How to access an attribute into a struct from a void pointer?

Question

I am given a task which i can't see what a function does in detail but i only know what id does generally.

So i have a function, let's call it "get_button", and in the documentation i know that "it returns a pointer to the button struct". I know the button struct, it's something like:

struct button{
    int size;
    char title;
    short keyID;
};

The function is something like:

void * get_button(int ID, int group)

But i can't know what it does.

Now i want to know the title, but playing with the complier i got that i have a void pointer in return. So how i am supposed to use this void pointer? ASAIK i have to use a static_cast to give a reference to it, but in this case is a struct; moreover i can't really understand what casting means and how to make the compiler know that i want a certain struct.

This is a particular case where i need to know how to get a parameter into a struct from a void pointer. I don't know if i should use a static_cast or whatever (so i can't understand the reference to it). Neither i need to know how to use propery a void pointer in this case! I just need to know how i can access the attribute in the struct. I've seen the other answers here and there but i couldn't find something valuable for this case.

Answer 1

i can't really understand what casting means

You answer yourself in your next question:

how to make the compiler know that i want a certain struct.

That's exactly it. Casting means that you tell the compiler, that the object has a specific type.

This is a particular case where i need to know how to get a parameter into a struct from a void pointer.

First, cast the void pointer into the pointer type of the object that it points to: *button . Then you can access the attribute normally, using the operator-> on the pointer of correct type.

I don't know if i should use a static_cast or whatever

When casting a void pointer to another pointer type, you should indeed use static_cast .

void* void_pointer = get_button(ID, group); // get the pointer
button* button_pointer = static_cast<button*>(void_pointer); // cast to the type of the pointed object
int size = button_pointer->size; // access the attribute through the pointer

Answer 2

Here is an example of a plain C old style cast with your struct to a void pointer:

#include <iostream>

using namespace std;

struct button{
    int size;
    char title;
    short keyID;
};

int main()
{
    button b;
    b.size  = 1;
    b.title = 'c';
    b.keyID = 2;
    void *p = &b;
    button *a = (button *)p;
    std::cout << a->size << std::endl;
    std::cout << a->title << std::endl;
    std::cout << a->keyID << std::endl;
    return 0;
}

Answer 3

A pointer is basically an address of where in memory something is, in your case a button struct. That address doesn't change when the type of the pointer is changed to void*, so if your library providing the get_button function guarantees that the returned pointer will in fact point to a valid button struct, you can safely cast it back to a button* in order to use it (think of it as giving the compiler some extra information).

You'd go about it as follows:

void *what_is_returned = get_button();
button *the_button = static_cast<button *>(what_is_returned);
char the_character_you_want = the_button->title;

The -> operator means "dereference and access member", it's a shorthand for dereferencing what the left hand side variable contains (looking up the address, like in *the_button) and then accessing the member (like in (*the_button).title), effectively adding the offset of the title member within the button struct to the button's address and fetching the char from there.