std::ranges::replace_copy() algorithm

od C++20

Simplified

// (1)
constexpr replace_copy_result<I, O>
    replace_copy( I first, S last, O result, const T1& old_value, const T2& new_value,
                  Proj proj = {} );

// (2)
constexpr replace_copy_result<ranges::borrowed_iterator_t<R>, O>
    replace_copy( R&& r, O result, const T1& old_value, const T2& 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 T2&>
• R - std::input_range
• T1 - (none)
• T2 - (none)
• Proj - (none)

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

Additionally, each overload has the following constraints:

• (1):

  indirectly_copyable<I, O>
  && indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T1*>
• (2):

  indirectly_copyable<ranges::iterator_t<R>, O>
  && indirect_binary_predicate<ranges::equal_to, projected<ranges::iterator_t<R>, Proj>, const T1*>

(The std:: namespace was ommitted here for readability)

Detailed

// (1)
template<
  std::input_iterator I,
  std::sentinel_for<I> S,
  class T1,
  class T2,
  std::output_iterator<const T2&> O,
  class Proj = std::identity
>
requires std::indirectly_copyable<I, O> &&
         std::indirect_binary_predicate<ranges::equal_to,
             std::projected<I, Proj>, const T1*>
constexpr replace_copy_result<I, O>
    replace_copy( I first, S last, O result, const T1& old_value, const T2& new_value,
                  Proj proj = {} );

// (2)
template<
  ranges::input_range R,
  class T1,
  class T2,
  std::output_iterator<const T2&> O,
  class Proj = std::identity
>
requires std::indirectly_copyable<ranges::iterator_t<R>, O> &&
         std::indirect_binary_predicate<ranges::equal_to,
             std::projected<ranges::iterator_t<R>, Proj>, const T1*>
constexpr replace_copy_result<ranges::borrowed_iterator_t<R>, O>
    replace_copy( R&& r, O result, const T1& old_value, const T2& new_value,
                  Proj proj = {} );

With the helper types defined as follows:

template< class I, class O >
using replace_copy_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 are equal to old_value with new_value.

  Uses std::invoke(proj, *(first + (i - result))) 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 of elements to replace.
new_value	The value to use as a replacement.
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 comparisons after projecting with proj.

Exceptions

(none)

Possible implementation

replace_copy(1) and replace_copy(2)

struct replace_copy_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S, class T1, class T2,
             std::output_iterator<const T2&> O, class Proj = std::identity>
    requires std::indirectly_copyable<I, O> &&
             std::indirect_binary_predicate<ranges::equal_to,
                 std::projected<I, Proj>, const T1*>
    constexpr ranges::replace_copy_result<I, O>
        operator()(I first, S last, O result, const T1& old_value, const T2& new_value,
                   Proj proj = {}) const
    {
        for (; first != last; ++first, ++result)
            *result = (std::invoke(proj, *first) == old_value) ? new_value : *first;
        return {std::move(first), std::move(result)};
    }

    template<ranges::input_range R, class T1, class T2,
             std::output_iterator<const T2&> O, class Proj = std::identity>
    requires std::indirectly_copyable<ranges::iterator_t<R>, O> &&
             std::indirect_binary_predicate<ranges::equal_to,
                 std::projected<ranges::iterator_t<R>, Proj>, const T1*>
    constexpr ranges::replace_copy_result<ranges::borrowed_iterator_t<R>, O>
        operator()(R&& r, O result, const T1& old_value, const T2& new_value,
                   Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::move(result),
                       old_value, new_value, std::move(proj));
    }
};

inline constexpr replace_copy_fn replace_copy {};

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