Question

template<typename T>
struct check
{
  static const bool value = false;
};

What I want to do is to have check<T>::value be true if and only if T is a std::map<A,B> or std::unordered_map<A,B> and both A and B be std::string. So basically check enables compile-time checking of the type T.
How do I do this?

Answer 1

Partial specialization for when you want to allow any comparator, hasher, key-equal-comparator and allocator:

template<class Comp, class Alloc>
struct check<std::map<std::string, std::string, Comp, Alloc>>{
  static const bool value = true;
};

template<class Hash, class KeyEq, class Alloc>
struct check<std::unordered_map<std::string, std::string, Hash, KeyEq, Alloc>>{
  static const bool value = true;
};

If you want to check if T used the default version of those types (aka only map<A,B> and not map<A,B,my_comp>, you can omit the template arguments and go with explicit specialization:

template<>
struct check<std::map<std::string, std::string>>{
  static const bool value = true;
};

template<>
struct check<std::unordered_map<std::string, std::string>>{
  static const bool value = true;
};

And if you want to generally check if it’s a std::map or std::unordered_map of any key/value combination (and comparator / hasher / etc.), you can go fully generic as taken from here:

#include <type_traits>

template < template <typename...> class Template, typename T >
struct is_specialization_of : std::false_type {};

template < template <typename...> class Template, typename... Args >
struct is_specialization_of< Template, Template<Args...> > : std::true_type {};

template<class A, class B>
struct or_ : std::integral_constant<bool, A::value || B::value>{};

template<class T>
struct check
  : or_<is_specialization_of<std::map, T>,
       is_specialization_of<std::unordered_map, T>>{};