for pointers is much more obvious: there's no other way to do call-by-reference, it's the best way to store a string, it's the best way to iterate through an array, etc.
One use of pointers (I won't mention things already covered in other people's posts) is to access memory that you haven't allocated. This isn't useful much for PC programming, but it's used in embedded programming to access memory mapped hardware devices.
Back in the old days of DOS, you used to be able to access the video card's video memory directly by declaring a pointer to:
unsigned char *pVideoMemory = (unsigned char *)0xA0000000;
Many embedded devices still use this technique.
In large part, pointers are arrays (in C/C++) - they are addresses in memory, and can be accessed like an array if desired (in "normal" cases).
Since they're the address of an item, they're small: they take up only the space of an address. Since they're small, sending them to a function is cheap. And then they allow that function to work on the actual item rather than a copy.
If you want to do dynamic storage allocation (such as for a linked-list), you must use pointers, because they're the only way to grab memory from the heap.
Pointers are important in many data structures whose design requires the ability to link or chain one "node" to another efficiently. You would not "choose" a pointer over say a normal data type like float, they simply have different purposes.
Pointers are useful where you require high performance and/or compact memory footprint.
The address of the first element in your array can be assigned to a pointer. This then allows you to access the underlying allocated bytes directly. The whole point of an array is to avoid you needing to do this though.
One way to use pointers over variables is to eliminate duplicate memory required. For example, if you have some large complex object, you can use a pointer to point to that variable for each reference you make. With a variable, you need to duplicate the memory for each copy.
In C++, if you want to use subtype polymorphism, you have to use pointers. See this post: C++ Polymorphism without pointers.
Really, when you think about it, this makes sense. When you use subtype polymorphism, ultimately, you don't know ahead of time which class's or subclass's implementation of the method will be invoked because you don't know what the actual class is.
This idea of having a variable that holds an object of an unknown class is incompatible with C++'s default (non-pointer) mode of storing objects on the stack, where the amount of space allocated directly corresponds to the class. Note: if a class has 5 instance fields versus 3, more space will need to be allocated.
Note that if you are using '&' to pass arguments by reference, indirection (i.e., pointers) is still involved behind the scenes. The '&' is just syntactic sugar that (1) saves you the trouble of using pointer syntax and (2) allows the compiler to be more strict (such as prohibiting null pointers).
Because copying big objects all over the places wastes time and memory.
Here's my anwser, and I won't promse to be an expert, but I've found pointers to be great in one of my libraries I'm trying to write. In this library (It's a graphics API with OpenGL:-)) you can create a triangle with vertex objects passed into them. The draw method takes these triangle objects, and well.. draws them based on the vertex objects i created. Well, its ok.
But, what if i change a vertex coordinate? Move it or something with moveX() in the vertex class? Well, ok, now i have to update the triangle, adding more methods and performance is being wasted because i have to update the triangle every time a vertex moves. Still not a big deal, but it's not that great.
Now, what if i have a mesh with tons of vertices and tons of triangles, and the mesh is rotateing, and moveing, and such. I'll have to update every triangle that uses these vertices, and probably every triangle in the scene because i wouldn't know which ones use which vertices. That's hugely computer intensive, and if I have several meshes ontop of a landscape, oh god! I'm in trouble, because im updateing every triangle almost every frame because these vertices are changing al the time!
With pointers, you don't have to update the triangles.
If I had three *Vertex objects per triangle class, not only am i saving room because a zillion triangles don't have three vertex objects which are large themselves, but also these pointers will always point to the Vertices they are meant to, no matter how often the vertices change. Since the pointers still point to the same vertex, the triangles don't change, and the update process is easier to handle. If I confused you, I wouldn't doubt it, I don't pretend to be an expert, just throwing my two cents into the discussion.
The need for pointers in C language is described here
The basic idea is that many limitations in the language (like using arrays, strings and modifying multiple variables in functions) could be removed by manipulating with the memory location of the data. To overcome these limitations, pointers were introduced in C.
Further, it is also seen that using pointers, you can run your code faster and save memory in cases where you are passing big data types (like a structure with many fields) to a function. Making a copy of such data types before passing would take time and would consume memory. This is another reason why programmers prefer pointers for big data types.
