std::replace_copy, std::replace_copy_if

 

C++

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Named requirements

Feature test macros (C++20)

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Concepts library (C++20)

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Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

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Filesystem library (C++17)

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Execution control library (C++26)

Technical specifications

Symbols index

External libraries

 

Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
Numeric operations
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

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)
	(3)
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 );			(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 );			(since C++26)
		(4)
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 );				(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 );				(since C++26)

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 all following conditions are satisfied:

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 pair of iterators defining the source 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
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.

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 (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) { for (; first != last; ++first) *d_first++ = (*first == old_value) ? new_value : *first; return d_first; }
replace_copy_if (3)
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
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	corrected to LegacyInputIterator

See also

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