#include <iostream>
#include <map>
using std::cout; using std::cin;
using std::endl; using std::map;
using std::string;
int main(){
    std::map<string, string> m1 = {{"h", "htop"},
                                   {"k", "ktop"},
                                   {"t", "ttop"},
                                   {"r", "rtop"},
                                   {"w", "wtop"},
                                   {"p", "ptop"},};
    string key = "h";
    auto item = m1.find(key);
    if (item != m1.end()) {
        cout << "Key exists!  -  {" <<
            item->first << ";" << item->second << "}\n";
    } else {
        cout << "Key does not exist!" << endl;
    return EXIT_SUCCESS;
Key exists!  -  {h;htop}
使用 contains 成员函数检查 C++ 映射中是否存在给定元素
如果用户需要确认具有给定值的一对是否存在于 map 对象中，可以使用成员函数 contains。自 C++20 版本以来，该函数一直是 std::map 容器的一部分，因此你应该知道编译器版本才能运行以下代码片段。contains 函数获取对键的引用，如果找到，则返回 bool 值 true。这个成员函数的时间复杂度也是对数的。
#include <iostream>
#include <map>
using std::cout; using std::cin;
using std::endl; using std::map;
using std::string;
int main(){
    std::map<string, string> m1 = {{"h", "htop"},
                                   {"k", "ktop"},
                                   {"t", "ttop"},
                                   {"r", "rtop"},
                                   {"w", "wtop"},
                                   {"p", "ptop"},};
    string key = "h";
    if (m1.contains(key)) {
        cout << "Key Exists!" << endl;
    } else {
        cout << "Key does not exist!" << endl;
    return EXIT_SUCCESS;
Key Exists!
或者，可以使用 cout 成员函数来检查具有给定键的元素对是否存在于 map 中。通常，cout 函数用于检索具有给定键的元素数量，但由于 std::map 仅存储唯一的键值，如果找到该元素，该过程将返回 1。否则，返回零值以指示未找到元素。

#include <iostream>
#include <map>
using std::cout; using std::cin;
using std::endl; using std::map;
using std::string;
int main(){
    std::map<string, string> m1 = {{"h", "htop"},
                                   {"k", "ktop"},
                                   {"t", "ttop"},
                                   {"r", "rtop"},
                                   {"w", "wtop"},
                                   {"p", "ptop"},};
    string key = "h";
    if (m1.count(key)) {
        cout << "Key Exists!" << endl;
    } else {
        cout << "Key does not exist!" << endl;
    return EXIT_SUCCESS;
Key Exists!
    
Author: Jinku Hu

                
Founder of DelftStack.com. Jinku has worked in the robotics and automotive industries for over 8 years. He sharpened his coding skills when he needed to do the automatic testing, data collection from remote servers and report creation from the endurance test. He is from an electrical/electronics engineering background but has expanded his interest to embedded electronics, embedded programming and front-/back-end programming.

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