编译期判断类是否存在成员函数

1. 检测类是否存在push_back

只能检测stl，因为用到了T::value_type，只在stl中存在
可以通过修改push_back的参数类型来判断其它类

#include <iostream>
#include <list>
#include <map>
#include <set>
#include <string>
#include <vector>

template <typename>
using void_t = void;

template <typename T, typename V = void>
struct has_push_back:std::false_type {};

template <typename T>
struct has_push_back<T, void_t<decltype(std::declval<T>().push_back(std::declval<typename T::value_type>()))>>:std::true_type {};

int main() {
    std::cout << has_push_back<std::list<int>>::value << std::endl;
    std::cout << has_push_back<std::map<int, int>>::value << std::endl;
    std::cout << has_push_back<std::set<int>>::value << std::endl;
    std::cout << has_push_back<std::string>::value << std::endl;
    std::cout << has_push_back<std::vector<int>>::value << std::endl;
    return 0;
}

2. 检测stringstream是否存在对应参数的重载函数

注意：不能判断引用类型的参数，或者修改T为T&

template <typename>
using void_t = void;

template <typename T, typename V = void>
struct has_ss_type : std::false_type {};

template <typename T>
struct has_ss_type <T, void_t<decltype(std::declval<std::stringstream>().operator<<(std::declval<T>()))>> : std::true_type {};

/*
template <typename T>
struct has_ss_type <T, void_t<decltype(std::declval<std::stringstream>().operator<<(std::declval<T&>()))>> : std::true_type {};
*/

int main() {
    std::cout << has_ss_type<int>::value << std::endl;
    std::cout << has_ss_type<double>::value << std::endl;
    std::cout << has_ss_type<std::string>::value << std::endl;
    std::cout << has_ss_type<std::vector<int>>::value << std::endl;
    return 0;
}

发布于 2023-12-31