std::ranges:: replace_copy, std::ranges:: replace_copy_if, std::ranges:: replace_copy_result, std::ranges:: replace_copy_if_result

From cppreference.com

Defined in header `<algorithm>`
Call signature
	(1)
template < std:: input_iterator I, std:: sentinel_for < I > S, class T1, class T2, std:: output_iterator < const T2 & > O, class Proj = std:: identity > requires std:: indirectly_copyable < I, O > && std:: indirect_binary_predicate < ranges:: equal_to , std :: projected < I, Proj > , const T1 * > constexpr replace_copy_result < I, O > replace_copy ( I first, S last, O result, const T1 & old_value, const T2 & new_value, Proj proj = { } ) ;			(since C++20) (until C++26)
template < std:: input_iterator I, std:: sentinel_for < I > S, class O, class Proj = std:: identity , class T1 = std :: projected_value_t < I, Proj > , class T2 = std:: iter_value_t < O > > requires std:: indirectly_copyable < I, O > && std:: indirect_binary_predicate < ranges:: equal_to , std :: projected < I, Proj > , const T1 * > && std:: output_iterator < O, const T2 & > constexpr replace_copy_result < I, O > replace_copy ( I first, S last, O result, const T1 & old_value, const T2 & new_value, Proj proj = { } ) ;			(since C++26)
		(2)
template < ranges:: input_range R, class T1, class T2, std:: output_iterator < const T2 & > O, class Proj = std:: identity > requires std:: indirectly_copyable < ranges:: iterator_t < R > , O > && std:: indirect_binary_predicate < ranges:: equal_to , std :: projected < ranges:: iterator_t < R > , Proj > , const T1 * > constexpr replace_copy_result < ranges:: borrowed_iterator_t < R > , O > replace_copy ( R && r, O result, const T1 & old_value, const T2 & new_value, Proj proj = { } ) ;				(since C++20) (until C++26)
template < ranges:: input_range R, class O, class Proj = std:: identity , class T1 = std :: projected_value_t < ranges:: iterator_t < R > , Proj > , class T2 = std:: iter_value_t < O > > requires std:: indirectly_copyable < ranges:: iterator_t < R > , O > && std:: indirect_binary_predicate < ranges:: equal_to , std :: projected < ranges:: iterator_t < R > , Proj > , const T1 * > && std:: output_iterator < O, const T2 & > constexpr replace_copy_result < ranges:: borrowed_iterator_t < R > , O > replace_copy ( R && r, O result, const T1 & old_value, const T2 & new_value, Proj proj = { } ) ;				(since C++26)
			(3)
template < std:: input_iterator I, std:: sentinel_for < I > S, class T, std:: output_iterator < const T & > O, class Proj = std:: identity , std:: indirect_unary_predicate < std :: projected < I, Proj >> Pred > requires std:: indirectly_copyable < I, O > constexpr replace_copy_if_result < I, O > replace_copy_if ( I first, S last, O result, Pred pred, const T & new_value, Proj proj = { } ) ;					(since C++20) (until C++26)
template < std:: input_iterator I, std:: sentinel_for < I > S, class O, class T = std:: iter_value_t < O > class Proj = std:: identity , std:: indirect_unary_predicate < std :: projected < I, Proj >> Pred > requires std:: indirectly_copyable < I, O > && std:: output_iterator < O, const T & > constexpr replace_copy_if_result < I, O > replace_copy_if ( I first, S last, O result, Pred pred, const T & new_value, Proj proj = { } ) ;					(since C++26)
				(4)
template < ranges:: input_range R, class T, std:: output_iterator < const T & > O, class Proj = std:: identity , std:: indirect_unary_predicate < std :: projected < ranges:: iterator_t < R > , Proj >> Pred > requires std:: indirectly_copyable < ranges:: iterator_t < R > , O > constexpr replace_copy_if_result < ranges:: borrowed_iterator_t < R > , O > replace_copy_if ( R && r, O result, Pred pred, const T & new_value, Proj proj = { } ) ;						(since C++20) (until C++26)
template < ranges:: input_range R, class O, class T = std:: iter_value_t < O > class Proj = std:: identity , std:: indirect_unary_predicate < std :: projected < ranges:: iterator_t < R > , Proj >> Pred > requires std:: indirectly_copyable < ranges:: iterator_t < R > , O > && std:: output_iterator < O, const T & > constexpr replace_copy_if_result < ranges:: borrowed_iterator_t < R > , O > replace_copy_if ( R && r, O result, Pred pred, const T & new_value, Proj proj = { } ) ;						(since C++26)
Helper types
template < class I, class O > using replace_copy_result = ranges:: in_out_result < I, O > ;					(5)	(since C++20)
template < class I, class O > using replace_copy_if_result = ranges:: in_out_result < I, O > ;					(6)	(since C++20)

Copies the elements from the source range [ first , last ) to the destination range beginning at result , replacing all elements satisfying specific criteria with new_value . The behavior is undefined if the source and destination ranges overlap.

1) Replaces all elements that are equal to old_value , using std:: invoke ( proj, * ( first + ( i - result ) ) ) == old_value to compare.

3) Replaces all elements for which the predicate pred evaluates to true , where the evaluating expression is std:: invoke ( pred, std:: invoke ( proj, * ( first + ( i - result ) ) ) ) .

2,4) Same as (1,3) , but uses r as the source range, as if using ranges:: begin ( r ) as first , and ranges:: end ( r ) as last .

The function-like entities described on this page are algorithm function objects (informally known as niebloids ), that is:

Explicit template argument lists cannot be specified when calling any of them.
None of them are visible to argument-dependent lookup .
When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Parameters

first, last	-	the range of elements to copy
r	-	the range of elements to copy
result	-	the beginning of the destination range
old_value	-	the value of elements to replace
new_value	-	the value to use as a replacement
pred	-	predicate to apply to the projected elements
proj	-	projection to apply to the elements.

