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

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std::forward_list

Member functions
forward_list::forward_list forward_list::~forward_list forward_list::operator= forward_list::assign forward_list::assign_range (C++23)   forward_list::get_allocator Element access forward_list::front Iterators forward_list::before_beginforward_list::cbefore_begin forward_list::beginforward_list::cbegin forward_list::endforward_list::cend Capacity forward_list::empty forward_list::max_size	Modifiers forward_list::clear forward_list::emplace_front forward_list::push_front forward_list::insert_after forward_list::emplace_after forward_list::erase_after forward_list::insert_range_after (C++23) forward_list::prepend_range (C++23) forward_list::pop_front forward_list::resize forward_list::swap Operations forward_list::merge forward_list::splice_after forward_list::removeforward_list::remove_if forward_list::reverse forward_list::unique forward_list::sort
Non-member functions
operator==operator<=> (C++20) swap(std::forward_list) erase(std::forward_list)erase_if(std::forward_list) (C++20)(C++20)	operator!=operator<operator>operator<=operator>= (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
Deduction guides (C++17)

 

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)]

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
LWG 4135	C++20	the return type of the lambda predicate used was deduced from predicate arguments	the return type is explicitly specified as bool

See also

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