Question

Why do I keep on getting the following error in this code in Visual C++ 2010, and how do I fix it while maintaining the type inference capability for the member variable?

error C2825: 'Foo<T>::value_type': must be a class or namespace when followed by '::'

template<class T>
struct Foo
{
    typedef typename T::value_type value_type;

    template<class M>
    void foo(M value_type::*member) const;   // error
};
struct S { typedef int value_type; };

int main() { Foo<S> s; }

Answer 1

The template parameter T turns out to be type S, therefore value_type turns out to be int (the nested-type in S). So how can you write value_type::*member? Note that it turns out to be int::*member which doesn’t make sense. int is not a class type.

I think you meant T::*member instead of value_type::*member.