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Example

Trait type

所谓trait type，指的是获得type。

boost graph/include/boost/graph/adjacency_list.hpp

//=========================================================================== // Selectors for the VertexList and EdgeList template parameters of // adjacency_list, and the container_gen traits class which is used // to map the selectors to the container type used to implement the // graph.

selector and trait。

struct vecS
{
};
struct listS
{
};
struct setS
{
};

template < class Selector, class ValueType > struct container_gen
{
};

template < class ValueType > struct container_gen< listS, ValueType >
{
    typedef std::list< ValueType > type;
};

template < class ValueType > struct container_gen< vecS, ValueType >
{
    typedef std::vector< ValueType > type;
};

template < class ValueType > struct container_gen< mapS, ValueType >
{
    typedef std::set< ValueType > type;
};

显然，在primary template中，没有提供type，只在specialization中提供了type。

Trait function

libstdc++-v3/include/std/limits

  /**
   *  @brief Properties of fundamental types.
   *
   *  This class allows a program to obtain information about the
   *  representation of a fundamental type on a given platform.  For
   *  non-fundamental types, the functions will return 0 and the data
   *  members will all be @c false.
  */
  template<typename _Tp>
    struct numeric_limits : public __numeric_limits_base

注释中，对default value的说明。

Trait variable

这种是最最简单的，即定义有成员变量来说明含义。

What is a Type Trait?

not

// (A)
template <bool X>
struct not
{
    static constexpr bool value {false};
};

// (B)
template <>
struct not<false>
{
    static constexpr bool value {true};
};

Comparing types

// (A)
template <typename T, typename U>
struct is_same_type
{
    static constexpr bool value {false};
};

// (B)
template <typename T>
struct is_same_type<T, T>
{
    static constexpr bool value {true};
};

template <typename T>
T myfunc(T x)
{
    if (is_same_type<T, FooType>::value) {
        /* do something which is completely FooType specific */
    } else {
        /* do the general thing */
    }
}

template <typename T>
T myfunc(T x)
{ /* do the general thing */ }

template <>
FooType myfunc(FooType x)
{ /* do something which is completely FooType specific */ }

Determining if T is a pointer type

template <typename T>
struct is_pointer
{
    static constexpr bool value {false};
};

template <typename T>
struct is_pointer<T*>
{
    static constexpr bool value {true};
};

library mapbox/ jni.hpp/include/jni/traits.hpp