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

Returns a range containing all elements with the given key in the container. The range is defined by two iterators, one pointing to the first element that is not less than the given key and another pointing to the first element greater than the given key.

Alternatively, the first iterator may be obtained with lower_bound() , and the second with upper_bound() .

1,2) Compares the keys to key.

3,4) Compares the keys to the value x.

This overload participates in overload resolution only if Compare is transparent. It allows calling this function without constructing an instance of Key.

#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::print
 (
 "Found {} characters with name \"{} {}\"\n", 
 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::print ("Found {} characters with surname \"{}\":\n", std::distance (begin, end), surname);
 for (const Names& names : std::ranges::subrange (begin, end))
 std::print (" {} {}\n", names.forename, names.surname);
}
 
int main()
{
 print_unique({"Maude", "Flanders"});
 print_unique({"Lisa", "Simpson"});
 print_by_surname("Simpson");
}

Output:

Found 0 characters with name "Maude Flanders"
Found 1 characters with name "Lisa Simpson"
Found 3 characters with surname "Simpson":
 Homer Simpson
 Lisa Simpson
 Marge Simpson