C++ Template Metaprogramming/

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Understanding and Defining Type Traits

Understanding and Defining Type Traits

Learn the concept of type traits and how they are used in template metaprogramming.

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In a nutshell, type traits are small class templates that contain a constant value whose value represents the answer to a question we ask about a type. An example of such a question is, Is this type a floating-point type? The technique for building type traits that provide such information about types relies on template specialization; we define a primary template as well as one or more specializations.

Let’s see how we can build a type trait that tells us, at compile-time, whether a type is a floating-point type:

template<typename T>
struct is_floating_point
{
   static const bool value = false;
};
template<>
struct is_floating_point<float>
{
   static const bool value = true;
};
template<>
struct is_floating_point<double>
{
   static const bool value = true;
};
template<>
struct is_floating_point<long double>
{
   static const bool value = true;
};
Building type traits to recognize floating-point types at compile-time

There are two things to notice here:

  • We have defined a primary template as well as several full specializations, one for each type that is a floating-point type.

  • The primary template has a static const boolean member initialized with the false value; the full specializations set the value of this member to true.

There is nothing more to building a type trait than this. The is_floating_point<T> type trait tells us whether a type is a ...