NAME¶

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

Synopsis¶

Defined in header <algorithm>
template< class ForwardIt, class
T >
(constexpr
void replace( ForwardIt first, since C++20)
ForwardIt last, (until C++26)

const T& old_value,
const T& new_value );
template< class ForwardIt, class
T = typename std::iterator_traits

<ForwardIt>::value_type > (since C++26)
constexpr void replace( ForwardIt
first, ForwardIt last,

const T&
old_value, const T& new_value );
template< class ExecutionPolicy,
class ForwardIt, class T >

void replace( ExecutionPolicy&& (since
policy, C++17)
ForwardIt first, (until
ForwardIt last, C++26)

const T& old_value,
const T& new_value );
template< class ExecutionPolicy,
class ForwardIt,

class T = typename
std::iterator_traits

<ForwardIt>::value_type > (since
void replace( ExecutionPolicy&& C++26)
policy,
ForwardIt first,
ForwardIt last,

const T& old_value,
const T& new_value );
template< class ForwardIt, class
UnaryPred, class T >
(1) (constexpr
void replace_if( ForwardIt first, since C++20)
ForwardIt last, (until
C++26)
UnaryPred p,
const T& new_value );
template< class ForwardIt, class
UnaryPred,

class T = typename (2)
std::iterator_traits
(since
<ForwardIt>::value_type> > C++26)
constexpr void replace_if(
ForwardIt first, ForwardIt last,

UnaryPred p, const T& new_value
);
template< class ExecutionPolicy,

class ForwardIt, class (3)
UnaryPred, class T > (since
void replace_if( C++17)
ExecutionPolicy&& policy, (until
ForwardIt first, C++26)
ForwardIt last,

UnaryPred p,
const T& new_value );
template< class ExecutionPolicy,
(4)
class ForwardIt, class
UnaryPred,
class T = typename
std::iterator_traits
(since
<ForwardIt>::value_type> > C++26)
void replace_if(
ExecutionPolicy&& policy,
ForwardIt first,
ForwardIt last,

UnaryPred p,
const T& new_value );

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

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
*first = new_value is invalid
(until C++20)
new_value is not writable to first
(since C++20), the program is ill-formed.

Parameters¶

first, last - the range of elements to process
old_value - the value of elements to replace
policy - the execution policy to use. See execution policy for details.
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
p - of type (possibly const) VT, where VT is the value type of ForwardIt,
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¶

-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
UnaryPred must meet the requirements of Predicate.

Return value¶

(none)

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.

Notes¶

Because the algorithm takes old_value and new_value by reference, it can 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
template<class ForwardIt,
class T = typename std::iterator_traits<ForwardIt>::value_type>
void replace(ForwardIt first, ForwardIt last,
const T& old_value, const T& new_value)
{
for (; first != last; ++first)
if (*first == old_value)
*first = new_value;
}
replace_if
template<class ForwardIt, class UnaryPred,
class T = typename std::iterator_traits<ForwardIt>::value_type>
void replace_if(ForwardIt first, ForwardIt last,
UnaryPred p, const T& new_value)
{
for (; first != last; ++first)
if (p(*first))
*first = new_value;
}

Example¶

// Run this code

#include <algorithm>
#include <array>
#include <complex>
#include <functional>
#include <iostream>

void println(const auto& seq)
{
for (const auto& e : seq)
std::cout << e << ' ';
std::cout << '\n';
}

int main()
{
std::array<int, 10> s{5, 7, 4, 2, 8, 6, 1, 9, 0, 3};

// Replace all occurrences of 8 with 88.
std::replace(s.begin(), s.end(), 8, 88);
println(s);

// Replace all values less than 5 with 55.
std::replace_if(s.begin(), s.end(),
std::bind(std::less<int>(), std::placeholders::_1, 5), 55);
println(s);

std::array<std::complex<double>, 2> nums{{{1, 3}, {1, 3}}};
#ifdef __cpp_lib_algorithm_default_value_type
std::replace(nums.begin(), nums.end(), {1, 3}, {4, 2});
#else
std::replace(nums.begin(), nums.end(), std::complex<double>{1, 3},
std::complex<double>{4, 2});
#endif
println(nums);
}

Output:¶

5 7 4 2 88 6 1 9 0 3
5 7 55 55 88 6 55 9 55 55
(4,2), (4,2)

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
T was required to be CopyAssignable (and
EqualityComparable required *first =
LWG 283 C++98 for replace), but the value type of new_value
ForwardIt is not to be valid instead
always T and T is not always writable to
ForwardIt

See also¶

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