C++ 可以重载 `operator<<` 来显示 map 吗？

Question

我重载了我的 operator<<如下所示:

friend ostream& operator<<(ostream& os, Data& e)
{
    // possible code for printing map..
    return os;
}

我有一个 map<string, vector<int>> table ，我的问题是，是否可以通过e.table访问并打印出 map ？ ？

Answer 1

是的，你可以。例如。

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

std::ostream & operator <<(std::ostream &os, 
                           const std::map<std::string, std::vector<int>> &m)
{
    for (const auto &p : m)
    {
        os << p.first << ": ";
        for (int x : p.second) os << x << ' ';
        os << std::endl;
    }

    return os;
}

int main()
{
    std::map<std::string, std::vector<int>> m =
    {
        { "A", { 1, 2, 3, 4, 5 } },
        { "Z", { 5, 4, 3, 2, 1 } }
    };

    std::cout << m << std::endl;
}

程序输出为

A: 1 2 3 4 5
Z: 5 4 3 2 1

或者

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

class Data
{
public:
    friend std::ostream & operator <<(std::ostream &os, const Data &d);

private:
    std::map<std::string, std::vector<int>> m =
    {
        { "A",{ 1, 2, 3, 4, 5 } },
        { "Z",{ 5, 4, 3, 2, 1 } }
    };
};

std::ostream & operator <<(std::ostream &os, const std::map<std::string, std::vector<int>> &m)
{
    for (const auto &p : m)
    {
        os << p.first << ": ";
        for (int x : p.second) os << x << ' ';
        os << std::endl;
    }

    return os;
}

std::ostream & operator <<(std::ostream &os, const Data &d)
{
    return os << d.m;
}

int main()
{
    Data d;
    std::cout << d << std::endl;
}