std::replace_if() algorithm

since C++20

// (1)
template< class ForwardIt, class UnaryPredicate, class T >
constexpr void replace_if( ForwardIt first, ForwardIt last,
                 UnaryPredicate p, const T& new_value );

// (2)
template< class ExecutionPolicy, class ForwardIt,
          class UnaryPredicate, class T >
void replace_if( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last,
                 UnaryPredicate p, const T& new_value );

since C++17

// (1)
template< class ForwardIt, class UnaryPredicate, class T >
void replace_if( ForwardIt first, ForwardIt last,
                 UnaryPredicate p, const T& new_value );

// (2)
template< class ExecutionPolicy, class ForwardIt,
          class UnaryPredicate, class T >
void replace_if( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last,
                 UnaryPredicate p, const T& new_value );

until C++17

// (1)
template< class ForwardIt, class UnaryPredicate, class T >
void replace_if( ForwardIt first, ForwardIt last,
                 UnaryPredicate p, const T& new_value );

• (1) Replaces all elements for which predicate p returns true with new_value in the range [first; last).

• (2) Same as (1), but executed according to policy.

  Overload Resolution

  These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>  (until C++20) std::is_execution_policy_v<std::replace_if_cvref_t<ExecutionPolicy>>  (since C++20) is true.

Parameters

first last	The range of elements to process.
new_value	The value to use as a replace_ifment.
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 InputIt, 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).

Type requirements

ForwardIt	LegacyForwardIterator
UnaryPredicate	Predicate

since C++20

*first

Must be writable to d_first.

until C++20

The expression *first = new_value must be valid.

Return value

(none)

Complexity

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

Exactly N applications of 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.

Possible implementation

replace_if (1)

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;
}

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).

Examples

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.

Main.cpp

#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