C++: What are scenarios where using pointers is a "Good Idea"(TM)?

Question

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.]

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

You can use them for linked lists, trees, etc. They're very important data structures.

Answer 5

A pointer has a special value, NULL, that reference's won't. I use pointers wherever NULL is a valid and useful value.

Answer 6

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.

Answer 7

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) {

Answer 8

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).

Answer 9

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.

Answer 10

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).

Answer 11

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.