overloading << for vector of pairs iterator

Question

I am currently trying to use find_if to find an element in a vector of pairs.I have tried searching on google how to overload << operator and it did give me a large amount of information on how to overload <<. It still left me confused how to achieve my goal. Below is the code i am using, i want to find the string supplied to the function in my vector of pairs s.

void Utils::findIt(string serchVal)
{
    vector<pair<string, HWND>>::iterator it = find_if(s.begin(), s.end(),
 [&serchVal](const pair<string, HWND>& element) { return element.first == serchVal; });

    cout << "find function found : " <<  *it << endl;

}

i have tried overloading the << operator like this.

template <typename T, typename D>
void operator<<(ostream& os, vector<pair(typename T, typename D)> &lst)
{
    cout << lst.begin.front();
}

i am not well versed in overloading functions and i am still new to vectors. so any help concerning this would be appreciated!

Answer 1

You do not need to overload operator<< for a std::vector<std::pair> here as std::find_if will return the iterator pointing to the element found in the std::vector , which in this case will be an iterator to a std::pair<std::string, HWND> , to print this via a std::ostream you could use,

template<typename _Ty1, typename _Ty2>
std::ostream& operator<<(std::ostream& _os, const std::pair<_Ty1, _Ty2>& _p) {
    _os << _p.first << ' ' << _p.second;
    return _os;
}

if you want to go down the operator<< overloading route. However, printing elements of a std::pair is trivial anyway so overloading the insertion operator is not entirely necessary here.

Answer 2

The complete code for your problem:

#include <iostream>
#include <vector>
#include <utility>

template <typename T, typename D>
std::ostream& operator<<(std::ostream& os, std::vector<std::pair<T, D>> &lst) {
    for (const auto &p : lst) {
        os << p.first << ", " << p.second;
    }
    return os;
}

int main() {
    std::vector<std::pair<int, int>> pairs = { { 1, 2 }, { 5, 6 } };
    std::cout << pairs << std::endl;
    return 0;
}

Step by step:

1. Usually it is better to return std::ostream& back to caller code so we change your void operator<< to std::ostream& operator<< prototype.

2. In the operator<< you just do what you want. If you pair 's data in the output - put it there:

    for (const auto &p : lst) { os << p.first << ", " << p.second; }