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 );

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

for (; n-- > 0; ++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.

The function-like entities described on this page are niebloids.

Parameters

first	The beginning of the range of elements to initialize.
n	The number of elements to construct.

Return value

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

Complexity

Linear in n.

Exceptions

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

Possible implementation

uninitialized_default_construct(1)

struct uninitialized_default_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 ValueType = std::remove_reference_t<std::iter_reference_t<I>>;
        if constexpr (std::is_trivially_default_constructible_v<ValueType>)
            return ranges::next(first, n); // skip initialization
        I rollback{first};
        try
        {
            for (; n-- > 0; ++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;
        }
    }
};

inline constexpr uninitialized_default_construct_n_fn uninitialized_default_construct_n{};

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 = std::ranges::uninitialized_default_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_default_construct_n
    // generally does not zero-fill the given uninitialized memory area.
    constexpr int etalon[]{1, 2, 3, 4, 5, 6};
    int v[]{1, 2, 3, 4, 5, 6};
    std::ranges::uninitialized_default_construct_n(std::begin(v), std::size(v));
    if (std::memcmp(v, etalon, sizeof(v)) == 0)
    {
        // Maybe undefined behavior, pending CWG 1997:
        // for (const int i : v) { std::cout << i << ' '; }
        for (const int i : etalon)
            std::cout << i << ' ';
    }
    else
        std::cout << "Unspecified!";
    std::cout << '\n';
}

Output

1 █▓▒░ █▓▒░
2 █▓▒░ █▓▒░
3 █▓▒░ █▓▒░
4 █▓▒░ █▓▒░
1 2 3 4 5 6