Question

I have run into some strange behavior. I am not sure if this should work or not. Note that I am not trying to make it work, I am rather concerned with theoretical side. I am using Visual C++ 2010. Observe the following code

template <class T_>
ostream &operator <<(std::ostream &out, 
        typename SequenceCheckResult<T_>::directiontype const &direction) {

    switch(direction) {
    case SequenceCheckResult<T_>::None:
        out<<"none o";
        break;
    case SequenceCheckResult<T_>::Horizontal:
        out<<"horizontal _";
        break;
    case SequenceCheckResult<T_>::Vertical:
        out<<"vertical |";
        break;
    case SequenceCheckResult<T_>::ForwardDiagonal:
        out<<"forward diagonal \\";
        break;
    case SequenceCheckResult<T_>::BackwardDiagonal:
        out<<"backward diagonal /";
        break;
    }

    return out;
}

Obviously the template class SequenceCheckResult contains an enum named directiontype. Now the code above does not get instantiated and integer value of the variable is printed. Even if I provide specialization, it still does not work. If I create the following function, it is used and the textual value is printed. Notice that the following code and int specialization of the above function only differs by template<> line and <int> following operator <<.

ostream &operator << (std::ostream &out, 
         SequenceCheckResult<int>::directiontype const &direction) {

    switch(direction) {
    case SequenceCheckResult<int>::None:
        out<<"none o";
        break;
    case SequenceCheckResult<int>::Horizontal:
        out<<"horizontal _";
        break;
    case SequenceCheckResult<int>::Vertical:
        out<<"vertical |";
        break;
    case SequenceCheckResult<int>::ForwardDiagonal:
        out<<"forward diagonal \\";
        break;
    case SequenceCheckResult<int>::BackwardDiagonal:
        out<<"backward diagonal /";
        break;
    }

    return out;
}

I wish to know why this happens, possible causes are:

• Compiler bug
• Implementation defines this behavior
• Standards define this behavior
• I am missing something

Thanks in advance

Answer 1

Your approach does not and cannot work. You cannot deduce a “container based on an element”!

For a simple argument, consider this thought experiment:

struct Foo { typedef int type; }
struct Bar { typedef int type; }

template <typename T> void deduce_me(typename T::type n) { }

Now if I call deduce_me(5), how are we supposed to deduce either Foo or Bar?