std::ranges::replace_copy_if() algorithm

od C++20

Simplified

// (1)
constexpr replace_copy_if_result<I, O>
    replace_copy_if( I first, S last, O result, Pred pred, const T& new_value,
                     Proj proj = {} );

// (2)
constexpr replace_copy_if_result<ranges::borrowed_iterator_t<R>, O>
    replace_copy_if( R&& r, O result, Pred pred, const T& new_value,
                     Proj proj = {} );

The type of arguments are generic and have the following constraints:

• I - std::input_iterator
• S - std::sentinel_for<I>
• O - std::output_iterator<const T& >
• R - std::input_range
• Proj - (none)
• T - (none)
• Pred:
  • (1) - std::indirect_unary_predicate<std::projected<I, Proj>>
  • (2) - std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>>

The Proj template argument has a default type of std::identity for all overloads.

Additionally, each overload has the following constraints:

• (1) - std::indirectly_copyable<I, O>
• (2) - std::indirectly_copyable<ranges::iterator_t<R>, O>

Detailed

// (1)
template<
  std::input_iterator I,
  std::sentinel_for<I> S,
  class T,
  std::output_iterator<const T&> O,
  class Proj = std::identity,
  std::indirect_unary_predicate<std::projected<I, Proj>> Pred
>
  requires std::indirectly_copyable<I, O>
constexpr replace_copy_if_result<I, O>
    replace_copy_if( I first, S last, O result, Pred pred, const T& new_value,
                     Proj proj = {} );

// (2)
template<
  ranges::input_range R,
  class T,
  std::output_iterator<const T&> O,
  class Proj = std::identity,
  std::indirect_unary_predicate< std::projected<ranges::iterator_t<R>, Proj>> Pred
>
  requires std::indirectly_copyable<ranges::iterator_t<R>, O>
constexpr replace_copy_if_result<ranges::borrowed_iterator_t<R>, O>
    replace_copy_if( R&& r, O result, Pred pred, const T& new_value,
                     Proj proj = {} );

With the helper types defined as follows:

template< class I, class O >
using replace_copy_if_result = ranges::in_out_result<I, O>;

• (1) Copies the elements from the source range [first; last) to the destination range beginning at result, replacing all elements that satisfy the predicate pred with new_value.

  Uses std::invoke(pred, std::invoke(proj, *(first + (i - result)))) == true to compare.

• (2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

Undefined Behaviour

The behavior is undefined

if the source and destination ranges overlap.

The function-like entities described on this page are niebloids.

Parameters

first last	The range of elements to copy.
r	The range of elements to copy.
result	The beginning of the destination range.
old_value	The value to replace.
pred	Unary predicate which returns true if the element should be ignored.
proj	The projection to apply to the elements.

Return value

{
  last,
  result + N
}

Where N is the size of the source range.

Complexity

Given N as:

• (1) - ranges::distance(first, last)
• (2) - ranges::distance(r)

Exactly N applications of the corresponding predicate pred and any projection proj.

Exceptions

(none)

Possible implementation

replace_copy_if(1)

struct replace_copy_if_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S, class T,
             std::output_iterator<const T&> O,
             class Proj = std::identity, std::indirect_unary_predicate<
                 std::projected<I, Proj>> Pred>
    requires std::indirectly_copyable<I, O>
    constexpr ranges::replace_copy_if_result<I, O>
        operator()(I first, S last, O result, Pred pred, const T& new_value,
                   Proj proj = {}) const
    {
        for (; first != last; ++first, ++result)
             *result = std::invoke(pred, std::invoke(proj, *first)) ? new_value : *first;
        return {std::move(first), std::move(result)};
    }

    template<ranges::input_range R, class T, std::output_iterator<const T&> O,
             class Proj = std::identity,
             std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred>
    requires std::indirectly_copyable<ranges::iterator_t<R>, O>
    constexpr ranges::replace_copy_if_result<ranges::borrowed_iterator_t<R>, O>
        operator()(R&& r, O result, Pred pred, const T& new_value,
                   Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::move(result),
                       std::move(pred), new_value, std::move(proj));
    }
};

inline constexpr replace_copy_if_fn replace_copy_if {};

Examples

Main.cpp

#include <algorithm>
#include <array>
#include <iostream>
#include <vector>

int main()
{
    auto print = [](const auto rem, const auto& v)
    {
        for (std::cout << rem << ": "; const auto& e : v)
            std::cout << e << ' ';
        std::cout << '\n';
    };

    std::vector<int> o;

    std::array p {1, 6, 1, 6, 1, 6};
    o.resize(p.size());
    print("p", p);
    std::ranges::replace_copy(p, o.begin(), 6, 9);
    print("o", o);

    std::array q {1, 2, 3, 6, 7, 8, 4, 5};
    o.resize(q.size());
    print("q", q);
    std::ranges::replace_copy_if(q, o.begin(), [](int x) { return 5 < x; }, 5);
    print("o", o);
}

Output

p: 1 6 1 6 1 6 
o: 1 9 1 9 1 9 
q: 1 2 3 6 7 8 4 5 
o: 1 2 3 5 5 5 4 5