c++: Basic Traits

Traits:
In C++ "traits" are basically a group of template classes that provide miscellaneous information about another type or data structure.

'Think of a trait as a small object whose main purpose is to carry information used by another object or algorithm to determine "policy" or "implementation details".'
- Bjarne Stroustrup

Traits are very common in C++. One of the more common ones is the string class its self. The string class uses traits to provide information about internationalization and character encoding. Often they are visible in the code and debugger output as a template parameter that has a default initialization via a second template.

In other cases the use of traits is more normal as in the the class "std::numeric_limits" and the other members of the limits.h header.

Boost also offers many interesting and helpful traits classes as an example here is the "is_void" traits class.

template< typename T > 
struct is_void{ 
  static const bool value = false;
};

template<> 
struct is_void< void >{ 
  static const bool value = true; 
};

As you can see this trait template's sole purpose to is provide information about what the input parameter type was. This is only really useful in a parts of the code which the programmer doesn't know what the input type was: Hence inside another template.

For more info refer to:
http://www.cantrip.org/traits.html
http://www.cplusplus.com/reference/std/limits/numeric_limits/