Question

How can I determine if a type is derived from a template class? In particular, I need to determine if a template parameter has std::basic_ostream as a base class. Normally std::is_base_of is the tool for the job. However, std::is_base_of only works for complete types not class templates.

I’m looking for something like this.

template< typename T >
bool is_based_in_basic_ostream( T&& t )
{
   if( std::is_base_of< std::basic_ostream< /*anything*/>, T >::value )
   {
      return true;
   }
   else
   {
      return false;
   }
}

I’m sure this can be done I can’t think how.

Answer 1

I’m not aware of a short and concise way. But you can abuse overloading again

template< typename T, typename U >
std::true_type is_based_impl( std::basic_ostream<T, U> const volatile& );
std::false_type is_based_impl( ... );

template< typename T >
bool is_based_in_basic_ostream( T&& t ) {
  return decltype(is_based_impl(t))::value;
}

It will only detect public inheritance. Note that you can instead detect derivation from ios_base, which may work for you equally well (this test will also be positive for input streams, so it’s only of limited applicability)

std::is_base_of<std::ios_base, T>