std::set<Key,Compare,Allocator>:: equal_range

#include <set>
#include <functional>
#include <print>
#include <ranges>
#include <string>
#include <string_view>
#include <tuple>

struct Names
{
    std::string forename, surname;
    friend auto operator<(const Names& lhs, const Names& rhs)
    {
        return std::tie(lhs.surname, lhs.forename) < std::tie(rhs.surname, rhs.forename);
    }
};

struct SurnameCompare
{
    std::string_view surname;
    
    friend bool operator<(const Names& lhs, const SurnameCompare& rhs)
    {
        return lhs.surname < rhs.surname;
    }
    
    friend bool operator<(const SurnameCompare& lhs, const Names& rhs)
    {
        return lhs.surname < rhs.surname;
    }
}; 

std::set<Names, std::less<>> characters
{
    {"Homer", "Simpson"},
    {"Marge", "Simpson"},
    {"Lisa", "Simpson"},
    {"Ned", "Flanders"},
    {"Joe", "Quimby"}
};

void print_unique(const Names& names)
{
    auto [begin, end] = characters.equal_range(names);
    std::println(
        "Found {} characters with name \"{} {}\"", 
        std::distance(begin, end), 
        names.forename, names.surname
    );
}

void print_by_surname(std::string_view surname)
{
    auto [begin, end] = characters.equal_range(SurnameCompare{surname});
    std::println("Found {} characters with surname \"{}\":", std::distance(begin, end), surname);
    for (const Names& names : std::ranges::subrange(begin, end))
        std::println("    {} {}", names.forename, names.surname);
}

int main()
{
    print_unique({"Maude", "Flanders"});
    print_unique({"Lisa", "Simpson"});
    print_by_surname("Simpson");
}