PS: Please refer the link provided for detailed explanation with sample code.
In java and C# all the object references are pointers, the thing with c++ is that you have more control on where you pointer points. Remember With great power comes grand responsibility.
Regarding your second question, generally you don't need to use pointers while programming, however there is one exception to this and that is when you make a public API.
The problem with C++ constructs that people generally use to replace pointers are very dependent on the toolset that you use which is fine when you have all the control you need over the source code, however if you compile a static library with visual studio 2008 for instance and try to use it in a visual studio 2010 you will get a ton of linker errors because the new project is linked with a newer version of STL which is not backwards compatible. Things get even nastier if you compile a DLL and give an import library that people use in a different toolset because in that case your program will crash sooner or later for no apparent reason.
So for the purpose of moving large data sets from one library to another you could consider giving a pointer to an array to the function that is supposed to copy the data if you don't want to force others to use the same tools that you use. The good part about this is that it doesn't even have to be a C-style array, you can use a std::vector and give the pointer by giving the address of the first element &vector[0] for instance, and use the std::vector to manage the array internally.
Another good reason to use pointers in C++ again relates to libraries, consider having a dll that cannot be loaded when your program runs, so if you use an import library then the dependency isn't satisfied and the program crashes. This is the case for instance when you give a public api in a dll alongside your application and you want to access it from other applications. In this case in order to use the API you need to load the dll from its' location (usually it's in a registry key) and then you need to use a function pointer to be able to call functions inside the DLL. Sometimes the people that make the API are nice enough to give you a .h file that contain helper functions to automate this process and give you all the function pointers that you need, but if not you can use LoadLibrary and GetProcAddress on windows and dlopen and dlsym on unix to get them (considering that you know the entire signature of the function).
In some cases, function pointers are required to use functions that are in a shared library (.DLL or .so). This includes performing stuff across languages, where oftentimes a DLL interface is provided.
Making compilers
Making scientific calculators, where you have an array or vector or string map of function pointers?
Trying to modify video memory directly - making your own graphics package
Making an API!
Data structures - node link pointers for special trees you are making
There are Lots of reasons for pointers. Having C name mangling especially is important in DLLs if you want to maintain cross-language compatibility.
I have a C++ class where I have a dynamically-allocated array of pointers to structs. I have a member function to "add an item" to this array by assigning an index of the array to the pointer to a dynamically allocated instance of the struct.
I have sort_arr initialized with sort_arr = new node *[this->max_items];.
In my assignment function I have sort_arr[this->num_items] = item; where the pointer is being passed as an argument with node *item.
In this function, I am able to access a member variable using (*sort_arr[i]).key_a (where i is the index), but once another item is added, this reference is no longer valid and causes a seg fault.
Is the pointer being deallocated, and if so, is it possible to prevent this?
EDIT: Sorry for the ambiguity here. I am trying to understand the problem generally and not specifically (in a pedagogical sort of way). I was hoping it was a problem with my conceptual approach. Given that it probably isn't, here are some more details:
node is defined as node **sort_arr; in the class declaration and then initialized by the constructor as sort_arr = new node *[this->max_items];. The insert method of the class executes: sort_arr[this->num_items] = item;, where item is passed with node *item.
It seems that after an item 'n2' is inserted after 'n1', 'n1' is no longer accessible via the reference (*sort_arr[num_items]).key_a. key_a is a member variable of the node struct.
EDIT 2: node *item is dynamically allocated outside of the class (in the main function).
The code you posted looks basically correct (if not the best way to do this sort of thing), but I can't tell what key_a is, or what context you are calling it in. Because of that, it's hard to tell exactly what the problem is. Posting the entire body of your function might be useful.
The only way something you allocated via new will be deallocated is if you (or some code you call) explicitly calls delete. That's pretty much the whole point of dynamic memory allocation, to allow your objects to live after the stack frame gets popped off.
My best guess with the current information is that you're trying to access a local value that got allocated on the stack after returning from the function. For example, this would cause a problem:
some_type* some_function(int i)
{
// ...
some_type p = (*sort_arr[i]).key_a; // p is a copy of key_a, allocated on the stack
// ...
some_type* result = &p;
return result;
}
In this scenario, p would be okay to return directly (if you changed the return type to some_type instead of some_type*), but you can't return a pointer to a local value. The local value is no longer valid after the function exits. This often causes a segfault.
