C++11 std::map访问的使用小结

#include <iostream>
#include <map>
#include <string>

int main() {
    std::map<int, std::string> fruitMap;
    
    // 插入初始元素
    fruitMap[1] = "Apple";
    fruitMap[2] = "Banana";

    // 访问存在的key
    std::cout << "Key 1: " << fruitMap[1] << std::endl;  // 输出 "Apple"

    // 访问不存在的key（会自动插入新元素！）
    std::cout << "Key 3 (new): " << fruitMap[3] << std::endl;  // 输出空串，此时fruitMap多了{3, ""}
    std::cout << "Now size: " << fruitMap.size() << std::endl;  // 输出3（原2个+新增1个）

    return 0;
}

#include <iostream>
#include <map>
#include <string>
#include <stdexcept>  // for std::out_of_range

int main() {
    std::map<int, std::string> fruitMap = {{1, "Apple"}, {2, "Banana"}};

    try {
        // 访问存在的key
        std::cout << "Key 2: " << fruitMap.at(2) << std::endl;  // 输出 "Banana"

        // 访问不存在的key（抛异常）
        std::cout << "Key 3: " << fruitMap.at(3) << std::endl;  
    } catch (const std::out_of_range& e) {
        std::cerr << "Error: " << e.what() << std::endl;  // 输出 "map::at"
    }

    return 0;
}

#include <iostream>
#include <map>
#include <string>

int main() {
    std::map<int, std::string> fruitMap = {{1, "Apple"}, {2, "Banana"}};

    // 查找存在的key
    auto it = fruitMap.find(1);
    if (it != fruitMap.end()) {
        std::cout << "Key 1: " << it->second << std::endl;  // 输出 "Apple"
    }

    // 查找不存在的key
    it = fruitMap.find(3);
    if (it == fruitMap.end()) {
        std::cout << "Key 3 not found." << std::endl;  // 输出提示
    }

    return 0;
}