为什么我不能拥有某些私有成员函数？

Question

Given a class, why can we implement certain private member functions, but not other private member functions? For instance, see below:

header file:
class A
{
    ....
    struct B
    {
    ....
    }
    int f(); 
    B* g();
}

cpp file:
int A::f(){...} // will compile
B*  A::g(){...} // will not compile!

The error is "Unknown type name 'B'", but my actual code has some more details.

Answer 1

B*  A::g(){...}

B is not visible outside A .

A::B*  A::g(){...}

This has nothing to do with private members. Since B is declared inside A , its not in global scope, hence you need to specify what B you are referring to.

Answer 2

Alternatively to the other answer you can use trailing return types

auto A::g() -> B* {...}

Before A::g() you have to use A::B . After A::g() you can simply use B .