NAME¶

std::replace_copy,std::replace_copy_if - std::replace_copy,std::replace_copy_if

Synopsis¶

Defined in header <algorithm>
template< class InputIt, class OutputIt,
class T >

OutputIt replace_copy( InputIt first, (1) (constexpr since
InputIt last, OutputIt d_first, C++20)

const T&
old_value, const T& new_value );
template< class ExecutionPolicy,

class ForwardIt1, class
ForwardIt2, class T >
ForwardIt2 replace_copy
( ExecutionPolicy&& policy, (2) (since C++17)
ForwardIt1 first, ForwardIt1 last,
ForwardIt2 d_first,

const T& old_value, const T&
new_value );
template< class InputIt, class OutputIt,
class UnaryPred, class T >
(constexpr
OutputIt replace_copy_if since C++20)
( InputIt first, InputIt last, (until C++26)
OutputIt d_first,

UnaryPred p, const T& new_value );
template< class InputIt, class OutputIt,
class UnaryPred,

class T = typename
std::iterator_traits
(since C++26)
<OutputIt>::value_type >
constexpr OutputIt replace_copy_if
( InputIt first, InputIt last,
OutputIt d_first,

UnaryPred p, const T& new_value );
template< class ExecutionPolicy, class
ForwardIt1, class ForwardIt2, (3)

class UnaryPred, class T > (since
ForwardIt2 replace_copy_if C++17)
( ExecutionPolicy&& policy, (until
ForwardIt1 first, ForwardIt1 last, C++26)
ForwardIt2 d_first,

UnaryPred p, const T& new_value );
template< class ExecutionPolicy, class
ForwardIt1, class ForwardIt2, (4)

class UnaryPred, class T =
typename std::iterator_traits
(since
<ForwardIt2>::value_type > C++26)
ForwardIt2 replace_copy_if
( ExecutionPolicy&& policy,
ForwardIt1 first, ForwardIt1 last,
ForwardIt2 d_first,

UnaryPred p, const T& new_value );

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.

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 participate in overload resolution only if

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 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 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.
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
p - 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.

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 using operator==.
3,4) Exactly \(\scriptsize N\)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 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;
}

Notes¶

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

Example¶

// Run this code

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

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

See also¶

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