std::unorde...iset::erase(3)

🌸 NAME 🌸

This is the std::unorde...iset::erase(3) page from the C++ Standard Libary — yes, that spelling is literally in the manual name and we are not fixing it, periodt 💅.

std::unordered_multiset::erase — std::unordered_multiset::erase

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✨ Synopsis ✨

okay bestie, here’s the overload lineup exactly as the page lays it out:

       iterator erase( iterator pos );
       (since C++11)
                                                                               (until
       C++23)
          iterator erase( iterator pos )
       (since C++23)
              requires(!std::same_as<iterator, const_iterator>);
          iterator erase( const_iterator pos );                        (1) (2)
       (since C++11)
          iterator erase( const_iterator first, const_iterator last );     (3)
       (since C++11)
          size_type erase( const Key& key );                               (4)
       (since C++11)
          template< class K >                                              (5)
       (since C++23)
          size_type erase( K&& x );

So what’s the vibe? erase removes specified elements from the container. The order of the remaining elements is preserved. (This makes it possible to erase individual elements while iterating through the container.) ✨

• 1,2) Removes the element at pos. Only one overload is provided if iterator and const_iterator are the same type.
• 3) Removes the elements in the range [first, last), which must be a valid range in *this.
• 4) Removes all elements with the key equivalent to key.
• 5) Removes all elements with key that compares equivalent to the value x. This overload participates in overload resolution only if Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a type, and neither iterator nor const_iterator is implicitly convertible from K. This assumes that such Hash is callable with both K and Key type, and that the KeyEqual is transparent, which, together, allows calling this function without constructing an instance of Key.

References and iterators to the erased elements are invalidated. Other iterators and references are not invalidated.

The iterator pos must be valid and dereferenceable. Thus the end() iterator (which is valid, but is not dereferenceable) cannot be used as a value for pos.

🫶 important little vibe check: pos has to actually point at something dereferenceable, so end() is a no-go for pos.

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🎀 Parameters 🎀

• pos — iterator to the element to remove.
• first, last — range of elements to remove.
• key — key value of the elements to remove.
• x — a value of any type that can be transparently compared with a key denoting the elements to remove.

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💖 Return value 💖

• 1-3) Iterator following the last removed element.
• 4) Number of elements removed.
• 5) Number of elements removed.

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😭 Exceptions 😭

• 1-3) Throws nothing.
• 4,5) Any exceptions thrown by the Hash and KeyEqual object.

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🔥 Complexity 🔥

Given an instance c of unordered_multiset:

• 1,2) Average case: constant, worst case: c.size().
• 3) Average case: std::distance(first, last), worst case: c.size().
• 4) Average case: c.count(key), worst case: c.size().
• 5) Average case: c.count(x), worst case: c.size().

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🧚 Notes 🧚

Feature-test macro time, bestie:

• Feature-test macro: __cpp_lib_associative_heterogeneous_erasure
• Value: 202110L
• Std: (C++23)
• Feature: Heterogeneous erasure in associative containers and unordered associative containers; overload (5)

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🌈 Example 🌈

// Run this code

#include <unordered_set>
#include <iostream>

int main()
{
    std::unordered_multiset<int> c = {1, 2, 3, 4, 1, 2, 3, 4};

    auto print = [&c]
    {
        std::cout << "c = { ";
        for (int n : c)
            std::cout << n << ' ';
        std::cout << "}\n";
    };
    print();

    std::cout << "Erase all odd numbers:\n";
    for (auto it = c.begin(); it != c.end();)
    {
        if (*it % 2 != 0)
            it = c.erase(it);
        else
            ++it;
    }
    print();

    std::cout << "Erase 1, erased count: " << c.erase(1) << '\n';
    std::cout << "Erase 2, erased count: " << c.erase(2) << '\n';
    std::cout << "Erase 2, erased count: " << c.erase(2) << '\n';
    print();
}

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🩹 Defect reports 🩹

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• LWG 2059 — Applied to: C++11. Behavior as published: there was ambiguity for overload (2). Correct behavior: added overload (1).
• LWG 2356 — Applied to: C++11. Behavior as published: the order of non-equivalent elements that are not erased was not guaranteed to be preserved. Correct behavior: required to be preserved.

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👑 See also 👑

• clear — clears the contents (public member function).

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That’s the whole page, bestie — source is http://cppreference.com, version/date is 2024.06.10, and the page title is std::unorde...iset::erase(3) 💜✨