CODEWITHSUNDEEP
c++templatestemplate-specializationpartial-specialization
Fred
Fred

Reputation: 636

c++ template specialization with std::enable_if not working

I've a simple primary function template that I want to partially specialize.

template< typename T >
void SetAttribute( const T& value )
{
  static_assert( false, "SetAttribute: wrong type!" );
}

template<> void SetAttribute( const bool& value ) {}

template<> void SetAttribute( const std::wstring& value ) {}

template< typename T >
void SetAttribute( const typename std::enable_if< std::is_integral< T >::value >::type& value ) {}

int main()
{
  SetAttribute( std::wstring( L"bla" ) );
  SetAttribute( bool( true ) );
  SetAttribute( std::uint32_t( 1 ) ); // error C2338: SetAttribute: wrong type!

  return 0;
}

When I compile this with VS 2015 Update 3, then I'll get an error on the 3d call (see comment). why? I don't understand why the 3d specialization is not used.

Thx Fred

Upvotes: 3

Views: 699

Answers (1)

Marco A.
Marco A.

Reputation: 43662

The problem is that you're using T in a non-deduced context

template< typename T >
void SetAttribute( const typename std::enable_if< std::is_integral< T >::value >::type& value ) {}
                                                                    ^

Functions are probably the wrong tool for this job (they cannot be partially specialized), a possible workaround if you insist in using functions could be a combination of tag dispatching and specializations

template<class T>
void SetAttribute(const T&, std::true_type) {}

template<class T>
void SetAttribute(const T& value, std::false_type)
{
  static_assert(std::is_integral<T>::value, "SetAttribute: wrong type!");
}

template< typename T >
void SetAttribute(const T& value)
{
  SetAttribute(value, std::is_integral<T>());
}

template<> void SetAttribute(const bool&) {}

template<> void SetAttribute(const std::wstring&) {}

Example

Quite unreadable if you ask me..

Upvotes: 1

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