std::replace_copy, std::replace_copy_if

Defined in header `<algorithm>`
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 );	(1)	(constexpr since C++20)
template< class InputIt, class OutputIt, class UnaryPred, class T > OutputIt replace_copy_if ( InputIt first, InputIt last, OutputIt d_first, UnaryPred p, const T& new_value );	(2)	(constexpr since C++20) (until C++26)
template< class InputIt, class OutputIt, class UnaryPred, class T = typename std::iterator_traits <OutputIt>::value_type > constexpr OutputIt replace_copy_if ( InputIt first, InputIt last, OutputIt d_first, UnaryPred p, const T& new_value );	(2)		(since C++26)
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 );	(3)	(since C++17)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class UnaryPred, class T > ForwardIt2 replace_copy_if ( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, UnaryPred p, const T& new_value );	(4)	(since C++17) (until C++26)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class UnaryPred, class T = typename std::iterator_traits <ForwardIt2>::value_type > ForwardIt2 replace_copy_if ( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, UnaryPred p, const T& new_value );	(4)		(since C++26)

Copies the elements from the source range [first, last) to the destination range d_first, while replacing all elements satisfying specific criteria with new_value.

1) replace_copy replaces all elements that are equal to old_value (using operator==).

2) replace_copy_if replaces all elements for which predicate p returns true.

3,4) Same as (1,2), but executed according to policy.

These overloads participate in overload resolution only if the value of the following expression is true:

`std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>`	(until C++20)
`std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>`	(since C++20)

If any of the results of the expressions *first and new_value is not writable to d_first, the program is ill-formed.

If the source and destination ranges overlap, the behavior is undefined.

Parameters

first, last	-	the pair of iterators defining the source range
d_first	-	the beginning of the destination range
old_value	-	the value of elements to replace
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
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.

Return value

The past-the-end iterator of the destination range.

Complexity

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

1) Exactly \(\scriptsize N\)N comparisons using operator==.

2) Exactly \(\scriptsize N\)N applications of the predicate p.

3) \(\scriptsize \mathcal{O}(N)\)𝓞(N) comparisons using operator==.

4) \(\scriptsize \mathcal{O}(N)\)𝓞(N) applications of the predicate p.

Exceptions

3,4) During the execution process:

If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_algorithm_default_value_type`	`202403`	(C++26)	List-initialization for algorithms (3,4)

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 UnaryPred, class T = typename std::iterator_traits<ForwardIt>::value_type> OutputIt replace_copy_if(InputIt first, InputIt last, OutputIt d_first, UnaryPred p, const T& new_value) { for (; first != last; ++first) d_first++ = p(first) ? new_value : *first; return d_first; }

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 UnaryPred,
         class T = typename std::iterator_traits<ForwardIt>::value_type>
OutputIt replace_copy_if(InputIt first, InputIt last, OutputIt d_first,
                         UnaryPred p, const T& new_value)
{
    for (; first != last; ++first)
        *d_first++ = p(*first) ? new_value : *first;
    return d_first;
}

Example

#include <algorithm>
#include <complex>
#include <iostream>
#include <vector>

void println(const auto& seq)
{
    for (const auto& e : seq)
        std::cout << e << ' ';
    std::cout << '\n';
}
 
int main()
{
    std::vector<short> src{3, 1, 4, 1, 5, 9, 2, 6, 5};
    println(src);
    std::vector<int> dst(src.size());
    std::replace_copy_if(src.cbegin(), src.cend(),
                         dst.begin(),
                         [](short n){ return n > 5; }, 0);
    println(dst);
    
    std::vector<std::complex<double>> src2{{1, 3}, {2, 4}, {3, 5}},
                                      dst2(src2.size());
    println(src2);
    #ifdef __cpp_lib_algorithm_default_value_type
        std::replace_copy_if(src2.cbegin(), src2.cend(), dst2.begin(),
            [](std::complex<double> z){ return std::abs(z) < 5; },
            {4, 2}); // Possible, since the T is deduced.
    #else
        std::replace_copy_if(src2.cbegin(), src2.cend(), dst2.begin(),
            [](std::complex<double> z){ return std::abs(z) < 5; },
            std::complex<double>{4, 2});
    #endif
    println(dst2);
}

Output:

3 1 4 1 5 9 2 6 5 
3 1 4 1 5 0 2 0 5 
(1,3) (2,4) (3,5) 
(4,2) (4,2) (3,5)

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 algorithms library, the template type parameters whose name ends with Iterator imply the type requirements of the corresponding iterator categories.

ranges::replace_copyranges::replace_copy_if (C++20)(C++20)	copies a range, replacing elements satisfying specific criteria with another value (algorithm function object)
replacereplace_if	replaces all values satisfying specific criteria with another value (function template & algorithm function object)
ranges::replaceranges::replace_if (C++20)(C++20)
removeremove_if	removes elements satisfying specific criteria (function template & algorithm function object)
ranges::removeranges::remove_if (C++20)(C++20)