std::remove_copy,std::remove_copy_if(3)

NAME
std::remove_copy,std::remove_copy_if - std::remove_copy,std::remove_copy_if

Synopsis
   Defined in header <algorithm>
   template< class InputIt, class OutputIt,
   class T >
                        (constexpr since
   OutputIt remove_copy( InputIt first, C++20)
   InputIt last, (until C++26)
                        OutputIt d_first,
   const T& value );
   template< class InputIt, class OutputIt,
             class T = typename
   std::iterator_traits
   <InputIt>::value_type > (since C++26)
   constexpr OutputIt remove_copy( InputIt
   first, InputIt last,
                        OutputIt
   d_first, const T& value );
   template< class ExecutionPolicy,
             class ForwardIt1, class
   ForwardIt2, class T >
   ForwardIt2 remove_copy( ExecutionPolicy&& (since C++17)
   policy, (until C++26)
                        ForwardIt1 first,
   ForwardIt1 last,
                        ForwardIt2
   d_first, const T& value );
   template< class ExecutionPolicy,
                        (1)
             class ForwardIt1, class
   ForwardIt2,
             class T = typename
   std::iterator_traits
   <ForwardIt1>::value_type > (since C++26)
   ForwardIt2 remove_copy( ExecutionPolicy&&
   policy,
                        ForwardIt1 first,
   ForwardIt1 last, (2)
                        ForwardIt2
   d_first, const T& value );
   template< class InputIt, class OutputIt,
   class UnaryPred >
   OutputIt remove_copy_if( InputIt first, (3) (constexpr since
   InputIt last, C++20)
                        OutputIt d_first,
   UnaryPred p );
   template< class ExecutionPolicy,
             class ForwardIt1, class
   ForwardIt2, class UnaryPred >
   ForwardIt2 remove_copy_if(
   ExecutionPolicy&& policy, (4) (since C++17)
                        ForwardIt1
   first, ForwardIt1 last,
                        ForwardIt2
   d_first, UnaryPred p );
   Copies elements from the range [first, last), to another range beginning at d_first,
   omitting the elements which satisfy specific criteria.
   1) Ignores all elements that are equal to value (using operator==).
   3) Ignores all elements for which predicate p returns true.
   2,4) Same as (1,3), but executed according to policy.
   These overloads participate in overload resolution only if
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until
                        C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since
                        C++20)
   If
   *d_first = *first is invalid
   (until C++20)
   *first is not writable to d_first
   (since C++20), the program is ill-formed.
   If source and destination ranges overlap, the behavior is undefined.

Parameters
   first, last - the range of elements to copy
   d_first - the beginning of the destination range
   value - the value of the elements not to copy
   policy - the execution policy to use. See execution policy for details.

Type requirements
   -
   InputIt must meet the requirements of LegacyInputIterator.
   -
   OutputIt must meet the requirements of LegacyOutputIterator.
   -
   ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.
   -
   UnaryPred must meet the requirements of Predicate.

Return value
   Iterator to the element past the last element copied.

Complexity
   Given \(\scriptsize N\)N as std::distance(first, last):
   1,2) Exactly \(\scriptsize N\)N comparisons with value using operator==.
   3,4) Exactly \(\scriptsize N\)N applications of the predicate p.
   For the overloads with an ExecutionPolicy, there may be a performance cost if
   ForwardIt1's value_type is not MoveConstructible.

Exceptions
   The overloads with a template parameter named ExecutionPolicy report errors as
   follows:
     * If execution of a function invoked as part of the algorithm throws an exception
       and ExecutionPolicy is one of the standard policies, std::terminate is called.
       For any other ExecutionPolicy, the behavior is implementation-defined.
     * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation
                        remove_copy (1)
   template<class InputIt, class OutputIt,
            class T = typename std::iterator_traits<InputIt>::value_type>
   constexpr OutputIt remove_copy(InputIt first, InputIt last,
                        OutputIt d_first, const T& value)
   {
       for (; first != last; ++first)
           if (!(*first == value))
               *d_first++ = *first;
       return d_first;
   }
                        remove_copy_if (3)
   template<class InputIt, class OutputIt, class UnaryPred>
   constexpr OutputIt remove_copy_if(InputIt first, InputIt last,
                        OutputIt d_first, UnaryPred p)
   {
       for (; first != last; ++first)
           if (!p(*first))
               *d_first++ = *first;
       return d_first;
   }

Notes
             Feature-test macro Value Std Feature
   __cpp_lib_algorithm_default_value_type 202403 (C++26) List-initialization for
                        algorithms (1,2)

Example
// Run this code
 #include <algorithm>
 #include <complex>
 #include <iomanip>
 #include <iostream>
 #include <iterator>
 #include <string>
 #include <vector>
 int main()
 {
     // Erase the hash characters '#' on the fly.
     std::string str = "#Return #Value #Optimization";
     std::cout << "before: " << std::quoted(str) << '\n';
     std::cout << "after: \"";
     std::remove_copy(str.begin(), str.end(),
                      std::ostream_iterator<char>(std::cout), '#');
     std::cout << "\"\n";
     // Erase {1, 3} value on the fly.
     std::vector<std::complex<double>> nums{{2, 2}, {1, 3}, {4, 8}, {1, 3}};
     std::remove_copy(nums.begin(), nums.end(),
                      std::ostream_iterator<std::complex<double>>(std::cout),
     #ifdef __cpp_lib_algorithm_default_value_type
                      {1, 3}); // T gets deduced
     #else
                      std::complex<double>{1, 3});
     #endif
 }

Output:
 before: "#Return #Value #Optimization"
 after: "Return Value Optimization"
 (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
                      T was required to be EqualityComparable, but required *d_first =
   LWG 779 C++98 the value type of ForwardIt is not always T *first
                        to be valid instead

See also
   remove removes elements satisfying specific criteria
   remove_if (function template)
   copy copies a range of elements to a new location
   copy_if (function template)
   (C++11)
   partition_copy copies a range dividing the elements into two groups
   (C++11) (function template)
   ranges::remove_copy copies a range of elements omitting those that satisfy
   ranges::remove_copy_if specific criteria
   (C++20) (niebloid)
   (C++20)