Does std::find use operator== for std::vector<std::pair<T, T>>? - iterator

Question

I tried to overload operator== for std::pair<int, int> so that only the first element of the pair would matter. Then, I'd like to use std::find to look for a particular element in std::vector<std::pair<int, int>>, using the overloaded operator==. But, it seems that, std::find is not using my overloaded operator==, though it is working in a simple comparison statement.
I expect the following code to output:
1
1
1
but I get:
1
1
0
Run on Linux, gcc 11.3:
#include <iostream>
#include <algorithm>
#include <vector>
#include <utility>
using namespace std;
typedef pair<int, int> p_int_t;
bool operator==(const p_int_t& p1, const p_int_t& p2)
{
return p1.first == p2.first;
}
int main()
{
vector<p_int_t> v;
v.push_back({1, 2});
v.push_back({1, 3});
p_int_t p(1, 4);
cout << (v[0] == p) << endl;
cout << (v[1] == p) << endl;
cout << (find(v.begin(), v.end(), p) != v.end()) << endl;
return 0;
}

Answer 1

The compiler does not select the free standing comparison operator because the type p_int_t is an alias, and it is not defined in the std namespace as std:: pair is. In other words, the compiler is looking for an operator with this signature: std::operator==(const std::pair<int, int>&, const std::pair<int, int>&); and finds it in algorithm.
You could declare your operator in the std namespace, which works, but is not recommended, or define p_int_t as a class, as in:
#include <algorithm>
#include <iostream>
#include <utility>
#include <vector>
using namespace std;
struct p_int_t : pair<int, int> {
using pair<int, int>::pair; // for c++=11 and later
p_int_t() : pair() {} // for c++98
p_int_t(int x, int y) : pair(x, y) {} // for c++98
friend bool operator==(const p_int_t& p1, const p_int_t& p2) {
return p1.first == p2.first;
}
};
int main() {
vector<p_int_t> v;
v.push_back({1, 2});
v.push_back({1, 3});
p_int_t p(1, 4);
cout << (v[0] == p) << endl;
cout << (v[1] == p) << endl;
cout << (find(v.begin(), v.end(), p) != v.end()) << endl;
return 0;
}
Code can be found here: https://godbolt.org/z/5dfPaaoMz
Having to redefine constructors is quite cumbersome, but you can also use std::find_if(), as in:
#include <algorithm>
#include <iostream>
#include <utility>
#include <vector>
using namespace std;
typedef pair<int, int> p_int_t;
struct compare_first {
p_int_t p;
compare_first(p_int_t x) : p(x) {}
bool operator()(const p_int_t& x) { return x.first == p.first; }
};
int main() {
vector<p_int_t> v;
v.push_back({1, 2});
v.push_back({1, 3});
p_int_t p(1, 4);
cout << (find_if(v.begin(), v.end(), compare_first(p)) != v.end()) << endl;
// or for c++11 or later...
cout << (find_if(v.begin(), v.end(), [&p](const p_int_t& x) { return p.first == x.first; }) != v.end()) << endl;
return 0;
}
Code here: https://godbolt.org/z/r87hdrrK9