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std::replace_copy, std::replace_copy_if

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
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replace_copyreplace_copy_if

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(C++20)(C++20)
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(C++17)

Sorting and related operations
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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 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)
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 );
(3) (constexpr since C++20)
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)

Copies the elements from the range [firstlast) 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.

Contents

[edit] 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.

[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 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 UnaryPred, class T>
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;
}

[edit] Example

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

#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

[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
  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.

[edit] See also

replaces all values satisfying specific criteria with another value
(function template) [edit]
removes elements satisfying specific criteria
(function template) [edit]
copies a range, replacing elements satisfying specific criteria with another value
(niebloid)[edit]