Pointers

[cypher5783]

Hey all,

I'm new to C++ but not to programming in general. I've knowledge in VB, PHP, and MySQL. But i've never had to deal with pointers before. This is a very new topic to me and it seems irrelevant and pointless. The only benefit i've been able to discern from the use of pointers is to return multiple variables from a function (which can also be done with globals) So can someone please explain why pointers are useful. Any advice on this topic will be appreciated. I've tried reading multiple books on the subject and it just doesn't seem to click.

[Chris_F]

Pointers don't exist for sake of advantage, they are a crucial tool in programming, and the only reason you haven't yet had to work with them is the fact that you've only used extremely high level languages which manage nicely to abstract them away. I don't know what you've written so far, but it's safe to say you can't go far without pointers.

...is to return multiple variables from a function (which can also be done with globals)...

I'm not sure how you came to this understanding, but the use of globals is generally a sign of bad design. They should be avoided whenever possible. I'm not exactly sure how you are using globals in this way, but it sounds like it is the worst violation of the use of globals (i.e. everywhere.)

[D_Drmmr]

If you use globals to communicate results from a function, it is hard to apply that function to different input data, because you would need to copy the results from the global to a local variable in order to retain the value (the next time the function gets called, it will be overwritten). And even that depends heavily on a single threaded design. Once you introduce multiple threads, you're completely lost with such a design.

Besides, when function arguments are pointers, you should always consider if a reference is not more appropriate. The difference (in the context of function arguments) is that a pointer can be NULL, whereas a reference must always refer to a valid object. If it doesn't make sense for the pointer to be NULL, you should use a reference and not a pointer.

The use of pointers will probably become clearer when you learn how to use classes. Some constructs, such as a linked list data structure, cannot be implemented in C++ without using pointers. Also, pointers come in real handy when you want to use classes polymorphically.
When you just consider functional programming, the main use of pointers is for optional (output) arguments of a function.

void ComputeDivAndMod(int x, int y, int* div, int* mod)
{
    if (div) {
        *div = x / y;
    }
    if (mod) {
        *mod = x % y;
    }
}

int main()
{
    int a = 10, b = 3, c, d;
    // if we only want div
    ComputeDivAndMod(a, b, &c, NULL);
    // if we only want mod
    ComputeDivAndMod(a, b, NULL, &d);
}

[Paul McKenzie]

So can someone please explain why pointers are useful.

void ChangeToZero( ??? )
{
    //???
}

int main()
{
   int x = 10;
   int y = 20;
   int z = 30;
   ChangeToZero( ??? );
}

Write a single function ChangeToZero that can take either x, y, or z, or any integer variable I desire, and change that variable's value to 0. Remember, you can only write one function to do this, and the function knows nothing about the x, y, and z that I've declared local to main(). Once you've solved that problem, then you know pointers.

Secondly, have you ever used scanf()? If so, what are the arguments you provide to get the input? Pointers. How about strcpy(), strlen(), strcat(), memcpy() -- ever used those functions? Again, what arguments must you provide? Pointers. Why do you think these functions require pointers as arguments?

Last, polymorphism, the thing that makes C++ what it is, requires usage of pointers and/or references to have this paradigm work. If you don't understand pointers, then you don't understand polymorphism.

Regards,

Paul McKenzie

[VladimirF]

Imagine that I have a great book with answers to all you questions. In a world without pointers, I would have to copy/paste it here for you, but if we DO have pointers, I would simply point you to this book

[GCDEF]

In c++ you can't take advantage of polymorphism without pointers.

When you allocate memory on the heap, the OS returns an address to the memory it's allocated. It's the only way you can access that memory.

Many data structures use pointers to connect related elements. A linked list is a good example of this.