是否可以检查类型或类的重载operator<<是否存在？

#include <iostream>
#include <type_traits>

// 定义一个检查operator<<是否存在的模板函数
template<typename T>
struct has_output_operator
{
    template<typename U>
    static auto test(U* p) -> decltype(std::declval<std::ostream&>() << *p, std::true_type());

    template<typename U>
    static auto test(...) -> std::false_type;

    static constexpr bool value = decltype(test<T>(nullptr))::value;
};

// 定义一个示例类
class MyClass
{
public:
    int value;

    // 重载operator<<
    friend std::ostream& operator<<(std::ostream& os, const MyClass& obj)
    {
        return os << "MyClass(" << obj.value << ")";
    }
};

int main()
{
    // 检查MyClass是否重载了operator<<
    if (has_output_operator<MyClass>::value)
    {
        std::cout << "MyClass has overloaded operator<<." << std::endl;
    }
    else
    {
        std::cout << "MyClass does not have overloaded operator<<." << std::endl;
    }

    return 0;
}

MyClass has overloaded operator<<.

MyClass does not have overloaded operator<<.