std::ranges::uninitialized_default_construct() algorithm

since C++20

Simplified

// (1)
I uninitialized_default_construct( I first, S last );

// (2)
ranges::borrowed_iterator_t<R>
  uninitialized_default_construct( R&& r );

The type of arguments are generic and have following constraints:

• I - no-throw-forward-iterator
• S - no-throw-sentinel-for<I>
• R - no-throw-forward-range

Additionally, each overload has the following constraints:

• (1) std::default_initializable<std::iter_value_t<I>>
• (2) std::default_initializable<ranges::range_value_t<R>>

Detailed

// (1)
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)
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>>;
  caution

  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.

Parameters

first last	The range of elements to initialize.
r	The range of elements to initialize.

Return value

An iterator equal to last.

Complexity

Linear in the distance between first and last.

Exceptions

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

Possible implementation

uninitialized_default_construct(1) and uninitialized_default_construct(2)

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{};

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.

Examples

Main.cpp

#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 etalon[]{'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, etalon, sizeof(v)) == 0)
    {
        std::cout << "  ";
        // Maybe undefined behavior, pending CWG 1997:
        // for (const char c : v) { std::cout << c << ' '; }
        for (const char c : etalon)
            std::cout << c << ' ';
    }
    else
        std::cout << "Unspecified\n";
}

Output

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