template private inheritance in vc++10 is not accessible

问题

The following code compiles using GCC 4.4.6 and Comeau 4.3.10.

#include <iostream>

struct A { int name; };
template<typename T> struct C : T { using T::name; };
struct B : private A { friend struct C<B>; };

int main()
{
    C<B> o;
    o.name = 0;
}

It gives the following error in VC++10:

main.cpp(4): error C2877: 'A::name' is not accessible from  'A'
main.cpp(10): error C2247: 'A::name' not accessible because 'B' uses 'private' to inherit from 'A'

What's a good cross-compiler workaround that allows o.name = 0;?

Note: Adding using A::name to B takes care of the problem, but publishes the A::name member to everyone, whereas it should only be visible to a particular template instantiation, namely C<B>.

回答1:

Work around is what @kerrekSB suggested, add using A::name; in class B:

struct A { int name; };
template<typename T> struct C : T { using T::name; };

struct B : private A { 
using A::name; 
friend struct C<B>;
};

your initial example didn't work cause class A is private to B and class C<B> is friend of B but when you access member name from object of C<B> , line using T::name; creates problem since the class B doesn't has any member name in it. it's scope search which find the member name when you try to access it via object of class B

Edit :

Adding using A::name to B takes care of the problem, but publishes the A::name member to everyone, whereas it should only be visible to a particular template instantiation, namely C

if that's the case , then simply declare statement using A::name; in private section in class B i.e

struct B : private A {
protected: using A::name; 
public:
friend struct C<B>;
};

回答2:

There seems to be a fundamental difference in visibility considerations between gcc and VC++ when using member using-declarations; check this simplified example without templates:

struct A { int name; };
struct B: private A { friend struct C; };
struct C: B {using B::name; };

int main()
{
   C o;
   o.name = 0;
}

It will compile on gcc but not on VC++ (with basically the same error as in the question). Will have to consult the standard on who is doing it right...

来源：https://stackoverflow.com/questions/12294422/template-private-inheritance-in-vc10-is-not-accessible

标签

c++

templates

visual-c++

private-inheritance