std::ranges::replace() algorithm

od C++20

Simplified

// (1)
constexpr I
    replace( I first, S last, const T1& old_value, const T2& new_value,
             Proj proj = {} );

// (2)
constexpr ranges::borrowed_iterator_t<R>
    replace( R&& r, const T1& old_value, const T2& new_value, Proj proj = {} );

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

• I - std::permutable
• S - std::sentinel_for<I>
• R - std::ranges::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_writable<I, const T2&>
  && indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T1*>
• (2):

  indirectly_writable<ranges::iterator_t<R>, const T2&>
  && 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,
  class Proj = std::identity
>
  requires std::indirectly_writable<I, const T2&> &&
           std::indirect_binary_predicate<ranges::equal_to,
            std::projected<I, Proj>, const T1*>
constexpr I
    replace( I first, S last, const T1& old_value, const T2& new_value,
             Proj proj = {} );

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

• (1) Replaces all elements that are equal to old_value with new_value, using std::invoke(proj, *i) == old_value 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.

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

Parameters

first last	The range of elements to process.
r	The range of elements to process.
old_value	The value to search for and replace.
new_value	The value to use as a replacement.
proj	Projection to apply to the elements.

Return value

An iterator equal to last.

Complexity

Exactly ranges::distance(first, last) applications of the projection proj.

Exceptions

(none)

Possible implementation

replace(1)

struct replace_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S, class T1, class T2,
             class Proj = std::identity>
    requires std::indirectly_writable<I, const T2&> && std::indirect_binary_predicate<
                 ranges::equal_to, std::projected<I, Proj>, const T1*>
    constexpr I
        operator()(I first, S last, const T1& old_value, const T2& new_value,
                   Proj proj = {}) const
    {
        for (; first != last; ++first)
            if (old_value == std::invoke(proj, *first))
                *first = new_value;
        return first;
    }

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

inline constexpr replace_fn replace {};

Notes

Because the algorithm takes old_value and new_value by reference, it may have unexpected behavior if either is a reference to an element of the range [first; last).

Examples

Main.cpp

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

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

    std::array p {1, 6, 1, 6, 1, 6};
    print(p);
    std::ranges::replace(p, 6, 9);
    print(p);

    std::array q {1, 2, 3, 6, 7, 8, 4, 5};
    print(q);
    std::ranges::replace_if(q, [](int x) { return 5 < x; }, 5);
    print(q);
}

Output

1 6 1 6 1 6
1 9 1 9 1 9
1 2 3 6 7 8 4 5
1 2 3 5 5 5 4 5