Why is the 'Declare before use' rule not required inside a class?

Question

I'm wondering why the declare-before-use rule of C++ doesn't hold inside a class.

Look at this example:

#ifdef BASE
struct Base {
#endif
    struct B;
    struct A {
        B *b;
        A(){ b->foo(); }
    };

    struct B {
        void foo() {}
    };
#ifdef BASE
};
#endif

int main( ) { return 0; }

If BASE is defined, the code is valid.

Within A's constructor I can use B::foo, which hasn't been declared yet.

Why does this work and, mostly, why only works inside a class?

Answer 1

That's because member functions are compiled only after the whole class definition has been parsed by the compiler, even when the function definition is written inline, whereas regular functions are compiled immediatedly after being read. The C++ standard requires this behaviour.