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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.

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; }

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

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