如何在没有无限递归的'=='运算符重载中检查空值？

#include<iostream>
#include <type_traits>

int main() {
    int* ptr = nullptr;
    if (std::is_null_pointer<decltype(ptr)>::value) {
        std::cout << "The pointer is null."<< std::endl;
    } else {
        std::cout << "The pointer is not null."<< std::endl;
    }
    return 0;
}

#include<iostream>
#include<memory>

int main() {
    std::shared_ptr<int> ptr;
    if (!ptr) {
        std::cout << "The shared_ptr is null."<< std::endl;
    } else {
        std::cout << "The shared_ptr is not null."<< std::endl;
    }
    return 0;
}

#include<iostream>
#include<vector>

int main() {
    std::vector<int> vec;
    if (vec.empty()) {
        std::cout << "The vector is empty."<< std::endl;
    } else {
        std::cout << "The vector is not empty."<< std::endl;
    }
    return 0;
}