std::ranges...:replace_if(3)

📦 version 2024.07.05, in nixpkgs unstable, opensuse-leap 16.0

🎀 NAME 🎀

okay bestie, this is the std::ranges...:replace_if(3) page from the C++ Standard Libary manual (that spelling is the manual's, not mine 💅). The name line is:

std::ranges::replace,std::ranges::replace_if — std::ranges::replace,std::ranges::replace_if

🌸 Synopsis 🌸

Defined in header <algorithm>. Call signature time — I reflowed the line breaks so it's readable, but every token, default, requires clause, and since/until tag is still here. No cap.

💅 `replace` — iterator form, `(1)`

(since C++20) (until C++26)

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 = {} );

(since C++26)

template< std::input_iterator I, std::sentinel_for<I> S,
          class Proj = std::identity,
          class T1 = std::projected_value_t<I, Proj>, class T2 = T1 >
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 = {} );

💅 `replace` — range form, `(2)`

(since C++20) (until C++26)

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 = {} );

(since C++26)

template< ranges::input_range R,
          class Proj = std::identity,
          class T1 = std::projected_value_t<ranges::iterator_t<R>, Proj>,
          class T2 = T1 >
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 = {} );

💅 `replace_if` — iterator form, `(3)`

(since C++20) (until C++26)

template< std::input_iterator I, std::sentinel_for<I> S,
          class T, class Proj = std::identity,
          std::indirect_unary_predicate<std::projected<I, Proj>> Pred >
requires std::indirectly_writable<I, const T&>
constexpr I replace_if( I first, S last, Pred pred,
                        const T& new_value, Proj proj = {} );

(since C++26)

template< std::input_iterator I, std::sentinel_for<I> S,
          class Proj = std::identity,
          class T = std::projected_value_t<I, Proj>,
          std::indirect_unary_predicate<std::projected<I, Proj>> Pred >
requires std::indirectly_writable<I, const T&>
constexpr I replace_if( I first, S last, Pred pred,
                        const T& new_value, Proj proj = {} );

💅 `replace_if` — range form, `(4)`

(since C++20) (until C++26)

template< ranges::input_range R, class T, class Proj = std::identity,
          std::indirect_unary_predicate<
              std::projected<ranges::iterator_t<R>, Proj>> Pred >
requires std::indirectly_writable<ranges::iterator_t<R>, const T&>
constexpr ranges::borrowed_iterator_t<R>
    replace_if( R&& r, Pred pred, const T& new_value, Proj proj = {} );

(since C++26)

template< ranges::input_range R, class Proj = std::identity,
          class T = std::projected_value_t<ranges::iterator_t<R>, Proj>,
          std::indirect_unary_predicate<
              std::projected<ranges::iterator_t<R>, Proj>> Pred >
requires std::indirectly_writable<ranges::iterator_t<R>, const T&>
constexpr ranges::borrowed_iterator_t<R>
    replace_if( R&& r, Pred pred, const T& new_value, Proj proj = {} );

So the vibe is: these algorithms replace all elements satisfying specific criteria with new_value in the range [first, last).

1) Replaces all elements that are equal to old_value, using std::invoke(proj, *i) == old_value to compare.
3) Replaces all elements for which the predicate pred evaluates to true, where the evaluating expression is std::invoke(pred, std::invoke(proj, *i)).
2,4) Same as (1,3), but uses r as the 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, that is:

Explicit template argument lists cannot be specified when calling any of them.
None of them are visible to argument-dependent lookup.
When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler extensions.

🫶 Parameters 🫶

first, last — the range of elements to process
r — the range of elements to process
old_value — the value of elements to replace
new_value — the value to use as a replacement
pred — predicate to apply to the projected elements
proj — projection to apply to the elements

👑 Return value 👑

An iterator equal to last.

⭐ Complexity ⭐

Exactly ranges::distance(first, last) applications of the corresponding predicate comp and any projection proj.

🍓 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).

🌈 Feature-test macro 🌈

__cpp_lib_algorithm_default_value_type — value 202403, Std (C++26), feature: List-initialization for algorithms (1-4)

🧚 Possible implementation 🧚

💅 `replace`

struct replace_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S, class
 Proj = std::identity,
             class T1 = std::projected_value_t<I, Proj>, class T2 =
 T1>
    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 Proj = std::identity
             class T1 = std::projected_value_t<ranges::iterator_t<R>,
 Proj>,
             class T2 = T1>
    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 {};

💅 `replace_if`

struct replace_if_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S,
             class Proj = std::identity, class T =
 std::projected_value_t<I, Proj>,
             std::indirect_unary_predicate<std::projected<I, Proj>>
 Pred>
    requires std::indirectly_writable<I, const T&>
    constexpr I operator()(I first, S last, Pred pred,
                           const T& new_value, Proj proj = {}) const
    {
        for (; first != last; ++first)
            if (!!std::invoke(pred, std::invoke(proj, *first)))
                *first = new_value;
        return std::move(first);
    }

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

inline constexpr replace_if_fn replace_if {};

🍓 Example 🍓

okay so this is the whole demo, bestie — run it and vibe check the output right after 💅

        // Run this code

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

        void println(const auto& v)
        {
            for (const auto& e : v)
                std::cout << e << ' ';
            std::cout << '\n';
        }

        int main()
        {
            namespace ranges = std::ranges;

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

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

            std::array<std::complex<double>, 2> nums{{{1, 3}, {1, 3}}};
            println(nums);
            #ifdef __cpp_lib_algorithm_default_value_type
                ranges::replace(nums, {1, 3}, {4, 2});
            #else
                ranges::replace(nums, std::complex<double>{1, 3},
       std::complex<double>{4, 2});
            #endif
            println(nums);
        }

🖨️ 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
        (1,3) (1,3)
        (4,2) (4,2)

👀 See also 👀

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

💖 and that's the whole vibe, bestie — source/version: cppreference.com, 2024.06.10. slay! 🎀

📦 where this page comes from

nixpkgs unstable: stdman 2024.07.05
opensuse-leap 16.0: stdman 2024.07.05-bp160.1.9 · MIT
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms

std::ranges...:replace_if(3)

🎀 NAME 🎀

🌸 Synopsis 🌸

💅 replace — iterator form, (1)

💅 replace — range form, (2)

💅 replace_if — iterator form, (3)

💅 replace_if — range form, (4)

🫶 Parameters 🫶

👑 Return value 👑

⭐ Complexity ⭐

🍓 Notes 🍓

🌈 Feature-test macro 🌈

🧚 Possible implementation 🧚

💅 replace

💅 replace_if

🍓 Example 🍓

🖨️ Output 🖨️

👀 See also 👀

💅 `replace` — iterator form, `(1)`

💅 `replace` — range form, `(2)`

💅 `replace_if` — iterator form, `(3)`

💅 `replace_if` — range form, `(4)`

💅 `replace`

💅 `replace_if`