使用C++17检测类成员是否存在

#include <iostream>
#include <type_traits>

// 检测成员函数 has_foo
template <typename T, typename = void>
struct has_foo : std::false_type {};

template <typename T>
struct has_foo<T, std::void_t<decltype(std::declval<T>().foo())>> : std::true_type {};

// 示例类
struct A {
    void foo() {}
};

struct B {};

int main() {
    std::cout << "A has foo: " << has_foo<A>::value << '\n'; // 输出: 1 (true)
    std::cout << "B has foo: " << has_foo<B>::value << '\n'; // 输出: 0 (false)
    return 0;
}

#include <iostream>
#include <type_traits>

// 检测私有成员函数 has_private_foo
template <typename T, typename = void>
struct has_private_foo : std::false_type {};

template <typename T>
struct has_private_foo<T, std::void_t<decltype(std::declval<T>().private_foo())>> : std::true_type {};

// 示例类
class A {
private:
    friend struct has_private_foo<A>;
    void private_foo() {}
};

class B {};

int main() {
    std::cout << "A has private_foo: " << has_private_foo<A>::value << '\n'; // 输出: 1 (true)
    std::cout << "B has private_foo: " << has_private_foo<B>::value << '\n'; // 输出: 0 (false)
    return 0;
}