Make sure that this->num_items as well asi is less than this->max_items and greater than -1, as this could be the cause of seg-fault.
Don't use dynamic arrays if it isn't for lecturing. Use a simple and save std::vector. It handles nearly everything that could go wrong. Try with that and see if there's still a seg-fault.
As noted, the code does appear to be correct. The problem was unrelated to the referenced code. I had been debugging some memory leaks and was deleting the referenced items, which was in turn causing the problem (quite obviously now that I see that).
I appreciate everyone's help and I'm sorry if I drove anyone crazy trying to find what was wrong.
This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
Common Uses For Pointers?
I am still learning the basics of C++ but I already know enough to do useful little programs.
I understand the concept of pointers and the examples I see in tutorials make sense to me. However, on the practical level, and being a (former) PHP developer, I am not yet confident to actually use them in my programs.
In fact, so far I have not felt the need to use any pointer. I have my classes and functions and I seem to be doing perfectly fine without using any pointer (let alone pointers to pointers). And I can't help feeling a bit proud of my little programs.
Still, I am aware that I am missing on one of C++'s most important feature, a double edged one: pointers and memory management can create havoc, seemingly random crashes, hard to find bugs and security holes... but at the same time, properly used, they must allow for clever and efficient programming.
So: do tell me what I am missing by not using pointers.
What are good scenarios where using pointers is a must?
What do they allow you to do that you couldn't do otherwise?
In which way to they make your programs more efficient?
And what about pointers to pointers???
[Edit: All the various answers are useful. One problem at SO is that we cannot "accept" more than one answer. I often wish I could. Actually, it's all the answers combined that help to understand better the whole picture. Thanks.]
I use pointers when I want to give a class access to an object, without giving it ownership of that object. Even then, I can use a reference, unless I need to be able to change which object I am accessing and/or I need the option of no object, in which case the pointer would be NULL.
This question has been asked on SO before. My answer from there:
I use pointers about once every six lines in the C++ code that I write. Off the top of my head, these are the most common uses:
When I need to dynamically create an object whose lifetime exceeds the scope in which it was created.
When I need to allocate an object whose size is unknown at compile time.
When I need to transfer ownership of an object from one thing to another without actually copying it (like in a linked list/heap/whatever of really big, expensive structs)
When I need to refer to the same object from two different places.
When I need to slice an array without copying it.
When I need to use compiler intrinsics to generate CPU-specific instructions, or work around situations where the compiler emits suboptimal or naive code.
When I need to write directly to a specific region of memory (because it has memory-mapped IO).
Pointers are commonly used in C++. Becoming comfortable with them, will help you understand a broader range of code. That said if you can avoid them that is great, however, in time as your programs become more complex, you will likely need them even if only to interface with other libraries.
Primarily pointers are used to refer to dynamically allocated memory (returned by new).
They allow functions to take arguments that cannot be copied onto the stack either because they are too big or cannot be copied, such as an object returned by a system call. (I think also stack alignment, can be an issue, but too hazy to be confident.)
In embedded programing they are used to refer to things like hardware registers, which require that the code write to a very specific address in memory.
Pointers are also used to access objects through their base class interfaces. That is if I have a class B that is derived from class A class B : public A {}. That is an instance of the object B could be accessed as if it where class A by providing its address to a pointer to class A, ie: A *a = &b_obj;
It is a C idiom to use pointers as iterators on arrays. This may still be common in older C++ code, but is probably considered a poor cousin to the STL iterator objects.
If you need to interface with C code, you will invariable need to handle pointers which are used to refer to dynamically allocated objects, as there are no references. C strings are just pointers to an array of characters terminated by the nul '\0' character.
Once you feel comfortable with pointers, pointers to pointers won't seem so awful. The most obvious example is the argument list to main(). This is typically declared as char *argv[], but I have seen it declared (legally I believe) as char **argv.
