std::replac...ace_copy_if(3)

Manual page: std::replac...ace_copy_if(3) — C++ Standard Libary.

💖 NAME 💖

std::replace_copy, std::replace_copy_if — the copy-and-replace algorithms. The page literally says std::replace_copy,std::replace_copy_if, so bestie, we are covering both 🫶.

✨ Synopsis ✨

Defined in header <algorithm>.

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 );

(1) — constexpr since C++20.

template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class T >
ForwardIt2 replace_copy( ExecutionPolicy&& policy,
                         ForwardIt1 first, ForwardIt1 last,
                         ForwardIt2 d_first,
                         const T& old_value, const T& new_value );

(2) — since C++17.

template< class InputIt, class OutputIt, class UnaryPred, class T >
OutputIt replace_copy_if( InputIt first, InputIt last, OutputIt d_first,
                          UnaryPred p, const T& new_value );

Annotation: (constexpr since C++20), (until C++26).

template< class InputIt, class OutputIt, class UnaryPred,
          class T = typename std::iterator_traits<OutputIt>::value_type >
constexpr OutputIt replace_copy_if( InputIt first, InputIt last,
                                    OutputIt d_first,
                                    UnaryPred p, const T& new_value );

Annotation: (since C++26).

template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2,
          class UnaryPred, class T >
ForwardIt2 replace_copy_if( ExecutionPolicy&& policy,
                            ForwardIt1 first, ForwardIt1 last,
                            ForwardIt2 d_first,
                            UnaryPred p, const T& new_value );

(3) — since C++17, until C++26.

template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2,
          class UnaryPred,
          class T = typename std::iterator_traits<ForwardIt2>::value_type >
ForwardIt2 replace_copy_if( ExecutionPolicy&& policy,
                            ForwardIt1 first, ForwardIt1 last,
                            ForwardIt2 d_first,
                            UnaryPred p, const T& new_value );

(4) — since C++26.

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.

• 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 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. That is not a vibe check you want to fail, bestie 💀

🌸 Parameters 🌸

• first, last — the range of elements to copy.
• d_first — the beginning of the destination range.
• 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 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).
• new_value — the value to use as replacement.

🦋 Type requirements 🦋

• InputIt must meet the requirements of LegacyInputIterator.
• OutputIt must meet the requirements of LegacyOutputIterator.
• ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.

🎀 Return value 🎀

Iterator to the element past the last element copied. Slay, that's literally it ✨.

🔥 Complexity 🔥

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

• 1,2) Exactly N comparisons 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.

🧚 Possible implementation 🧚

replace_copy

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;
}

replace_copy_if

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 ⭐

Feature-test macro: __cpp_lib_algorithm_default_value_type — value 202403 — std (C++26) — feature: List-initialization for algorithms (3,4).

🍓 Example 🍓

// Run this code

#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);
}

🌈 Output 🌈

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.

• LWG 283 — applied to C++98. Behavior as published: T was required to be CopyAssignable (and EqualityComparable for replace_copy), but the value type of InputIt is not always T. Correct behavior: removed the requirement.
• LWG 337 — applied to C++98. Behavior as published: replace_copy_if only required InputIt to meet the requirements of LegacyIterator^[1]. Correct behavior: corrected to LegacyInputIterator.

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 💜

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

xoxo, your Unix bestie 💅 — source/version: http://cppreference.com · 2024.06.10 · page std::replac...ace_copy_if(3).