SFINAE compiler troubles

Question

The following code of mine should detect whether T has begin and end methods:

template 
struct is_contai         


        

Answer 1

Why are you going to all that effort? If you want to check if U::begin() exists, why not try it?

template 
struct is_container
{
    template  static char test(U* u,
       typename U::const_iterator b = ((U*)0)->begin(),
       typename U::const_iterator e = ((U*)0)->end());
    template  static long test(...);

    enum { value = (1 == sizeof test(0)) };
};

In addition to checking for the existance of U::begin() and U::end(), this also checks whether they return something that is convertible to a const_iterator. It also avoids the pitfall highlighted by Stephan T. Lavavej by using a call expression that must be supported, instead of assuming a particular signature.

[edit] Sorry, this relied on VC10's template instantiation. Better approach (puts the existance check in the argument types, which do participate in overloading):

template  struct is_container
{
    // Is.
    template 
    static char test(U* u, 
                     int (*b)[sizeof(typename U::const_iterator()==((U*)0)->begin())] = 0,
                     int (*e)[sizeof(typename U::const_iterator()==((U*)0)->end())] = 0);
    // Is not.
    template  static long test(...);

    enum { value = (1 == sizeof test(0)) };
};