std::ranges...construct_n(3)

💖 std::ranges::uninitialized_value_construct_n(3) — C++ Standard Libary 💖

okay so bestie, welcome to the std::ranges...construct_n(3) manual page — the std::ranges::uninitialized_value_construct_n situation in the C++ Standard Libary. yes, the header spelling said “Libary,” we’re keeping the lore. 💅

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🌸 NAME 🌸

std::ranges::uninitialized_value_construct_n — and the NAME line is literally repeating it, iconic behavior: std::ranges::uninitialized_value_construct_n - std::ranges::uninitialized_value_construct_n.

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✨ Synopsis ✨

Defined in header <memory> — Call signature vibes:

template< no-throw-forward-iterator I >

requires std::default_initializable<std::iter_value_t<I>>
(since C++20)

I uninitialized_value_construct_n( I first, std::iter_difference_t<I>
n );

So it constructs n objects of type std::iter_value_t<I> in the uninitialized memory area starting at first by value-initialization, as if by:

for (; n-- > 0; ++first)
    ::new (static_cast<void*>(std::addressof(*first)))
        std::remove_reference_t<std::iter_reference_t<I>>();

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order. 🫶

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.

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🎀 Parameters 🎀

• first — the beginning of the range of elements to initialize
• n — the number of elements to construct

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🦋 Return value 🦋

The end of the range of objects (i.e., ranges::next(first, n)).

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🌈 Complexity 🌈

Linear in n.

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💜 Exceptions 💜

The exception thrown on construction of the elements in the destination range, if any.

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⭐ Notes ⭐

An implementation may improve the efficiency of the ranges::uninitialized_value_construct_n, e.g. by using ranges::fill_n, if the value type of the range is TrivialType and CopyAssignable.

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🧚 Possible implementation 🧚

Here’s the possible implementation, bestie — copy it exactly, no “fixing” the vibes:

struct uninitialized_value_construct_n_fn
{
    template<no-throw-forward-iterator I>
    requires std::default_initializable<std::iter_value_t<I>>
    I operator()(I first, std::iter_difference_t<I> n) const
    {
        using T = std::remove_reference_t<std::iter_reference_t<I>>;
        if constexpr (std::is_trivial_v<T> &&
std::is_copy_assignable_v<T>)
            return ranges::fill_n(first, n, T());
        I rollback{first};
        try
        {
            for (; n-- > 0; ++first)
                ::new (const_cast<void*>(static_cast<const volatile
void*>
                    (std::addressof(*first)))) T();
            return first;
        }
        catch (...) // rollback: destroy constructed elements
        {
            for (; rollback != first; ++rollback)
                ranges::destroy_at(std::addressof(*rollback));
            throw;
        }
    }
};

inline constexpr uninitialized_value_construct_n_fn
uninitialized_value_construct_n{};

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🌸 Example 🌸

// Run this code

#include <iostream>
#include <memory>
#include <string>

int main()
{
    struct S { std::string m{"█▓▒░ █▓▒░ █▓▒░ "}; };

    constexpr int n{4};
    alignas(alignof(S)) char out[n * sizeof(S)];

    try
    {
        auto first{reinterpret_cast<S*>(out)};
        auto last = std::ranges::uninitialized_value_construct_n(first,
n);

        auto count{1};
        for (auto it{first}; it != last; ++it)
            std::cout << count++ << ' ' << it->m << '\n';

        std::ranges::destroy(first, last);
    }
    catch (...)
    {
        std::cout << "Exception!\n";
    }

    // Notice that for "trivial types" the
uninitialized_value_construct_n
    // zero-initializes the given uninitialized memory area.
    int v[]{1, 2, 3, 4, 5, 6, 7, 8};
    std::cout << ' ';
    for (const int i : v)
        std::cout << i << ' ';
    std::cout << "\n ";
    std::ranges::uninitialized_value_construct_n(std::begin(v),
std::size(v));
    for (const int i : v)
        std::cout << i << ' ';
    std::cout << '\n';
}

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🍓 Output: 🍓

1 █▓▒░ █▓▒░ █▓▒░
2 █▓▒░ █▓▒░ █▓▒░
3 █▓▒░ █▓▒░ █▓▒░
4 █▓▒░ █▓▒░ █▓▒░
1 2 3 4 5 6 7 8
0 0 0 0 0 0 0 0

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💀 Defect reports 💀

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• DR LWG 3870 — Applied to: C++20. Behavior as published: this algorithm might create objects on a const storage. Correct behavior: kept disallowed.

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👑 See also 👑

• ranges::uninitialized_value_construct (C++20) — constructs objects by value-initialization in an uninitialized area of memory, defined by a range (niebloid).
• ranges::uninitialized_default_construct (C++20) — constructs objects by default-initialization in an uninitialized area of memory, defined by a range (niebloid).
• ranges::uninitialized_default_construct_n (C++20) — constructs objects by default-initialization in an uninitialized area of memory, defined by a start and count (niebloid).
• uninitialized_value_construct_n (C++17) — constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count (function template).

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💜 that’s the whole page, bestie! source/version/date sign-off: http://cppreference.com, 2024.06.10, and the page tag std::ranges...construct_n(3). bye! 💜