Instance of private nested class c++ [duplicate]

Question

I have the following code

class A
{
private:
    class B
    {
    public:
        void f()
        {
            printf("Test");
        }
    };
public:
    B g() 
    {
        return B(); 
    }
};


int main()
{
    A a;
    A::B b; // Compilation error C2248
    A::B b1 = a.g(); //Compilation error C2248
    auto b2 = a.g(); // OK
    a.g(); // OK 
    b2.f(); // OK. Output is "Test"
}

As you can see I have class A and private nested class B. Without using auto I can't create instance of A::B outside A, but with auto I can. Can somebody explain what wrong here? I use VC++ 12.0, 13.0, 14.0 (always same behavior)

Answer 1

The type B is accessible only to A and friends of A, which means that other code cannot refer to it. On the other hand template type deduction works even for private types, which is needed if you ever wanted to use your private type in any form of template inside A's code.

The auto feature is based on template type deduction and follows the same rules, allowing for the call auto b2 = a.g();.