Question

In the following snippet of code, I’ve overloaded the operator== to compare my pair type with string. But for some reason, the compiler isn’t finding my operator as a match for the find function. Why not?

Edit: Thanks for all the suggestions for alternatives, but I’d still like to understand why. The code looks like it should work; I’d like to know why it doesn’t.

#include <vector>
#include <utility>
#include <string>
#include <algorithm>

typedef std::pair<std::string, int> RegPair;
typedef std::vector<RegPair> RegPairSeq;

bool operator== (const RegPair& lhs, const std::string& rhs)
{
    return lhs.first == rhs;
}

int main()
{
    RegPairSeq sequence;
    std::string foo("foo");
    // stuff that's not important
    std::find(sequence.begin(), sequence.end(), foo);
    // g++: error: no match for 'operator==' in '__first. __gnu_cxx::__normal_iterator<_Iterator, _Container>::operator* [with _Iterator = std::pair<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int>*, _Container = std::vector<std::pair<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int>, std::allocator<std::pair<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int> > >]() == __val'
    // clang++: error: invalid operands to binary expression ('std::pair<std::basic_string<char>, int>' and 'std::basic_string<char> const')
}

Answer 1

The problem is that std::find is a function template and it uses argument-dependent lookup (ADL) to find the right operator== to use.

Both of the arguments are in the std namespace (std::pair<std::string, int> and std::string), so ADL starts by looking in the std namespace. There it finds some operator== (which one, it doesn’t matter; there are lots in the Standard Library and if you’ve included <string>, at least the one that compares two std::basic_string<T> objects could be found).

Because an operator== overload is found in the std namespace, ADL stops searching enclosing scopes. Your overload, which is located in the global namespace, is never found. Name lookup occurs before overload resolution; it doesn’t matter during name lookup whether the arguments match.