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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../container/flat_multimap/erase_if.html below:

std::erase_if (std::flat_multimap) - cppreference.com

template< class Key, class T, class Compare, class KeyContainer, class MappedContainer, class Pred > std::flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>::size_type erase_if( std::flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>& c, Pred pred ); (since C++23)
(constexpr since C++26)

Erases all elements that satisfy the predicate pred from c.

The predicate pred is satisfied if the expression bool(pred(std::pair<const Key&, const T&>(e))) is true, where e is some element in c.

If Key or T is not MoveAssignable, the behavior is undefined.

[edit] Parameters c - container adaptor from which to erase pred - predicate that returns true if the element should be erased [edit] Return value

The number of erased elements.

[edit] Complexity

Exactly c.size() applications of the predicate pred.

Exceptions

If erase_if throws, c remains in valid but unspecified (maybe empty) state.

Notes

The algorithm is stable, that is, the order of elements that are not deleted remains unchanged.

[edit] Example

#include <iostream>
#include <flat_map>
 
void println(auto rem, const auto& 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::flat_multimap<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)
    {
        const auto& [key, value] = item;
        return (key & 1) == 1;
    });
 
    println("Erase items with odd keys:\n", data);
    std::cout << count << " items removed.\n";
}

Output:

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

[edit] See also

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