Question

I have a class template that intends to use its parameter K as the key to a map.

Is there any way to restrict the template parameter to be a type that complies with the Key in std::map?

I realize that even without such constraint, the compiler would spit out a pile of template errors like K having no operator < (), but it would be nice if I can make my code more obvious in specifying requirements.

C++11 solutions are welcome.

template< typename K >
class Foo
{
  // lots of other code here...

  private:
    std::map< K, size_t > m_map;
};

Answer 1

It depends on what you mean by “complies.” If you want to verify that K has a < operator then you might try the Boost Concept Checking Library.

#include "boost/concept_check.hpp"

template< typename K >
class Foo
{
  BOOST_CONCEPT_ASSERT((boost::LessThanComparable< K >));

  // lots of other code here...

  private:
    std::map< K, size_t > m_map;
};

However if you want to verify that < defines a StrictWeakOrdering on K, that would require testing run-time behaviour under all possible inputs.