std::ranges::uninitialized_value_construct_n

template <no-throw-forward-iterator I>
requires std::default_initializable<std::iter_value_t<I>>
I uninitialized_value_construct_n( I first, std::iter_difference_t<I> n );

Constructs n objects of type std::iter_value_t<I> in the uninitialized memory area starting at first by value-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:

• Explicit template argument lists may not be specified when calling any of them.
• None of them is visible to argument-dependent lookup.
• When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.

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

Parameters

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 improve the efficiency of the ranges::uninitialized_value_construct_n, e.g. by using ranges::fill_n, if the value type of the range is TrivialType and CopyAssignable.

Possible implementation

struct uninitialized_value_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 T = std::remove_reference_t<std::iter_reference_t<I>>;
        if constexpr (std::is_trivial_v<T> && std::is_copy_assignable_v<T>)
            return ranges::fill_n(first, n, T());
        I rollback {first};
        try {
            for (; n-- > 0; ++first)
                ::new (const_cast<void*>(static_cast<const volatile void*>
                        (std::addressof(*first)))) T();
            return first;
        } catch (...) { // rollback: destroy constructed elements
            for (; rollback != first; ++rollback)
                ranges::destroy_at(std::addressof(*rollback));
            throw;
        }
    }
};
 
inline constexpr uninitialized_value_construct_n_fn uninitialized_value_construct_n{};

Example

#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_value_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_value_construct_n
    // zero-initializes the given uninitialized memory area.
    int v[] { 1, 2, 3, 4, 5, 6, 7, 8 };
    std::cout << ' ';
    for (const int i : v) { std::cout << i << ' '; }
    std::cout << "\n ";
    std::ranges::uninitialized_value_construct_n(std::begin(v), std::size(v));
    for (const int i : v) { std::cout << i << ' '; }
    std::cout << '\n';
}

1 █▓▒░ █▓▒░ █▓▒░ 
2 █▓▒░ █▓▒░ █▓▒░ 
3 █▓▒░ █▓▒░ █▓▒░ 
4 █▓▒░ █▓▒░ █▓▒░
1 2 3 4 5 6 7 8
0 0 0 0 0 0 0 0

See also