The declaration is C style, but it says that I have array of pointers to pointers to char. Which is interpreted as a arbitrary sized array (the size is carried by argc) of C style strings (character arrays terminated by the nul '\0' character).
If you haven't felt a need for pointers, I wouldn't spend a lot of time worrying about them until a need arises.
That said, one of the primary ways pointers can contribute to more efficient programming is by avoiding copies of actual data. For example, let's assume you were writing a network stack. You receive an Ethernet packet to be processed. You successively pass that data up the stack from the "raw" Ethernet driver to the IP driver to the TCP driver to, say, the HTTP driver to something that processes the HTML it contains.
If you're making a new copy of the contents for each of those, you end up making at least four copies of the data before you actually get around to rendering it at all.
Using pointers can avoid a lot of that -- instead of copying the data itself, you just pass around a pointer to the data. Each successive layer of the network stack looks at its own header, and passes a pointer to what it considers the "payload" up to the next higher layer in the stack. That next layer looks at its own header, modifies the pointer to show what it considers the payload, and passes it on up the stack. Instead of four copies of the data, all four layers work with one copy of the real data.
A big use for pointers is dynamic sizing of arrays. When you don't know the size of the array at compile time, you will need to allocate it at run-time.
int *array = new int[dynamicSize];
If your solution to this problem is to use std::vector from the STL, they use dynamic memory allocation behind the scenes.
There are several scenarios where pointers are required:
If you are using Abstract Base Classes with virtual methods. You can hold a std::vector and loop through all these objects and call a virtual method. This REQUIRES pointers.
You can pass a pointer to a buffer to a method reading from a file etc.
You need a lot of memory allocated on the heap.
It's a good thing to care about memory problems right from the start. So if you start using pointers, you might as well take a look at smart pointers, like boost's shared_ptr for example.
What are good scenarios where using pointers is a must?
Interviews. Implement strcpy.
What do they allow you to do that you couldn't do otherwise?
Use of inheritance hierarchy. Data structures like Binary trees.
In which way to they make your programs more efficient?
They give more control to the programmer, for creating and deleting resources at run time.
And what about pointers to pointers???
A frequently asked interview question. How will you create two dimensional array on heap.
A pointer has a special value, NULL, that reference's won't. I use pointers wherever NULL is a valid and useful value.
I just want to say that i rarely use pointers. I use references and stl objects (deque, list, map, etc).
A good idea is when you need to return an object where the calling function should free or when you dont want to return by value.
List<char*>* fileToList(char*filename) { //dont want to pass list by value
ClassName* DataToMyClass(DbConnectionOrSomeType& data) {
//alternatively you can do the below which doesnt require pointers
void DataToMyClass(DbConnectionOrSomeType& data, ClassName& myClass) {
Thats pretty much the only situation i use but i am not thinking that hard. Also if i want a function to modify a variable and cant use the return value (say i need more then one)
bool SetToFiveIfPositive(int**v) {
You can use them for linked lists, trees, etc.
They're very important data structures.
In general, pointers are useful as they can hold the address of a chunk of memory. They are especially useful in some low level drivers where they are efficiently used to operate on a piece of memory byte by byte. They are most powerful invention that C++ inherits from C.
As to pointer to pointer, here is a "hello-world" example showing you how to use it.
#include <iostream>
void main()
{
int i = 1;
int j = 2;
int *pInt = &i; // "pInt" points to "i"
std::cout<<*pInt<<std::endl; // prints: 1
*pInt = 6; // modify i, i = 6
std::cout<<i<<std::endl; // prints: 6
int **ppInt = &pInt; // "ppInt" points to "pInt"
std::cout<<**ppInt<<std::endl; // prints: 6
**ppInt = 8; // modify i, i = 8
std::cout<<i<<std::endl; // prints: 8
*ppInt = &j; // now pInt points to j
*pInt = 10; // modify j, j = 10
std::cout<<j<<std::endl; // prints: 10
}
As we see, "pInt" is a pointer to integer which points to "i" at the beginning. With it, you can modify "i". "ppInt" is a pointer to pointer which points to "pInt". With it, you can modify "pInt" which happens to be an address. As a result, "*ppInt = &j" makes "pInt" points to "j" now. So we have all the results above.