NAME¶

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

Synopsis¶

Defined in header <algorithm>
Call signature
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&> (since
&& C++20)
std::indirect_binary_predicate (until
<ranges::equal_to, C++26)
std::projected<I, Proj>, const T1*>
constexpr I replace( I first, S last, const T1&
old_value,

const T2& new_value, Proj
proj = {} );
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&>
&& (since
std::indirect_binary_predicate C++26)
<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 = {} );
template< ranges::input_range R,

class T1, class T2, class Proj =
std::identity >
requires
std::indirectly_writable<ranges::iterator_t<R>,
const T2&> && (since
std::indirect_binary_predicate C++20)
<ranges::equal_to, (until
C++26)
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 = {}
);
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&> && (since
std::indirect_binary_predicate C++26)
<ranges::equal_to,

std::projected<ranges::iterator_t<R>, Proj>,
const T1*>
constexpr ranges::borrowed_iterator_t<R> (1)
replace( R&& r, const T1& old_value,

const T2& new_value, Proj proj = {}
);
template< std::input_iterator I,
std::sentinel_for<I> S,

class T, class Proj = std::identity,
(since
std::indirect_unary_predicate<std::projected<I, C++20)
Proj>> Pred > (until
requires std::indirectly_writable<I, const T&> C++26)
constexpr I replace_if( I first, S last, Pred
pred,
(2)
const T& new_value, Proj
proj = {} );
template< std::input_iterator I,
std::sentinel_for<I> S,

class Proj = std::identity,
class T = std::projected_value_t<I,
Proj>,
(since
std::indirect_unary_predicate<std::projected<I, C++26)
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 = {} );
template< ranges::input_range R, class T, class
Proj = std::identity, (3)

std::indirect_unary_predicate<

std::projected<ranges::iterator_t<R>, Proj>> (since
Pred > C++20)
requires (until
std::indirectly_writable<ranges::iterator_t<R>, C++26)
const T&>
constexpr ranges::borrowed_iterator_t<R>

replace_if( R&& r, Pred pred, const T&
new_value, Proj proj = {} );
template< ranges::input_range R, class Proj =
std::identity, (4)

class T =
std::projected_value_t<ranges::iterator_t<R>,
Proj>,
std::indirect_unary_predicate<
(since
std::projected<ranges::iterator_t<R>, Proj>> C++26)
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 = {} );

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 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.

In practice, they may be implemented as function objects, or with special compiler
extensions.

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)

See also¶

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