Defined in header <memory>
Call signature
template< no-throw-forward-iterator I, no-throw-sentinel-for<I> S >
requires std::default_initializable<std::iter_value_t<I>>
I uninitialized_value_construct( I first, S last );
(1) (since C++20)
template< no-throw-forward-range R >
requires std::default_initializable<ranges::range_value_t<R>>
uninitialized_value_construct( R&& r );
(2) (since C++20)
1) Constructs objects of type std::iter_value_t<I> in the uninitialized storage designated by the range [firstlast) by value-initialization, as if by
for (; first != last; ++first)
    ::new (static_cast<void*>(std::addressof(*first)))
If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
2) Same as (1), but uses r as the range, as if using ranges::begin(r) as first, and ranges::end(r) as last.

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.


first, last - iterator-sentinel pair denoting the range of elements to value-initialize
r - the range of the elements to value-initialize

Return value

An iterator equal to last.


Linear in the distance between first and last.


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


An implementation may improve the efficiency of the ranges::uninitialized_value_construct, e.g. by using ranges::fill, if the value type of the range is TrivialType and CopyAssignable.

Possible implementation

struct uninitialized_value_construct_fn
    template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S>
    requires std::default_initializable<std::iter_value_t<I>>
    I operator()(I first, S last) 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(first, last, T());
        I rollback{first};
            for (; !(first == last); ++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)
    template<no-throw-forward-range R>
    requires std::default_initializable<ranges::range_value_t<R>>
    operator()(R&& r) const
        return (*this)(ranges::begin(r), ranges::end(r));
inline constexpr uninitialized_value_construct_fn uninitialized_value_construct{};


#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)];
        auto first{reinterpret_cast<S*>(out)};
        auto last{first + n};
        std::ranges::uninitialized_value_construct(first, last);
        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
    // zero-fills the given uninitialized memory area.
    int v[]{0, 1, 2, 3};
    std::cout << ' ';
    for (const int i : v)
        std::cout << ' ' << static_cast<char>(i + 'A');
    std::cout << "\n ";
    std::ranges::uninitialized_value_construct(std::begin(v), std::end(v));
    for (const int i : v)
        std::cout << ' ' << static_cast<char>(i + 'A');
    std::cout << '\n';


1 ▄▀▄▀▄▀▄▀
2 ▄▀▄▀▄▀▄▀
3 ▄▀▄▀▄▀▄▀
4 ▄▀▄▀▄▀▄▀
  A B C D
  A A A A

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

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

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