std::fill(3)

✨ NAME ✨

Bestie, welcome to std::fill(3) from the C++ Standard Library — it's the algorithm that assigns value to every element in [first, last). The NAME line is literally std::fill - std::fill, which is giving “the name is the name” energy, and honestly? Iconic. 💅

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🦋 Synopsis 🦋

Defined in header <algorithm>.

template< class ForwardIt, class T >
constexpr void fill( ForwardIt first, ForwardIt last, const T& value );

constexpr since C++20; (until C++26).

template< class ForwardIt, class T = typename std::iterator_traits<ForwardIt>::value_type >
constexpr void fill( ForwardIt first, ForwardIt last, const T& value );

(since C++26) — the default-template-parameter glow-up ✨

template< class ExecutionPolicy, class ForwardIt, class T >
void fill( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& value );

(1) (since C++17) (until C++26).

template< class ExecutionPolicy, class ForwardIt, class T = typename std::iterator_traits<ForwardIt>::value_type >
void fill( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& value );

(2) (since C++26).

1. Assigns the given value to all elements in the range [first, last).
2. Same as (1), but executed according to policy. This overload participates 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 value is not writable to first, the program is ill-formed. 🚨

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🌸 Parameters 🌸

• first, last — the range of elements to modify 🛠️
• value — the value to be assigned 💖
• policy — the execution policy to use. See execution policy for details. 📜

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🎀 Type requirements 🎀

• ForwardIt must meet the requirements of LegacyForwardIterator.

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⭐ Complexity ⭐

Exactly std::distance(first, last) assignments. No cap, no extra credit, just that many. 💯

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🔥 Exceptions 🔥

The overload with a template parameter named ExecutionPolicy reports 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.

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🧚 Possible implementation 🧚

fill

template<class ForwardIt,
         class T = typename std::iterator_traits<ForwardIt>::value_type>
void fill(ForwardIt first, ForwardIt last, const T& value)
{
    for (; first != last; ++first)
        *first = value;
}

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💜 Notes 💜

• __cpp_lib_algorithm_default_value_type — value 202403, Std C++26, Feature: “List-initialization for algorithms (1,2)”.

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🍓 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<int> v{0, 1, 2, 3, 4, 5, 6, 7, 8};
    println(v);

    // set all of the elements to 8
    std::fill(v.begin(), v.end(), 8);
    println(v);

    std::vector<std::complex<double>> nums{{1, 3}, {2, 2}, {4, 8}};
    println(nums);
    #ifdef __cpp_lib_algorithm_default_value_type
        std::fill(nums.begin(), nums.end(), {4, 2});
    #else
        std::fill(nums.begin(), nums.end(), std::complex<double>{4, 2});
    #endif
    println(nums);
}

Output:

0 1 2 3 4 5 6 7 8
8 8 8 8 8 8 8 8 8
(1,3) (2,2) (4,8)
(4,2) (4,2) (4,2)

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🩹 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, but T is not always writable to ForwardIt. Correct behavior: required to be writable instead.

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👑 See also 👑

• fill_n — copy-assigns the given value to N elements in a range (function template)
• copy — copies a range of elements to a new location (function template)
• copy_if — (C++11) (function template)
• generate — assigns the results of successive function calls to every element in a range (function template)
• transform — applies a function to a range of elements, storing results in a destination range (function template)
• ranges::fill — assigns a range of elements a certain value (C++20) (niebloid)

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Made with 💖 from http://cppreference.com, version/date 2024.06.10, for the std::fill(3) manual page. byeeeee 🌈🫶