# std::remove_copy, std::remove_copy_if

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< 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 ); |  | (constexpr since C++20) (until C++26)
template< class InputIt, class OutputIt, class T = typename std::iterator_traits <InputIt>::value_type > constexpr OutputIt remove_copy( InputIt first, InputIt last, OutputIt d_first, const T& value ); |  | (since C++26)
| (2) |
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class T > ForwardIt2 remove_copy( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, const T& value ); |  | (since C++17) (until C++26)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class T = typename std::iterator_traits <ForwardIt1>::value_type > ForwardIt2 remove_copy( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, const T& value ); |  | (since C++26)
template< class InputIt, class OutputIt, class UnaryPred > OutputIt remove_copy_if( InputIt first, InputIt last, OutputIt d_first, UnaryPred p ); | (3) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class UnaryPred > ForwardIt2 remove_copy_if( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, UnaryPred 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 (using operator==).
3) Ignores all elements for which predicate p returns true.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if all following conditions are satisfied:

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 *d_first = *first is invalid(until C++20)*first is not writable to d_first(since C++20), the program is ill-formed.

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
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `OutputIt` must meet the requirements of LegacyOutputIterator.
- `ForwardIt1, ForwardIt2` must meet the requirements of LegacyForwardIterator.
- `UnaryPred` must meet the requirements of Predicate.

### Return value

Iterator to the element past the last element copied.

### Complexity

Given \(\scriptsize N\)N as std::distance (first, last):

1,2) Exactly \(\scriptsize N\)N comparisons with value using operator==.
3,4) Exactly \(\scriptsize N\)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 (1)

    template<class InputIt, class OutputIt,
             class T = typename std::iterator_traits<InputIt>::value_type>
    constexpr 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 (3)

    template<class InputIt, class OutputIt, class UnaryPred>
    constexpr OutputIt remove_copy_if(InputIt first, InputIt last,
                                      OutputIt d_first, UnaryPred p)
    {
        for (; first != last; ++first)
            if (!p(*first))
                *d_first++ = *first;
        return d_first;
    }

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_algorithm_default_value_type` | `202403` | (C++26) | List-initialization for algorithms (1,2)

### Example

Run this code

    #include <algorithm>
    #include <complex>
    #include <iomanip>
    #include <iostream>
    #include <iterator>
    #include <string>
    #include <vector>

    int main()
    {
        // Erase the hash characters '#' on the fly.
        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";

        // Erase {1, 3} value on the fly.
        std::vector<std::complex<double>> nums{{2, 2}, {1, 3}, {4, 8}, {1, 3}};
        std::remove_copy(nums.begin(), nums.end(),
                         std::ostream_iterator<std::complex<double>>(std::cout),
        #ifdef __cpp_lib_algorithm_default_value_type
                         {1, 3}); // T gets deduced
        #else
                         std::complex<double>{1, 3});
        #endif
    }

Output:

    before: "#Return #Value #Optimization"
    after:  "Return Value Optimization"
    (2,2)(4,8)

### 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

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 (algorithm function object)

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