std::ranges...t_construct(3)

✨ NAME ✨

okay so bestie, gather round 🫶 — this is the manual page for std::ranges::uninitialized_default_construct, and the header we’re folding in near the top says std::ranges...t_construct(3), from the C++ Standard Libary manual. The NAME line itself is a whole echo: std::ranges::uninitialized_default_construct - std::ranges::uninitialized_default_construct. No cap, it really said it twice 💅.

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💫 Synopsis 💫

Defined in header <memory> ✨

Call signature 👇

(1) (since C++20)

template< no-throw-forward-iterator I, no-throw-sentinel-for<I> S >
    requires std::default_initializable<std::iter_value_t<I>>
I uninitialized_default_construct( I first, S last );

(2) (since C++20)

template< no-throw-forward-range R >
    requires std::default_initializable<ranges::range_value_t<R>>
ranges::borrowed_iterator_t<R>
uninitialized_default_construct( R&& r );

1) Constructs objects of type std::iter_value_t<I> in the uninitialized storage designated by the range [first, last) by default-initialization, as if by

for (; first != last; ++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 😭.

2) Same as (1), 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.

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🎀 Parameters 🎀

• first, last — iterator-sentinel pair denoting the range of the elements to initialize.
• r — the range of the elements to initialize.

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🦋 Return value 🦋

An iterator equal to last. periodt 💅

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🌸 Complexity 🌸

Linear in the distance between first and last.

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💥 Exceptions 💥

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

bestie, if construction throws, the already-built objects get destroyed in an unspecified order — it’s giving cleanup-on-fail energy 🫶

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📝 Notes 📝

An implementation may skip the objects construction (without changing the observable effect) if no non-trivial default constructor is called while default-initializing a std::iter_value_t<I> object, which can be detected by std::is_trivially_default_constructible_v.

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🛠️ Possible implementation 🛠️

struct uninitialized_default_construct_fn
{
    template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S>
    requires std::default_initializable<std::iter_value_t<I>>
    I operator()(I first, S last) const
    {
        using ValueType = std::remove_reference_t<std::iter_reference_t<I>>;
        if constexpr (std::is_trivially_default_constructible_v<ValueType>)
            return ranges::next(first, last); // skip initialization
        I rollback{first};
        try
        {
            for (; !(first == last); ++first)
                ::new (const_cast<void*>(static_cast<const volatile void*>
                    (std::addressof(*first)))) ValueType;
            return first;
        }
        catch (...) // rollback: destroy constructed elements
        {
            for (; rollback != first; ++rollback)
                ranges::destroy_at(std::addressof(*rollback));
            throw;
        }
    }

    template<no-throw-forward-range R>
    requires std::default_initializable<ranges::range_value_t<R>>
    ranges::borrowed_iterator_t<R>
    operator()(R&& r) const
    {
        return (*this)(ranges::begin(r), ranges::end(r));
    }
};

inline constexpr uninitialized_default_construct_fn uninitialized_default_construct{};

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🧪 Example 🧪

// Run this code

#include <cstring>
#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{first + n};

        std::ranges::uninitialized_default_construct(first, last);

        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_default_construct
    // generally does not zero-fill the given uninitialized memory area.
    constexpr char sample[]{'A', 'B', 'C', 'D', '\n'};
    char v[]{'A', 'B', 'C', 'D', '\n'};
    std::ranges::uninitialized_default_construct(std::begin(v), std::end(v));
    if (std::memcmp(v, sample, sizeof(v)) == 0)
    {
        std::cout << "  ";
        // Maybe undefined behavior, pending CWG 1997:
        // for (const char c : v) { std::cout << c << ' '; }
        for (const char c : sample)
            std::cout << c << ' ';
    }
    else
        std::cout << "Unspecified\n";
}

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🖨️ Possible output 🖨️

1 ▄▀▄▀▄▀▄▀
2 ▄▀▄▀▄▀▄▀
3 ▄▀▄▀▄▀▄▀
4 ▄▀▄▀▄▀▄▀
  A B C D

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🩹 Defect reports 🩹

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

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

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and that’s the whole page, bestie 🫶 source/version: http://cppreference.com · date: 2024.06.10 · page: std::ranges...t_construct(3) — now go default-initialize something iconic 💅✨