std::map::equal_range(3)

💅 std::map::equal_range(3) 💅

okay so welcome to the std::map::equal_range(3) page from the C++ Standard Libary manual (yes bestie, “Libary” — we respect the typo 💀). This is the one where we pull out a whole lil range of matching keys, and honestly? it’s giving useful.

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🌸 NAME 🌸

std::map::equal_range — std::map::equal_range

(yes, the name literally mirrors itself. iconic behavior.)

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✨ Synopsis ✨

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 points to the first element that is not less than key, and the other points to the first element greater than key. Alternatively, you can get the first iterator 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 only participates in overload resolution if the qualified-id Compare::is_transparent is valid and denotes a type. It lets you call this function without constructing an instance of Key.

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🎀 Parameters 🎀

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

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💖 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.

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🔥 Complexity 🔥

Logarithmic in the size of the container.

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🦋 Notes 🦋

Feature-test macro vibes:

• __cpp_lib_generic_associative_lookup — value 201304L, std (C++14), feature: heterogeneous comparison lookup in associative containers, for overloads (3,4)

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🌈 Example 🌈

Run this code, bestie (the original literally says // Run this code, so go off):

#include <iostream>
#include <map>

int main()
{
    const std::map<int, const char*> m
    {
        {0, "zero"},
        {1, "one"},
        {2, "two"}
    };

    auto p = m.equal_range(1);
    for (auto& q = p.first; q != p.second; ++q)
        std::cout << "m[" << q->first << "] = " << q->second << '\n';

    if (p.second == m.find(2))
        std::cout << "end of equal_range (p.second) is one-past p.first\n";
    else
        std::cout << "unexpected; p.second expected to be one-past p.first\n";

    auto pp = m.equal_range(-1);
    if (pp.first == m.begin())
        std::cout << "pp.first is iterator to first not-less than -1\n";
    else
        std::cout << "unexpected pp.first\n";

    if (pp.second == m.begin())
        std::cout << "pp.second is iterator to first element greater-than -1\n";
    else
        std::cout << "unexpected pp.second\n";

    auto ppp = m.equal_range(3);
    if (ppp.first == m.end())
        std::cout << "ppp.first is iterator to first not-less than 3\n";
    else
        std::cout << "unexpected ppp.first\n";

    if (ppp.second == m.end())
        std::cout << "ppp.second is iterator to first element greater-than 3\n";
    else
        std::cout << "unexpected ppp.second\n";
}

Output:

m[1] = one
end of equal_range (p.second) is one-past p.first
pp.first is iterator to first not-less than -1
pp.second is iterator to first element greater-than -1
ppp.first is iterator to first not-less than 3
ppp.second is iterator to first element greater-than 3

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👑 See also 👑

• find — finds element with specific key — (public member function)
• contains — checks if the container contains element with specific key — (C++20) (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)

kisses, this page is from http://cppreference.com, dated 2024.06.10, and it’s the std::map::equal_range(3) page 🫶