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std::replace, std::replace_if

Defined in header `<algorithm>`
	(1)
template< class ForwardIt, class T > void replace( ForwardIt first, ForwardIt last, const T& old_value, const T& new_value );		(until C++20)
template< class ForwardIt, class T > constexpr void replace( ForwardIt first, ForwardIt last, const T& old_value, const T& new_value );		(since C++20)
template< class ExecutionPolicy, class ForwardIt, class T > void replace( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& old_value, const T& new_value );	(2)	(since C++17)
	(3)
template< class ForwardIt, class UnaryPredicate, class T > void replace_if( ForwardIt first, ForwardIt last, UnaryPredicate p, const T& new_value );		(until C++20)
template< class ForwardIt, class UnaryPredicate, class T > constexpr void replace_if( ForwardIt first, ForwardIt last, UnaryPredicate p, const T& new_value );		(since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate, class T > void replace_if( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryPredicate p, const T& new_value );	(4)	(since C++17)

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 operator==).

3) Replaces all elements for which predicate p returns true.

2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless

`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)

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.
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 `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.
-`UnaryPredicate` must meet the requirements of Predicate.

The expression `*first = new_value` must be valid.	(until C++20)
`new_value` must be writable to `first`.	(since C++20)

Return value

(none).

Complexity

Given N as std::distance(first, last):

1,2) exactly N comparisons with old_value using operator==

3,4) exactly 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).

Possible implementation

replace
template<class ForwardIt, class T> 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 UnaryPredicate, class T> void replace_if(ForwardIt first, ForwardIt last, UnaryPredicate p, const T& new_value) { for (; first != last; ++first) if (p(first)) first = new_value; }

replace

template<class ForwardIt, class T>
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 UnaryPredicate, class T>
void replace_if(ForwardIt first, ForwardIt last,
                UnaryPredicate p, const T& new_value)
{
    for (; first != last; ++first)
        if (p(*first))
            *first = new_value;
}

Example

The following code at first replaces all occurrences of 8 with 88 in a vector of integers. Then it replaces all values less than 5 with 55.

#include <algorithm>
#include <array>
#include <functional>
#include <iostream>
 
int main()
{
    std::array<int, 10> s {5, 7, 4, 2, 8, 6, 1, 9, 0, 3};
 
    std::replace(s.begin(), s.end(), 8, 88);
 
    for (int a : s)
        std::cout << a << ' ';
    std::cout << '\n';
 
    std::replace_if(s.begin(), s.end(), 
                    std::bind(std::less<int>(), std::placeholders::_1, 5), 55);
 
    for (int a : s)
        std::cout << a << ' ';
    std::cout << '\n';
}

Output:

5 7 4 2 88 6 1 9 0 3
5 7 55 55 88 6 55 9 55 55

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`), but the value type of `ForwardIt` is not always `T` and `T` is not always writable to `ForwardIt`	required `*first = new_value` to be valid instead

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