Does class/function order matter in C++?

Question

I'm beginning to learn C++. In the IDE codeblocks, this compiles:

#include <iostream>
using namespace std;

struct A {};

struct B {
    A a;
}

void hi() {
    cout << "hi" << endl;
}

int main() {
    hi();
    return 0;
}

But this doesn't:

struct B {
    A a;
}

struct A {};

int main() {
    hi();
    return 0;
}

void hi() {
    cout << "hi" << endl;
}

It gives me the errors:

error: 'A' does not name a type
error: 'hi' was not declared in this scope

Should class/function order matter in C++? I thought it doesn't. Please clarify the issue.

Functions should at least be declared before being used. But once you declared them, the order does not matter (or very marginally). For short functions, it might be *slightly* better to group related functions (eg `f` before `g` if `g` calls `f`), perhaps because of cache issues. But this is often irrelevant (and the compiler will often reflush the generated function order). — Basile Starynkevitch, Oct 02 '14 at 18:37
I would suggest that you get a book on C++ instead of trying to learn the language by playing in an IDE. — Raymond Chen, Oct 02 '14 at 19:27

score 48 · Accepted Answer · edited Nov 07 '19 at 22:13

Yes, you must at least declare the class/function before you use/call it, even if the actual definition does not come until afterwards.

That is why you often declare the classes/functions in header files, then #include them at the top of your cpp file. Then you can use the classes/functions in any order, since they have already been effectively declared.

Note in your case you could have done this. (working example)

void hi();    // This function is now declared

struct A; // This type is now declared

struct B {
    A* a; // we can now have a pointer to it
};

int main() {
    hi();
    return 0;
}

void hi() {    // Even though the definition is afterwards
    cout << "hi" << endl;
}

struct A {}; // now A has a definition

Does class/function order matter in C++?

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