std::erase,std::erase_if(std::forward_list)(3)

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

Synopsis
   Defined in header <forward_list>
   template< class T, class Alloc, class U >
                        (since C++20)
   std::forward_list<T, Alloc>::size_type (until C++26)
       erase( std::forward_list<T, Alloc>& c, const U& value );
   template< class T, class Alloc, class U = T >
   std::forward_list<T, Alloc>::size_type (1) (since C++26)
       erase( std::forward_list<T, Alloc>& c, const U& value );
   template< class T, class Alloc, class Pred >
   std::forward_list<T, Alloc>::size_type (2) (since C++20)
       erase_if( std::forward_list<T, Alloc>& c, Pred pred );
   1) Erases all elements that compare equal to value from the container. Equivalent to
   return c.remove_if([&](auto& elem) { 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
           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
   pred - 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 << "{ ";
     for (const auto& x : c)
         std::cout << 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) }

See also
   remove removes elements satisfying specific criteria
   remove_if (function template)
   remove removes elements satisfying specific criteria
   remove_if (public member function)