std::ranges:: replace, std::ranges:: replace_if

From cppreference.com

Defined in header `<algorithm>`
Call signature
	(1)
template < std:: input_iterator I, std:: sentinel_for < I > S, class T1, class T2, class Proj = std:: identity > requires std:: indirectly_writable < I, const T2 & > && std:: indirect_binary_predicate < ranges:: equal_to , std :: projected < I, Proj > , const T1 * > constexpr I replace ( I first, S last, 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 Proj = std:: identity , class T1 = std :: projected_value_t < I, Proj > , class T2 = T1 > requires std:: indirectly_writable < I, const T2 & > && std:: indirect_binary_predicate < ranges:: equal_to , std :: projected < I, Proj > , const T1 * > constexpr I replace ( I first, S last, const T1 & old_value, const T2 & new_value, Proj proj = { } ) ;			(since C++26)
		(2)
template < ranges:: input_range R, class T1, class T2, class Proj = std:: identity > requires std:: indirectly_writable < ranges:: iterator_t < R > , const T2 & > && std:: indirect_binary_predicate < ranges:: equal_to , std :: projected < ranges:: iterator_t < R > , Proj > , const T1 * > constexpr ranges:: borrowed_iterator_t < R > replace ( R && r, const T1 & old_value, const T2 & new_value, Proj proj = { } ) ;				(since C++20) (until C++26)
template < ranges:: input_range R, class Proj = std:: identity , class T1 = std :: projected_value_t < ranges:: iterator_t < R > , Proj > , class T2 = T1 > requires std:: indirectly_writable < ranges:: iterator_t < R > , const T2 & > && std:: indirect_binary_predicate < ranges:: equal_to , std :: projected < ranges:: iterator_t < R > , Proj > , const T1 * > constexpr ranges:: borrowed_iterator_t < R > replace ( R && r, 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, class Proj = std:: identity , std:: indirect_unary_predicate < std :: projected < I, Proj >> Pred > requires std:: indirectly_writable < I, const T & > constexpr I replace_if ( I first, S last, 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 Proj = std:: identity , class T = std :: projected_value_t < I, Proj > , std:: indirect_unary_predicate < std :: projected < I, Proj >> Pred > requires std:: indirectly_writable < I, const T & > constexpr I replace_if ( I first, S last, Pred pred, const T & new_value, Proj proj = { } ) ;					(since C++26)
				(4)
template < ranges:: input_range R, class T, class Proj = std:: identity , std:: indirect_unary_predicate < std :: projected < ranges:: iterator_t < R > , Proj >> Pred > requires std:: indirectly_writable < ranges:: iterator_t < R > , const T & > constexpr ranges:: borrowed_iterator_t < R > replace_if ( R && r, Pred pred, const T & new_value, Proj proj = { } ) ;						(since C++20) (until C++26)
template < ranges:: input_range R, class Proj = std:: identity , class T = std :: projected_value_t < ranges:: iterator_t < R > , Proj > , std:: indirect_unary_predicate < std :: projected < ranges:: iterator_t < R > , Proj >> Pred > requires std:: indirectly_writable < ranges:: iterator_t < R > , const T & > constexpr ranges:: borrowed_iterator_t < R > replace_if ( R && r, Pred pred, const T & new_value, Proj proj = { } ) ;						(since C++26)

Replaces all elements satisfying specific criteria with new_value in the range [ first , last ) .

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

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

2,4) Same as (1,3) , but uses r as the 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 process
r	-	the range of elements to process
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

An iterator equal to last .

Complexity

Exactly ranges:: distance ( first, last ) applications of the corresponding predicate comp and any projection proj .

Notes

Because the algorithm takes old_value and new_value by reference, it may have unexpected behavior if either is a reference to an element of the range [ first , last ) .

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

Possible implementation

replace

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

replace_if

struct replace_if_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S,
             class Proj = std::identity, class T = std::projected_value_t<I, Proj>,
             std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
    requires std::indirectly_writable<I, const T&>
    constexpr I operator()(I first, S last, Pred pred,
                           const T& new_value, Proj proj = {}) const
    {
        for (; first != last; ++first)
            if (!!std::invoke(pred, std::invoke(proj, *first)))
                *first = new_value;
        return std::move(first);
    }
 
    template<ranges::input_range R, class Proj = std::identity,
             class T = std::projected_value_t<ranges::iterator_t<R>, Proj>
             std::indirect_unary_predicate
                 <std::projected<ranges::iterator_t<R>, Proj>> Pred>
    requires std::indirectly_writable<ranges::iterator_t<R>, const T&>
    constexpr ranges::borrowed_iterator_t<R>
        operator()(R&& r, Pred pred, const T& new_value, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::move(pred),
                       new_value, std::move(proj));
    }
};
 
inline constexpr replace_if_fn replace_if {};

Example

Run this code

#include <algorithm>
#include <array>
#include <complex>
#include <iostream>
 
void println(const auto& v)
{
    for (const auto& e : v)
        std::cout << e << ' ';
    std::cout << '\n';
}
 
int main()
{
    namespace ranges = std::ranges;
 
    std::array p{1, 6, 1, 6, 1, 6};
    println(p);
    ranges::replace(p, 6, 9);
    println(p);
 
    std::array q{1, 2, 3, 6, 7, 8, 4, 5};
    println(q);
    ranges::replace_if(q, [](int x) { return 5 < x; }, 5);
    println(q);
 
    std::array<std::complex<double>, 2> nums{{{1, 3}, {1, 3}}};
    println(nums);
    #ifdef __cpp_lib_algorithm_default_value_type
        ranges::replace(nums, {1, 3}, {4, 2});
    #else
        ranges::replace(nums, std::complex<double>{1, 3}, std::complex<double>{4, 2});
    #endif
    println(nums);
}

Output:

1 6 1 6 1 6
1 9 1 9 1 9
1 2 3 6 7 8 4 5
1 2 3 5 5 5 4 5
(1,3) (1,3)
(4,2) (4,2)

Compiler support
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Language
Standard library
Standard library headers
Named requirements
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
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

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