std::ranges::uninitialized_default_construct_n

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Defined in header <memory>
Call signature
template< no-throw-forward-iterator I >

requires std::default_initializable<std::iter_value_t<I>>

I uninitialized_default_construct_n( I first, std::iter_difference_t<I> n );
 (since C++20)

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>>;

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:

In practice, they may be implemented as function objects, or with special compiler extensions.

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.

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.

Possible implementation

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{};

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 = 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 sample[]{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, sample, sizeof(v)) == 0)
    {
        // Maybe undefined behavior, pending CWG 1997:
        // for (const int i : v) { std::cout << i << ' '; }
        for (const int i : sample)
            std::cout << i << ' ';
    }
    else
        std::cout << "Unspecified!";
    std::cout << '\n';
}

Possible output: 

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

Defect reports

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

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

See also

(C++20)
 constructs objects by default-initialization in an uninitialized area of memory, defined by a range
(niebloid)
(C++20)
 constructs objects by value-initialization in an uninitialized area of memory, defined by a range
(niebloid)
(C++20)
 constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count
(niebloid)
(C++17)
 constructs objects by default-initialization in an uninitialized area of memory, defined by a start and a count
(function template)
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