How to specialize Iterator by its value type, in C++?

Question

Is it possible to specialize an Iterator template parameter by its value_type?

I have a function with the following prototype.

template<typename InputIterator>
void f(InputIterator first, InputIterator last);

And I want to handle specially if InputIterator::value_type is SomeSpecificType.

Answer 1

You can use some intermediate structs to get the partial template specialisation that you need. Something like this should do the trick

template<typename T, typename V>
struct f_impl
{
  static void f( T first, T last ) {...}; //Default version
};

template<typename T>
struct f_impl<T, SomeSpecificType>
{
   static void f(T first,T last) {...}; //Specialisation
};

template<typename InputIterator> void f(InputIterator first, InputIterator last)
{
  f_impl<
      InputIterator,
      typename std::iterator_traits<InputIterator>::value_type
  >::f(first,last);
};

Answer 2

Using SFINAE, assuming enable_if[_c] and is_same are either from Boost or <type_traits> (and are appropriately qualified with either boost:: or std:: respectively):

template<typename InputIterator>
typename enable_if<
    !is_same<
        typename std::iterator_traits<InputIterator>::value_type,
        SomeSpecificType
    >::value
>::type
f(InputIterator first, InputIterator last)
{
    // Default implementation.
}

template<typename InputIterator>
typename enable_if<
    is_same<
        typename std::iterator_traits<InputIterator>::value_type,
        SomeSpecificType
    >::value
>::type
f(InputIterator first, InputIterator last)
{
    // Special case
}

In the Boost case, use boost::enable_if_c for something similar to the above. You can use boost::enable_if and get rid of the ::value but then must also use e.g. boost::disable_if.