std:: replace_copy, std:: replace_copy_if

Copies the elements from the range [ first , last ) to another range beginning at d_first , while replacing all elements satisfying specific criteria with new_value .

If any of the results of the expressions * first and new_value is not writable to d_first , the program is ill-formed.

If the source and destination ranges overlap, the behavior is undefined.

Parameters

Return value

Iterator to the element past the last element copied.

Complexity

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

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

template<class InputIt, class OutputIt, class T>
OutputIt replace_copy(InputIt first, InputIt last, OutputIt d_first,
                      const T& old_value, const T& new_value)
{
    for (; first != last; ++first)
        *d_first++ = (*first == old_value) ? new_value : *first;
    return d_first;
}

template<class InputIt, class OutputIt, class UnaryPred,
         class T = typename std::iterator_traits<ForwardIt>::value_type>
OutputIt replace_copy_if(InputIt first, InputIt last, OutputIt d_first,
                         UnaryPred p, const T& new_value)
{
    for (; first != last; ++first)
        *d_first++ = p(*first) ? new_value : *first;
    return d_first;
}

Notes

Example

#include <algorithm>
#include <complex>
#include <iostream>
#include <vector>
 
void println(const auto& seq)
{
    for (const auto& e : seq)
        std::cout << e << ' ';
    std::cout << '\n';
}
 
int main()
{
    std::vector<short> src{3, 1, 4, 1, 5, 9, 2, 6, 5};
    println(src);
    std::vector<int> dst(src.size());
    std::replace_copy_if(src.cbegin(), src.cend(),
                         dst.begin(),
                         [](short n){ return n > 5; }, 0);
    println(dst);
 
    std::vector<std::complex<double>> src2{{1, 3}, {2, 4}, {3, 5}},
                                      dst2(src2.size());
    println(src2);
    #ifdef __cpp_lib_algorithm_default_value_type
        std::replace_copy_if(src2.cbegin(), src2.cend(), dst2.begin(),
            [](std::complex<double> z){ return std::abs(z) < 5; },
            {4, 2}); // Possible, since the T is deduced.
    #else
        std::replace_copy_if(src2.cbegin(), src2.cend(), dst2.begin(),
            [](std::complex<double> z){ return std::abs(z) < 5; },
            std::complex<double>{4, 2});
    #endif
    println(dst2);
}

3 1 4 1 5 9 2 6 5 
3 1 4 1 5 0 2 0 5 
(1,3) (2,4) (3,5) 
(4,2) (4,2) (3,5)

Defect reports

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

1. ↑ The actual defect in the C++ standard is that the template parameter InputIterator was misspecified as Iterator . This affects the type requirements because the C++ standard states that for the function templates in the algorithms library, the template type parameters whose name ends with Iterator imply the type requirements of the corresponding iterator categories.

See also