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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 (const_cast<void*>(static_cast<const volatile 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.

Contents

[edit] Parameters

first - the beginning of the range of elements to initialize
n - the number of elements to construct

[edit] Return value

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

[edit] Complexity

Linear in n.

[edit] Exceptions

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

[edit] 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.

[edit] 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{};

[edit] Example

#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';
}

Possible output:

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

[edit] See also

constructs objects by default-initialization in an uninitialized area of memory, defined by a range
(niebloid) [edit]
constructs objects by value-initialization in an uninitialized area of memory, defined by a range
(niebloid) [edit]
constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count
(niebloid) [edit]
constructs objects by default-initialization in an uninitialized area of memory, defined by a start and a count
(function template) [edit]