Return value

{ last, result + N } , where

1,3) N = ranges:: distance ( first, last ) ;

2,4) N = ranges:: distance ( r ) .

Complexity

Exactly N applications of the corresponding predicate comp and any projection proj .

Possible implementation

replace_copy (1,2)

struct replace_copy_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S,
             class O, class Proj = std::identity,
             class T1 = std::projected_value_t<I, Proj>,
             class T2 = std::iter_value_t<O>>
    requires std::indirectly_copyable<I, O> &&
             std::indirect_binary_predicate
                 <ranges::equal_to, std::projected<I, Proj>, const T1*> &&
             std::output_iterator<O, const T2&>
    constexpr ranges::replace_copy_result<I, O>
        operator()(I first, S last, O result, const T1& old_value,
                   const T2& new_value, Proj proj = {}) const
    {
        for (; first != last; ++first, ++result)
            *result = (std::invoke(proj, *first) == old_value) ? new_value : *first;
        return {std::move(first), std::move(result)};
    }
 
    template<ranges::input_range R, class O, class Proj = std::identity,
             class T1 = std::projected_value_t<ranges::iterator_t<R>, Proj>,
             class T2 = std::iter_value_t<O>>
    requires std::indirectly_copyable<ranges::iterator_t<R>, O> &&
             std::indirect_binary_predicate
                 <ranges::equal_to,
                  std::projected<ranges::iterator_t<R>, Proj>, const T1*>
    constexpr ranges::replace_copy_result<ranges::borrowed_iterator_t<R>, O>
        operator()(R&& r, O result, const T1& old_value,
                   const T2& new_value, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::move(result),
                       old_value, new_value, std::move(proj));
    }
};
 
inline constexpr replace_copy_fn replace_copy {};

replace_copy_if (3,4)

struct replace_copy_if_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S,
             class O, class T = std::iter_value_t<O>
             class Proj = std::identity,
             std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
    requires std::indirectly_copyable<I, O> && std::output_iterator<O, const T&>
    constexpr ranges::replace_copy_if_result<I, O>
        operator()(I first, S last, O result, Pred pred,
                   const T& new_value, Proj proj = {}) const
    {
        for (; first != last; ++first, ++result)
             *result = std::invoke(pred, std::invoke(proj, *first)) ? new_value : *first;
        return {std::move(first), std::move(result)};
    }
 
    template<ranges::input_range R, class O, class T = std::iter_value_t<O>
             class Proj = std::identity,
             std::indirect_unary_predicate
                 <std::projected<ranges::iterator_t<R>, Proj>> Pred>
    requires std::indirectly_copyable<ranges::iterator_t<R>, O> &&
             std::output_iterator<O, const T&>
    constexpr ranges::replace_copy_if_result<ranges::borrowed_iterator_t<R>, O>
        operator()(R&& r, O result, Pred pred,
                   const T& new_value, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::move(result),
                       std::move(pred), new_value, std::move(proj));
    }
};
 
inline constexpr replace_copy_if_fn replace_copy_if {};

Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_algorithm_default_value_type`	202403	(C++26)	list-initialization for algorithms ( 1-4 )

Example

Run this code

#include <algorithm>
#include <array>
#include <complex>
#include <iostream>
#include <vector>
 
void println(const auto rem, const auto& v)
{
    for (std::cout << rem << ": "; const auto& e : v)
        std::cout << e << ' ';
    std::cout << '\n';
}
 
int main()
{    
    std::vector<int> o;
 
    std::array p{1, 6, 1, 6, 1, 6};
    o.resize(p.size());
    println("p", p);
    std::ranges::replace_copy(p, o.begin(), 6, 9);
    println("o", o);
 
    std::array q{1, 2, 3, 6, 7, 8, 4, 5};
    o.resize(q.size());
    println("q", q);
    std::ranges::replace_copy_if(q, o.begin(), [](int x) { return 5 < x; }, 5);
    println("o", o);
 
    std::vector<std::complex<short>> r{{1, 3}, {2, 2}, {4, 8}};
    std::vector<std::complex<float>> s(r.size());
    println("r", r);
    #ifdef __cpp_lib_algorithm_default_value_type
        std::ranges::replace_copy(r, s.begin(),
                                  {1, 3}, // T1 gets deduced
                                  {2.2, 4.8}); // T2 gets deduced
    #else
        std::ranges::replace_copy(r, s.begin(),
                                  std::complex<short>{1, 3},
                                  std::complex<float>{2.2, 4.8});
    #endif
    println("s", s);
 
    std::vector<std::complex<double>> b{{1, 3}, {2, 2}, {4, 8}},
                                      d(b.size());
    println("b", b);
    #ifdef __cpp_lib_algorithm_default_value_type
        std::ranges::replace_copy_if(b, d.begin(),
            [](std::complex<double> z){ return std::abs(z) < 5; },
            {4, 2}); // Possible, since the T is deduced.
    #else
        std::ranges::replace_copy_if(b, d.begin(),
            [](std::complex<double> z){ return std::abs(z) < 5; },
            std::complex<double>{4, 2});
    #endif
    println("d", d);
}

Output:

p: 1 6 1 6 1 6
o: 1 9 1 9 1 9
q: 1 2 3 6 7 8 4 5
o: 1 2 3 5 5 5 4 5
r: (1,3) (2,2) (4,8)
s: (2.2,4.8) (2,2) (4,8)
b: (1,3) (2,2) (4,8)
d: (4,2) (4,2) (4,8)

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Standard library
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Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
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Filesystem library (C++17)
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Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

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