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

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

This question already has answers here:

Do class functions/variables have to be declared before being used? (4 answers)

Closed 10 months ago.

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?

задан underscore_d 19 December 2017 в 14:09

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

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