std::replace_copy, std::replace_copy_if

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)

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.

[edit] Return value

Iterator to the element past the last element copied.

[edit] 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

[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 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

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;
}

[edit] Example

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

Run this code

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

[edit] 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

↑ 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.

[edit] See also

replacereplace_if	replaces all values satisfying specific criteria with another value (function template) [edit]
removeremove_if	removes elements satisfying specific criteria (function template) [edit]
ranges::replace_copyranges::replace_copy_if (C++20)(C++20)	copies a range, replacing elements satisfying specific criteria with another value (niebloid) [edit]

[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.

[1]

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std::replace_copy, std::replace_copy_if

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[edit] Parameters