# std::remove_copy, std::remove_copy_if

From cppreference.com
< 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.

Run this code

    #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

remove remove_if | removes elements satisfying specific criteria (function template)
copy copy_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_copy ranges::remove_copy_if (C++20)(C++20) | copies a range of elements omitting those that satisfy specific criteria (niebloid)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/algorithm/remove_copy&oldid=149984"