std::ranges::uninitialized_value_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_value_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 value-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:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

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 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

Run this code

#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';
}

Output:

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

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

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