std:: erase, std:: erase_if (std::forward_list)

From cppreference.com

Defined in header `<forward_list>`
	(1)
template < class T, class Alloc, class U > std:: forward_list < T, Alloc > :: size_type erase ( std:: forward_list < T, Alloc > & c, const U & value ) ;			(since C++20) (until C++26)
template < class T, class Alloc, class U = T > std:: forward_list < T, Alloc > :: size_type erase ( std:: forward_list < T, Alloc > & c, const U & value ) ;			(since C++26)
template < class T, class Alloc, class Pred > std:: forward_list < T, Alloc > :: size_type erase_if ( std:: forward_list < T, Alloc > & c, Pred pred ) ;		(2)	(since C++20)

1) Erases all elements that compare equal to value from the container. Equivalent to return c. remove_if ( [ & ] ( const auto & elem ) - > bool { return elem == value ; } ) ; .

2) Erases all elements that satisfy the predicate pred from the container. Equivalent to return c. remove_if ( pred ) ; .

Parameters

c	-	container from which to erase
value	-	value to be removed
pred	-	unary predicate which returns true if the element should be erased. The expression pred ( v ) must be convertible to bool for every argument `v` of type (possibly const) `T` , regardless of value category , and must not modify `v` . Thus, a parameter type of T & is not allowed , nor is T unless for `T` a move is equivalent to a copy (since C++11) .

Return value

The number of erased elements.

Complexity

Linear.

Notes

Unlike std::forward_list::remove , erase accepts heterogeneous types and does not force a conversion to the container's value type before invoking the == operator.

Feature-test macro	Value	Std	Feature
`__cpp_lib_algorithm_default_value_type`	202403	(C++26)	List-initialization for algorithm ( 1 )

Example

Run this code

#include <complex>
#include <iostream>
#include <numeric>
#include <string_view>
#include <forward_list>
 
void println(std::string_view comment, const auto& c)
{
    std::cout << comment << '[';
    bool first{true};
    for (const auto& x : c)
        std::cout << (first ? first = false, "" : ", ") << x;
    std::cout << "]\n";
}
 
int main()
{
    std::forward_list<char> cnt(10);
    std::iota(cnt.begin(), cnt.end(), '0');
    println("Initially, cnt = ", cnt);
 
    std::erase(cnt, '3');
    println("After erase '3', cnt = ", cnt);
 
    auto erased = std::erase_if(cnt, [](char x) { return (x - '0') % 2 == 0; });
    println("After erase all even numbers, cnt = ", cnt);
    std::cout << "Erased even numbers: " << erased << '\n';
 
    std::forward_list<std::complex<double>> nums{{2, 2}, {4, 2}, {4, 8}, {4, 2}};
    #ifdef __cpp_lib_algorithm_default_value_type
        std::erase(nums, {4, 2});
    #else
        std::erase(nums, std::complex<double>{4, 2});
    #endif
    println("After erase {4, 2}, nums = ", nums);
}

Output:

Initially, cnt = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
After erase '3', cnt = [0, 1, 2, 4, 5, 6, 7, 8, 9]
After erase all even numbers, cnt = [1, 5, 7, 9]
Erased even numbers: 5
After erase {4, 2}, nums = [(2,2), (4,8)]

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remove remove_if	removes elements satisfying specific criteria (function template)
ranges::remove ranges::remove_if (C++20) (C++20)	removes elements satisfying specific criteria (algorithm function object)
remove remove_if	removes elements satisfying specific criteria (public member function)