本题要求你自行实现一个 map 类，使以下示例代码能够正常运行。

#include <iostream>
#include <string>
#include <string_view>

class map;

void print_map(std::string_view comment, const map<std::string, int>& m)
{
    std::cout << comment;
    // Iterate using C++17 facilities
    for (const auto& [key, value] : m)
        std::cout << '[' << key << "] = " << value << "; ";

// C++11 alternative:
//  for (const auto& n : m)
//      std::cout << n.first << " = " << n.second << "; ";
//
// C++98 alternative:
//  for (map<std::string, int>::const_iterator it = m.begin(); it != m.end(); ++it)
//      std::cout << it->first << " = " << it->second << "; ";

    std::cout << '\n';
}

int main()
{
    // Create a map of three (string, int) pairs
    map<std::string, int> m{{"CPU", 10}, {"GPU", 15}, {"RAM", 20}};

    print_map("1) Initial map: ", m);

    m["CPU"] = 25; // update an existing value
    m["SSD"] = 30; // insert a new value
    print_map("2) Updated map: ", m);

    // Using operator[] with non-existent key always performs an insert
    std::cout << "3) m[UPS] = " << m["UPS"] << '\n';
    print_map("4) Updated map: ", m);

    m.erase("GPU");
    print_map("5) After erase: ", m);

    for (auto it = m.begin(); it != m.end(); )
    {
        if (it->second > 25)
            it = m.erase(it);  // erase returns iterator to next element
        else
            ++it;
    }
    print_map("6) After erase: ", m);
    std::cout << "7) m.size() = " << m.size() << '\n';

    m.clear();
    std::cout << std::boolalpha << "8) Map is empty: " << m.empty() << '\n';
}

正确的输出应为：

1) Initial map: [CPU] = 10; [GPU] = 15; [RAM] = 20;
2) Updated map: [CPU] = 25; [GPU] = 15; [RAM] = 20; [SSD] = 30;
3) m[UPS] = 0
4) Updated map: [CPU] = 25; [GPU] = 15; [RAM] = 20; [SSD] = 30; [UPS] = 0;
5) After erase: [CPU] = 25; [RAM] = 20; [SSD] = 30; [UPS] = 0;
6) After erase: [CPU] = 25; [RAM] = 20; [UPS] = 0;
7) m.size() = 3
8) Map is empty: true