在类模板中找不到声明的类型

Question

I'd appreciate help resolving the following error. I'm getting a 'type Attributes not found in struct Frobnigator<Foo> error' even though struct Frobnigator declares such a member, as shown below (see also ideone.com ).

struct Foo{};
struct Bar{};

/////////////////////////////////////////////////////////////////////////////

template<typename P>
struct Attributes
{
};

template<>
struct Attributes<Foo>
{
};

/////////////////////////////////////////////////////////////////////////////

template<typename P>
struct Frobnigator
{
    Attributes<P>   attributes;
};

/////////////////////////////////////////////////////////////////////////////

template<typename P>
struct OuterHandler
{
    typedef Frobnigator<P>  Frob;

    template<typename T>
    struct InnerHandler;

    void doStuff();
};

template<>
struct OuterHandler<Foo>
{
    typedef Frobnigator<Foo>    Frob;

    template<typename T>
    struct InnerHandler;

    void doStuff();
};

/////////////////////////////////////////////////////////////////////////////

template<typename P>
template<typename T>
struct OuterHandler<P>::InnerHandler
{   
    typename T::Attributes attributes;
    InnerHandler(){}
};


template<typename T>
struct OuterHandler<Foo>::InnerHandler
{
    typename T::Attributes attributes;
    InnerHandler(){}
};

/////////////////////////////////////////////////////////////////////////////

template<typename P>
void OuterHandler<P>::doStuff()
{
    InnerHandler<Frob>();
}

void OuterHandler<Foo>::doStuff()
{
    InnerHandler<Frob>();
}

/////////////////////////////////////////////////////////////////////////////

int main()
{
    return 0;
}

/////////////////////////////////////////////////////////////////////////////

Visual studio error message

Test.cpp(62) : error C2039: 'Attributes' : is not a member of 'Frobnigator<P>'
        with
        [
            P=Foo
        ]
        Test.cpp(76) : see reference to class template instantiation 'OuterHandler<Foo>::InnerHandler<T>' being compiled
        with
        [
            T=OuterHandler<Foo>::Frob
        ]

g++ (GCC) 4.5.3 error message

Test.cpp: In instantiation of ‘OuterHandler<Foo>::InnerHandler<Frobnigator<Foo> >’:
Test.cpp:76:21:   instantiated from here
Test.cpp:62:25: error: no type named ‘Attributes’ in ‘struct Frobnigator<Foo>’

Answer 1

What's confusing? Frobnigator<T> indeed has no type member named Attributes .

If you're using C++11, try:

typename decltype(T::attributes) attributes;

Otherwise, and probably clearer anyway:

template<typename P>
struct Frobnigator
{
    typedef Attributes<P> attributes_type;
    attributes_type attributes;
};

Then you can use Frobnigator<T>::attributes_type anywhere.

Answer 2

If I understand it right, your are looking for something similar implementation as graph_traits<Graph> in Boost.

#include <iostream>
using namespace std;

struct Foo {};

template<typename T>
struct Attributes
{
};

template<>
struct Attributes<Foo>
{
};

template<typename T>
struct Frobnigator
{
  typedef Attributes<T> Attr;
  Attr attributes;
};

template<typename T>
struct traits
{
  typedef typename T::Attr Attributes;
};

int main()
{
  traits<Frobnigator<Foo> >::Attributes a;
  return 0;
}