std::erase_...ordered_map)(3)

💖 std::erase_if(std::unordered_map) — C++ Standard Libary manual 💖

bestie, you're on the std::erase_...ordered_map)(3) page and we're about to make erasing from an unordered_map feel iconic. no cap.

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💖 NAME 💖

std::erase_if(std::unordered_map) — literally std::erase_if(std::unordered_map). it's the erase-if bestie for unordered_map, and it's here to serve. 💅

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✨ Synopsis ✨

Defined in header <unordered_map>.

template< class Key, class T, class Hash, class KeyEqual, class Alloc,
          class Pred >
std::unordered_map<Key, T, Hash, KeyEqual, Alloc>::size_type
    erase_if( std::unordered_map<Key, T, Hash, KeyEqual, Alloc>& c,
              Pred pred );

(since C++20) — yes bestie, this one is from C++20. ✨

What it does: Erases all elements that satisfy the predicate pred from c. all the matching ones, gone. 🫶

Equivalent to:

auto old_size = c.size();
for (auto first = c.begin(), last = c.end(); first != last;)
{
    if (pred(*first))
        first = c.erase(first);
    else
        ++first;
}
return old_size - c.size();

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🎀 Parameters 🎀

• c — container from which to erase.
• pred — predicate that returns true if the element should be erased.

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🫶 Return value 🫶

The number of erased elements. it literally hands you back that count, bestie. 💖

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⚡ Complexity ⚡

Linear. that's the whole vibe. ✨

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🌈 Example 🌈

the page says // Run this code, so here's the code exactly:

#include <iostream>
#include <unordered_map>

void println(auto rem, auto const& container)
{
    std::cout << rem << '{';
    for (char sep[]{0, ' ', 0}; const auto& [key, value] : container)
        std::cout << sep << '{' << key << ", " << value << '}', *sep =
       ',';
    std::cout << "}\n";
}

int main()
{
    std::unordered_map<int, char> data
    {
        {1, 'a'}, {2, 'b'}, {3, 'c'}, {4, 'd'},
        {5, 'e'}, {4, 'f'}, {5, 'g'}, {5, 'g'},
    };
    println("Original:\n", data);

    const auto count = std::erase_if(data, [](const auto& item)
    {
        auto const& [key, value] = item;
        return (key & 1) == 1;
    });

    println("Erase items with odd keys:\n", data);
    std::cout << count << " items removed.\n";
}

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🌸 Possible output 🌸

Original:
{{5, e}, {4, d}, {3, c}, {2, b}, {1, a}}
Erase items with odd keys:
{{4, d}, {2, b}}
3 items removed.

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👀 See also 👀

• remove — removes elements satisfying specific criteria
• remove_if (function template)

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💖 source/version/date check: this page is from http://cppreference.com, dated 2024.06.10, and the footer page is std::erase_...ordered_map)(3) — okay bye, love u, go erase something with a predicate! 💅🫶