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std::replace, std::replace_if - cppreference.com

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 all following conditions are satisfied:

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

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). ​

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.

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.

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).

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;
}

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;
}

#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)

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

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