std::remove_copy, std::remove_copy_if

< cpp‎ | algorithm
Defined in header <algorithm>
(1)
template< class InputIt, class OutputIt, class T >

OutputIt remove_copy( InputIt first, InputIt last,

                      OutputIt d_first, const T& value );
 (until C++20)
template< class InputIt, class OutputIt, class T >

constexpr OutputIt remove_copy( InputIt first, InputIt last,

                                OutputIt d_first, const T& value );
 (since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class T >

ForwardIt2 remove_copy( ExecutionPolicy&& policy,
                        ForwardIt1 first, ForwardIt1 last,

                        ForwardIt2 d_first, const T& value );
 (2) (since C++17)
(3)
template< class InputIt, class OutputIt, class UnaryPredicate >

OutputIt remove_copy_if( InputIt first, InputIt last,

                         OutputIt d_first, UnaryPredicate p );
 (until C++20)
template< class InputIt, class OutputIt, class UnaryPredicate >

constexpr OutputIt remove_copy_if( InputIt first, InputIt last,

                                   OutputIt d_first, UnaryPredicate p );
 (since C++20)
template< class ExecutionPolicy, class ForwardIt1,

          class ForwardIt2, class UnaryPredicate >
ForwardIt2 remove_copy_if( ExecutionPolicy&& policy,
                           ForwardIt1 first, ForwardIt1 last,

                           ForwardIt2 d_first, UnaryPredicate p );
 (4) (since C++17)

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.
3) Ignores all elements for which predicate p returns true.
2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless

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 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.
-
UnaryPredicate must meet the requirements of Predicate.

The expression *d_first = *first must be valid.

 (until C++20)

*first must be writable to d_first.

 (since C++20)

Return value

Iterator to the element past the last element copied.

Complexity

Given N as std::distance(first, last):

1,2) exactly N comparisons with value using operator==
3,4) exactly 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 
template<class InputIt, class OutputIt, class T>
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 
template<class InputIt, class OutputIt, class UnaryPredicate>
OutputIt remove_copy_if(InputIt first, InputIt last, OutputIt d_first, UnaryPredicate p)
{
    for (; first != last; ++first)
        if (!p(*first))
            *d_first++ = *first;
    return d_first;
}

Example

The following code outputs a string while erasing the hash characters '#' on the fly.

#include <algorithm>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <string>
 
int main()
{
    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";
}

Output:

before: "#Return #Value #Optimization"
after:  "Return Value Optimization"

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 779 C++98 T was required to be EqualityComparable, but
the value type of ForwardIt is not always T		 required *d_first = *first
to be valid instead

See also

 removes elements satisfying specific criteria
(function template)
(C++11)
 copies a range of elements to a new location
(function template)
(C++11)
 copies a range dividing the elements into two groups
(function template)
(C++20)(C++20)
 copies a range of elements omitting those that satisfy specific criteria
(niebloid)