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

 

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std::set

Member functions

set::set set::~set            set::operator= set::get_allocator

Iterators set::beginset::cbegin (C++11) set::endset::cend (C++11) set::rbeginset::crbegin (C++11)   set::rendset::crend (C++11) Capacity set::size set::max_size set::empty Observers set::key_comp set::value_comp Modifiers set::clear set::erase set::swap set::extract (C++17) set::merge (C++17) set::insert set::insert_range (C++23) set::emplace (C++11) set::emplace_hint (C++11) Lookup set::count set::find set::contains (C++20)    set::equal_range set::lower_bound set::upper_bound

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operator==operator<=> (C++20) std::swap(std::set) erase_if(std::set) (C++20)   operator!=operator<operator>operator<=operator>= (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)

Deduction guides(C++17)

 

std::pair<iterator, iterator> equal_range( const Key& key );	(1)
std::pair<const_iterator, const_iterator> equal_range( const Key& key ) const;	(2)
template< class K > std::pair<iterator, iterator> equal_range( const K& x );	(3)	(since C++14)
template< class K > std::pair<const_iterator, const_iterator> equal_range( const K& x ) const;	(4)	(since C++14)

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 key and another pointing to the first element greater than 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 the qualified-id Compare::is_transparent is valid and denotes a type. It allows calling this function without constructing an instance of Key.

Parameters

key	-	key value to compare the elements to
x	-	alternative value that can be compared to Key

Return value

std::pair containing a pair of iterators defining the wanted range: the first pointing to the first element that is not less than key and the second pointing to the first element greater than key.

If there are no elements not less than key, past-the-end (see end()) iterator is returned as the first element. Similarly if there are no elements greater than key, past-the-end iterator is returned as the second element.

Complexity

Logarithmic in the size of the container.

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_generic_associative_lookup	201304L	(C++14)	Heterogeneous comparison lookup in associative containers, for overloads (3,4)

Example

Run this code

#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

See also

find	finds element with specific key (public member function)
contains (C++20)	checks if the container contains element with specific key (public member function)
count	returns the number of elements matching specific key (public member function)
upper_bound	returns an iterator to the first element greater than the given key (public member function)
lower_bound	returns an iterator to the first element not less than the given key (public member function)
equal_range	returns range of elements matching a specific key (function template)