std::traits学习

类型判断和退化（获取原始类型）的原理就是利用模板的特例化。根据调用模板的特例化，在特例化模板中实现判断的逻辑或者退化的逻辑。

一、类型判断

判断整型数据的模板类

#include <iostream>  

namespace zk {
    template<typename T, T Val>
    struct integral_constant {
        static constexpr T value = Val;
    };

    using false_type = integral_constant<bool, false>;
    using true_type = integral_constant<bool, true>;

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

    template<>
    struct isInteger<std::int8_t> :true_type {};
    template<>
    struct isInteger<std::int16_t> :true_type {};
    template<>
    struct isInteger<std::int32_t> :true_type {};
    template<>
    struct isInteger<std::int64_t> :true_type {};

    template<>
    struct isInteger<std::uint8_t> :true_type {};
    template<>
    struct isInteger<std::uint16_t> :true_type {};
    template<>
    struct isInteger<std::uint32_t> :true_type {};
    template<>
    struct isInteger<std::uint64_t> :true_type {};

    template<typename T>
    constexpr bool is_integer_v = isInteger<T>::value;
}

int main() {
    if (zk::is_integer_v<std::string>) {
        std::cout << "is integer" << std::endl;
    }
    else {
        std::cout << "not integer" << std::endl;
    }
    if (zk::is_integer_v<std::int32_t>) {
        std::cout << "is integer" << std::endl;
    }
    if (zk::is_integer_v<std::uint32_t>) {
        std::cout << "is integer" << std::endl;
    }
    if (zk::is_integer_v<std::int64_t>) {
        std::cout << "is integer" << std::endl;
    }
    return 0;
}

其中

template<typename T, T Val>
struct integral_constant {
    static constexpr T value = Val;
};

申明定义了一个模板类，存放了一个由模板参数决定的静态常量成员变量。然后用这个模板类，申明并定义了false_type和true_type

using false_type = bool_constrait<bool, false>;
using true_type = bool_constrait<bool, true>;

然后定义一个模板类，默认匹配到模板类时，不是int整型

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

当匹配到特例化类时，是int整型

    template<>
    struct isInteger<std::int8_t> :true_type {};
    template<>
    struct isInteger<std::int16_t> :true_type {};
    template<>
    struct isInteger<std::int32_t> :true_type {};
    template<>
    struct isInteger<std::int64_t> :true_type {};

    template<>
    struct isInteger<std::uint8_t> :true_type {};
    template<>
    struct isInteger<std::uint16_t> :true_type {};
    template<>
    struct isInteger<std::uint32_t> :true_type {};
    template<>
    struct isInteger<std::uint64_t> :true_type {};

没写完，还有short、long、longlong等类型

二、退化（获取原始类型）

有时需要获取到一个对象的原始类型，不含指针、不含引用、不含右值引用等

#include <iostream>  

namespace zk {
    template<typename T>
    struct decay {
        using type = T;
    };

    template<typename T>
    struct decay<T&> {
        using type = T;
    };
    template<typename T>
    struct decay<T&&> {
        using type = T;
    };
    template<typename T>
    struct decay<T*> {
        using type = T;
    };
    template<typename T>
    struct decay<std::shared_ptr<T>> {
        using type = T;
    };

    template<typename T>
    using decay_t = typename decay<T>::type;
}

int main() {
    std::cout << "type : " << typeid(zk::decay_t<int>).name() << std::endl;
    std::cout << "type : " << typeid(zk::decay_t<int*>).name() << std::endl;
    std::cout << "type : " << typeid(zk::decay_t<std::shared_ptr<int>>).name() << std::endl;
    std::cout << "type : " << typeid(zk::decay_t<std::int32_t>).name() << std::endl;
    std::cout << "type : " << typeid(zk::decay_t<std::int32_t*>).name() << std::endl;
    std::cout << "type : " << typeid(zk::decay_t<std::shared_ptr<std::int32_t>>).name() << std::endl;
    std::cout << "type : " << typeid(zk::decay_t<std::string>).name() << std::endl;
    std::cout << "type : " << typeid(zk::decay_t<char*>).name() << std::endl;
    std::cout << "type : " << typeid(zk::decay_t<float*>).name() << std::endl;
    zk::decay_t<char*> a = 97;
    std::cout << "a : " << a << std::endl;
    return 0;
}

只移除了指针、引用和右值引用和共享指针。std::traits还可以还原出std::function、std::string等类型。我这里就没实现这两个了。