std::enable_if : parameter vs template parameter

I'm building some input checker that needs to have specific functions for integer and/or double (for example 'isPrime' should only be available for integers).

If I'm using enable_if as a parameter it's working perfectly :

template <class T>
class check
{
public:
   template< class U = T>
   inline static U readVal(typename std::enable_if<std::is_same<U, int>::value >::type* = 0)
   {
      return BuffCheck.getInt();
   }

   template< class U = T>
   inline static U readVal(typename std::enable_if<std::is_same<U, double>::value >::type* = 0)
   {
      return BuffCheck.getDouble();
   }   
};

but if I'm using it as a template paramater (as demonstrated on http://en.cppreference.com/w/cpp/types/enable_if )

template <class T>
class check
{
public:
   template< class U = T, class = typename std::enable_if<std::is_same<U, int>::value>::type >
   inline static U readVal()
   {
      return BuffCheck.getInt();
   }

   template< class U = T, class = typename std::enable_if<std::is_same<U, double>::value>::type >
   inline static U readVal()
   {
      return BuffCheck.getDouble();
   }
};

then I have the following error :

error: ‘template<class T> template<class U, class> static U check::readVal()’ cannot be overloaded
error: with ‘template<class T> template<class U, class> static U check::readVal()’

I can't figure out what is wrong in the second version.

Edit Report

The problem is that the compiler sees 2 overloads of the same method, both which contain the same arguments(none, in this case) and the same return value. You can't provide such definition. The cleanest way to do this is to use SFINAE on the function's return value:

template <class T>
class check
{
public:
   template< class U = T>
   static typename std::enable_if<std::is_same<U, int>::value, U>::type readVal()
   {
      return BuffCheck.getInt();
   }

   template< class U = T>
   static typename std::enable_if<std::is_same<U, double>::value, U>::type readVal()
   {
      return BuffCheck.getDouble();
   }
};

That way, you're providing 2 different overloads. One returns an int, the other one returns a double, and only one can be instantiated using a certain T.

answered 2012-06-15T18:18:55.403

std::enable_if : parameter vs template parameter

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