c++如何实现一个简单的模板元编程库_c++仿std::type_traits

struct true_type {
    static constexpr bool value = true;
};

struct false_type {
    static constexpr bool value = false;
};

template <bool B>
using bool_constant = typename std::conditional<B, true_type, false_type>::type;

template <bool B>
struct bool_constant {
    using type = bool_constant<B>;
    static constexpr bool value = B;
};

template <typename T, typename U>
struct is_same : false_type {};

template <typename T>
struct is_same<T, T> : true_type {};

template <typename T>
struct is_integral : false_type {};

template<> struct is_integral<bool> : true_type {};
template<> struct is_integral<char> : true_type;
template<> struct is_integral<unsigned char> : true_type;
template<> struct is_integral<signed char> : true_type;
// ... 其他如 short, int, long, long long 等

template <bool B, typename T = void>
struct enable_if {
    using type = T;
};

template <typename T>
struct enable_if<false, T> {}; // 不定义 type

template <typename T>
typename enable_if<is_integral<T>::value, T>::type
add(T a, T b) {
    return a + b;
}

template <typename T>
struct remove_const {
    using type = T;
};

template <typename T>
struct remove_const<const T> {
    using type = T;
};

template <typename T>
struct remove_volatile {
    using type = T;
};

template <typename T>
struct remove_volatile<volatile T> {
    using type = T;
};

template <typename T>
struct remove_cv {
    using type = typename remove_volatile<typename remove_const<T>::type>::type;
};

template <typename T>
struct remove_reference {
    using type = T;
};

template <typename T>
struct remove_reference<T&> {
    using type = T;
};

template <typename T>
struct remove_reference<T&&> {
    using type = T;
};

template <typename T>
struct is_arithmetic :
    bool_constant<
        is_integral<T>::value ||
        std::is_same<T, float>::value ||
        std::is_same<T, double>::value ||
        std::is_same<T, long double>::value
    > {};

template <typename T>
inline constexpr bool is_same_v = is_same<T, U>::value;

template <typename T>
using remove_reference_t = typename remove_reference<T>::type;