# std::replace_copy, std::replace_copy_if

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class InputIt, class OutputIt, class T > OutputIt replace_copy( InputIt first, InputIt last, OutputIt d_first, const T& old_value, const T& new_value ); |  | (until C++20)
template< class InputIt, class OutputIt, class T > constexpr OutputIt replace_copy( InputIt first, InputIt last, OutputIt d_first, const T& old_value, const T& new_value ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class T > ForwardIt2 replace_copy( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, const T& old_value, const T& new_value ); | (2) | (since C++17)
| (3) |
template< class InputIt, class OutputIt, class UnaryPredicate, class T > OutputIt replace_copy_if( InputIt first, InputIt last, OutputIt d_first, UnaryPredicate p, const T& new_value ); |  | (until C++20)
template< class InputIt, class OutputIt, class UnaryPredicate, class T > constexpr OutputIt replace_copy_if( InputIt first, InputIt last, OutputIt d_first, UnaryPredicate p, const T& new_value ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class UnaryPredicate, class T > ForwardIt2 replace_copy_if( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, UnaryPredicate p, const T& new_value ); | (4) | (since C++17)

Copies the elements from the range `[`first`,` last`)` to another range beginning at d_first, while replacing all elements satisfying specific criteria with new_value. If the source and destination ranges overlap, the behavior is undefined.

1) Replaces all elements that are equal to old_value (using `operator==`).
3) Replaces 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)

### Parameters

first, last | - | the range of elements to copy
d_first | - | the beginning of the destination range
old_value | - | the value of elements to replace
policy | - | the execution policy to use. See execution policy for details.
p | - | unary predicate which returns ​true if the element value should be replaced. The expression p(v) must be convertible to bool for every argument `v` of type (possibly const) `VT`, where `VT` is the value type of `InputIt`, regardless of value category, and must not modify `v`. Thus, a parameter type of VT&is not allowed, nor is VT unless for `VT` a move is equivalent to a copy (since C++11). ​
new_value | - | the value to use as replacement
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `OutputIt` must meet the requirements of LegacyOutputIterator.
- `ForwardIt1, ForwardIt2` must meet the requirements of LegacyForwardIterator.
- The results of the expressions *first and new_value must be writable to d_first.

### 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 old_value using `operator==`
3,4) exactly `N` applications of the predicate p

### 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

replace_copy
    template<class InputIt, class OutputIt, class T>
    OutputIt replace_copy(InputIt first, InputIt last, OutputIt d_first,
                          const T& old_value, const T& new_value)
    {
        for (; first != last; ++first)
            *d_first++ = (*first == old_value) ? new_value : *first;
        return d_first;
    }
replace_copy_if
    template<class InputIt, class OutputIt,
             class UnaryPredicate, class T>
    OutputIt replace_copy_if(InputIt first, InputIt last, OutputIt d_first,
                             UnaryPredicate p, const T& new_value)
    {
        for (; first != last; ++first)
            *d_first++ = p(*first) ? new_value : *first;
        return d_first;
    }

### Example

The following copy-prints a vector, replacing all values over 5 with 99 on the fly.

Run this code

    #include <algorithm>
    #include <iostream>
    #include <iterator>
    #include <vector>

    int main()
    {
        std::vector<int> v {5, 7, 4, 2, 8, 6, 1, 9, 0, 3};
        std::replace_copy_if(v.begin(), v.end(),
                             std::ostream_iterator<int>(std::cout, " "),
                             [](int n){ return n > 5; }, 99);
        std::cout << '\n';
    }

Output:

    5 99 4 2 99 99 1 99 0 3

### 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 283 | C++98 | `T` was required to be CopyAssignable (and EqualityComparable for `replace_copy`), but the value type of `InputIt` is not always `T` | removed the requirement
LWG 337 | C++98 | `replace_copy_if` only required `InputIt` to meet the requirements of LegacyIterator[1] | corrected to LegacyInputIterator

1. ↑ The actual defect in the C++ standard is that the template parameter `InputIterator` was misspecified as `Iterator`. This affects the type requirements because the C++ standard states that for the function templates in the alrogithms library, the template type parameters whose name ends with `Iterator` imply the type requirements of the corresponding iterator categories.

### See also

replace replace_if | replaces all values satisfying specific criteria with another value (function template)
remove remove_if | removes elements satisfying specific criteria (function template)
ranges::replace_copy ranges::replace_copy_if (C++20)(C++20) | copies a range, replacing elements satisfying specific criteria with another value (niebloid)

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