Question

While going through some allegro tutorials I found an odd call.

int al_init();

The function al_init() initializes allegro so that the allegro functions can be used. What is with the int al_init(); line? If I change this line of the code to exclude int it works the same, but if I take out the line altogether it does not work. What is this line doing? The only thing I can imagine is that it creates an integer and assigns it the return value of the al_init() function, with that likely being -1 for failure and 0 for success etc. But if that is what this is doing, then how can you even check the return value?

Answer 1

In C/C++ there are function declarations and function definitions:

Declarations look like these:

int a_function();
void another_function();
double yet_another_function();

Explanation:

• The identifier before the function name (e.g. int, void, double) describes the type of value returned by the function.
• If you do not specify one, it defaults to int (which is why it works when you remove int from int al_init(), but not when you remove the declaration altogether)
• void means it’s not supposed to return a value (even though technically it can, but that’s for rarer cases)

Definitions look like these:

int a_function() {
    std::cout << "hello world!";
    int x = 1;
    int y = 2;
    return (x + y);
}

Notice the difference:

• Declarations end with ;
• Definitions are followed by a block of code enclosed by braces: { and }, but no ;!
• In some cases, you do not need to declare a function. If it’s at a point in the code where the definition has already been seen, the definition can substitute for the declaration (this leads us to the next point…)
• The purpose of declaration is to tell the program that, for example, there is a function named a_function, it expects no arguments, and it returns a value of type int.