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std::remove_copy, std::remove_copy_if

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. Source and destination ranges cannot overlap.

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>> (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> (since C++20) is true.

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.

Return value

Iterator to the element past the last element copied.

Complexity

Exactly std::distance(first, last) applications of the predicate.

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

First version
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;
}
Second version
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 <iterator>
#include <string>
#include <iostream>
#include <iomanip>
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"

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)

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