Traits class (idea)

A technique used in C++ for reducing the number of parameters to a given template.

If a template you are creating has a large number of parameters (say, more than three), you might want to declare the template like this:

template <class FIRST, class SECOND, class THIRD, class FOURTH>
class foo
{
 typedef FIRST first_type;
 typedef SECOND second_type;
 typedef THIRD third_type;
 third_type fourth_func (first_type const &first, second_type cont &second)
 {
  if (first != 0) return FOURTH(second); else return 0;
 }
}; 

If you do, you are making problems for yourself. Given:

template <class F, class I>
class Floor
{
 I operator(F const &f) { return floor (f); }
}

You will have to refer to a foo<char,float,int,Floor<float, int> > everywhere in your code.

If you do this instead:

struct foo_traits1
{
 typedef char first_type;
 typedef float second_type;
 typedef int third_type;
 typedef Floor <float, int> FOURTH;
};

template <class TRAITS>
class foo
{
 typedef typename TRAITS::first_type first_type;
 typedef typename TRAITS::second_type second_type;
 typedef typename TRAITS::third_type third_type;
 third_type fourth_func (first_type const &first, second_type cont &second)
 {
  if (first != 0) return TRAITS::FOURTH(second); else return 0;
 }
}; 

you can now refer to a foo<foo_traits1> class everywhere you need that particular foo in the code. This will make maintaining and debugging your code easier. If you have a second implementation, you would declare another traits type (foo_traits2?) and put its name in the angle brackets.

The best-known traits type is the char_traits<> template used for implementing standard strings and iostreams.