operator<< for STL types

test.cpp

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

// Adding forward declarations would make this compile

// template <typename K, typename V>
// std::ostream& operator<< (std::ostream&, const std::map<K, V>&);

// template <typename T>
// std::ostream& operator<< (std::ostream&, const std::vector<T>&);

// Implementation

template <typename K, typename V>
std::ostream& operator<< (std::ostream& o, const std::map<K, V>& map)
{
    const char* sep = "{";
    for (const auto& x : map)
    {
        o << sep << "{" << x.first << ", " << x.second << "}";
        sep = ", ";
    }
    return o << "}";
}

template <typename T>
std::ostream& operator<< (std::ostream& o, const std::vector<T>& vec)
{
    const char* sep = "{";
    for (const auto& x : vec)
    {
        o << sep << x;
        sep = ", ";
    }
    return o << "}";
}

// Usage

int main()
{
    std::map<int, std::vector<int>> t = {{1, {2, 3}}, {4, {5}}};
    std::cout << t << std::endl;
    return 0;
}