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

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

template< class Key, class Compare, class KeyContainer, class Pred > std::flat_set<Key, Compare, KeyContainer>::size_type erase_if( std::flat_set<Key, Compare, KeyContainer>& 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::as_const(e))) is true, where e is some element in c.

If Key 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_set>
 
void println(auto rem, const auto& container)
{
    std::cout << rem << '{';
    for (char sep[]{0, ' ', 0}; const auto& item : container)
        std::cout << sep << item, *sep = ',';
    std::cout << "}\n";
}
 
int main()
{
    std::flat_set data{3, 3, 4, 5, 5, 6, 6, 7, 2, 1, 0};
    println("Original:\n", data);
 
    auto divisible_by_3 = [](const auto& x) { return (x % 3) == 0; };
 
    const auto count = std::erase_if(data, divisible_by_3);
 
    println("Erase all items divisible by 3:\n", data);
    std::cout << count << " items erased.\n";
}

Output:

Original:
{0, 1, 2, 3, 4, 5, 6, 7}
Erase all items divisible by 3:
{1, 2, 4, 5, 7}
3 items erased.

[edit] See also

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