std::enable_if : parameter vs template parameter

Question

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.

Answer 1

Default template arguments are not part of the signature of a template (so both definitions try to define the same template twice). Their parameter types are part of the signature, however. So you can do

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