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std::ranges::uninitialized_value_construct

Defined in header <memory>

Call signature

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>> ranges::borrowed_iterator_t<R> uninitialized_value_construct( R&& r );	(2)	(since C++20)

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>>
ranges::borrowed_iterator_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 [first, last) by value-initialization, as if by

for (; first != last; ++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.

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:

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, 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.

Complexity

Linear in the distance between first and last.

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, 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};
        try
        {
            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)
                ranges::destroy_at(std::addressof(*rollback));
            throw;
        }
    }
 
    template<no-throw-forward-range R>
    requires std::default_initializable<ranges::range_value_t<R>>
    ranges::borrowed_iterator_t<R>
    operator()(R&& r) const
    {
        return (*this)(ranges::begin(r), ranges::end(r));
    }
};
 
inline constexpr uninitialized_value_construct_fn uninitialized_value_construct{};

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{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';
}

Output:

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

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