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Pointers and structures

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TheDarkSide
Jr. Member

Posts: 36

Unleash The Force

Pointers and structures
« on: February 03, 2010, 05:26:07 pm »

Hi, everyone. I have a question for you.
I'm quite good at playin' around with C++. But I'm doubtful about pointers, which I think it's the hardest topic about C++.
I know that pointers point to an address in memory. The address of a variable in taken thanks to the "&" symbol. I also know that the value pointed by a pointer is taken by the "*" symbol.

Let's analyse this case

Code:

void savefile(char *filename) {}
int main(){
int a;
savefile(a);
}

In this case I understand that I assign to the VALUE OF THE POINTER the value of a
just like *filename = a

I was just wondering why this does not work with structures

eg.

Code:

struct f {
char x;
};

void savefile(struct f *filename) {}
int main(){

struct f a; // I declare give A the attributes of the F structure.
savefile(&a);
}

Why do I have to put the REFERENCE OP. & when using a structure in the void parameter, and why should not, when using a variable in the void parameter?

savefile(a) //if variable
savefile(&a) //if structure

I hope I was clear enough. Thanks


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hymerman
Global Moderator
Gamedev guru

Posts: 6750

booboo-be-doo

Re: Pointers and structures
« Reply #1 on: February 04, 2010, 01:09:18 pm »

I think you've mostly got the right idea, but... not quite. Your first example won't compile, as an int can't be implicitly converted to a pointer type. A pointer may be just a memory address but you won't be allowed (aside from reinterpret_cast) to convert any old number to one. Except 0 but we'll gloss over that. What would work would be taking the address of a ("&a") and passing it to savefile, since an int is convertible to a char (a char is really a numeric type, despite sounding like 'character'), savefile is expecting a pointer to one, and you're giving it a pointer. I don't think that's what you want though since the char* you want is probably an array of chars rather than a pointer to just one.

In your second example, it seems you're confused about types; "struct f ..." is a type declaration; after that, any time you write "f" that's a type. You don't need to put "struct" in front of it any more (though Visual Studio accepts this). When you write "f a;", you're creating on the stack a variable of type f, called a. It's not a pointer to an f, it's just an f (it's what's called a 'value' type). Savefile is expecting a pointer to an f, so if you call it as "savefile(a)", you're passing in a value where a pointer is expected, and it won't compile. You need the & in front of a to take its address, which you can then pass in to savefile where the pointer will have that address. Note that if savefile's parameter was "f filename" rather than "f* filename", you could pass in a as it is since it's now the correct type.

Also I don't know what you mean by "void parameter" - firstly you can't have a parameter that's void, you can have a function that takes void (i.e. takes no arguments, like your main), and secondly savefile doesn't take void.

Last thing; where you say "// if variable // if structure", they're both variables, but one is of class type (structs are class types) and one is of type int, a primitive type. Their type has nothing to do with whether you have to take their address though.

Hope that helps, I appreciate some of that's a bit confusing but then C++ is a confusing language


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Anders
Sr. Member

Posts: 356

I'm a llama!

Re: Pointers and structures
« Reply #2 on: February 04, 2010, 10:42:17 pm »

As Hymerman wrote you just type void when you have a function that you do not want to send or return any data.

void EndGame (void); //Dont send any data to EndGame function and nothing in return

void Test (int a); //Send an integer we call a to Test function

int TestMore (void); //TestMore returns an integer but doesn't get any data

The reason that you use the & in the function call is that the function savefile is expecting a pointer, not the direct value. Let's say you have a sack of gold. Sending a pointer to a function (as with the &) is like giving someone a map to where the sack of gold is. Sending the data directly to a function would be like giving someone the sack of gold directly.

Don't worry, pointers take a while to grasp but are very powerful.

You could also do like this:

Code:

f *Ptr=NULL; //This is a pointer to the structure f
savefile (Ptr);

But please note that the Ptr pointer in the end is empty so you have to allocate memory for it first and fill it with valid data.


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hymerman
Global Moderator
Gamedev guru

Posts: 6750

booboo-be-doo

Re: Pointers and structures
« Reply #3 on: February 05, 2010, 11:40:12 am »

"This is a pointer to the structure f" would be better written as "This is a pointer to an instance of type f". I know that's what you meant of course, I just don't want Mr. TheDarkSide getting confused


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Anders
Sr. Member

Posts: 356

I'm a llama!

Re: Pointers and structures
« Reply #4 on: February 05, 2010, 01:17:07 pm »

Quote from: hymerman on February 05, 2010, 11:40:12 am

Yep, it's good that you clarify that, thanks Grin


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