std::conditional

是在阅读stackoverflow Can't use enum class as unordered_map key的回答时，其中使用了std::conditional:

#include <unordered_map>
#include <iostream>
#include <type_traits>

/**
 * @brief 着色器
 *
 */
class Shader
{
public:
    enum class Type
    {
        Vertex = 1, Geometry = 2, Fragment = 3
    };
    //...
};

struct EnumClassHash
{
    template<typename T>
    std::size_t operator()(T t) const
    {
        std::cout << __FILE__ << " " << __LINE__ << " " << std::endl;
        return static_cast<std::size_t>(t);
    }
};

template<typename Key>
using HashType = typename std::conditional<std::is_enum<Key>::value, EnumClassHash, std::hash<Key>>::type;

template<typename Key, typename T>
using MyUnorderedMap = std::unordered_map<Key, T, HashType<Key>>;

int main()
{
    MyUnorderedMap<int, int> myMap2;
    MyUnorderedMap<Shader::Type, int> myMap3;
}
// g++ --std=c++11 test.cpp

上述使用 std::conditional 构建了一个类型抽象层。

cppreference std::conditional

Possible implementation

template<bool B, class T, class F> struct conditional { typedef T type; }; 

template<class T, class F> struct conditional<false, T, F> { typedef F type; };