检查C++中的字符串是否为"null“

#include <iostream>
#include <string>

int main() {
    std::string str = "null";
    if (str == "null") {
        std::cout << "The string is 'null'." << std::endl;
    } else {
        std::cout << "The string is not 'null'." << std::endl;
    }
    return 0;
}

#include <iostream>
#include <cstring>

int main() {
    const char* str = "null";
    if (std::strcmp(str, "null") == 0) {
        std::cout << "The string is 'null'." << std::endl;
    } else {
        std::cout << "The string is not 'null'." << std::endl;
    }
    return 0;
}

#include <iostream>
#include <string>
#include <algorithm>
#include <cctype>

// 去除字符串两端的空白字符
std::string trim(const std::string& str) {
    size_t first = str.find_first_not_of(' ');
    if (std::string::npos == first) {
        return str;
    }
    size_t last = str.find_last_not_of(' ');
    return str.substr(first, (last - first + 1));
}

int main() {
    std::string str = " null ";
    str = trim(str);
    if (str == "null") {
        std::cout << "The string is 'null'." << std::endl;
    } else {
        std::cout << "The string is not 'null'." << std::endl;
    }
    return 0;
}