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# std::remove_copy, std::remove_copy_if

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt, class OutputIt, class T > OutputIt remove_copy( InputIt first, InputIt last, OutputIt d_first, const T& value ); | (1) |
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)
template< class InputIt, class OutputIt, class UnaryPredicate > OutputIt remove_copy_if( InputIt first, InputIt last, OutputIt d_first, UnaryPredicate p ); | (3) |
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

### [edit] 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 `InputIterator`.
- `OutputIt` must meet the requirements of `OutputIterator`.
- `ForwardIt1, ForwardIt2` must meet the requirements of `ForwardIterator`.
- `UnaryPredicate` must meet the requirements of `Predicate`.

### [edit] Return value

Iterator to the element past the last element copied.

### [edit] 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`.

### [edit] 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 three 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.

### [edit] 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;
    }

### [edit] Example

The following code outputs a string while erasing the spaces on the fly.

Run this code

    #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

### [edit] See also

remove remove_if | removes elements satisfying specific criteria (function template) [edit]
copy copy_if (C++11) | copies a range of elements to a new location (function template) [edit]

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