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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. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> 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 last - first 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; }

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 spaces on the fly.

#include <algorithm>
#include <iterator>
#include <string>
#include <iostream>
int main()
{
    std::string str = "Text with some   spaces";
    std::cout << "before: " << str << "\n";
 
    std::cout << "after:  ";
    std::remove_copy(str.begin(), str.end(),
                     std::ostream_iterator<char>(std::cout), ' ');
    std::cout << '\n';
}

Output:

before: Text with some   spaces
after:  Textwithsomespaces

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)