std::ranges::uninitialized_value_construct

Defined in header `<memory>`
Call signature
template< /nothrow-forward-iterator/ I, /nothrow-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) (constexpr since C++26)
template< /nothrow-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) (constexpr since C++26)
template< /execution-policy/ Ep, /nothrow-random-access-iterator/ I, /nothrow-sized-sentinel-for/<I> S > requires std::default_initializable<std::iter_value_t<I>> I uninitialized_value_construct( Ep&& policy, I first, S last );	(3)	(since C++26)
template< /execution-policy/ Ep, /nothrow-sized-random-access-range/ R > requires std::default_initializable<ranges::range_value_t<R>> ranges::borrowed_iterator_t<R> uninitialized_value_construct( Ep&& policy, R&& r );	(4)	(since C++26)

For the definition of /*execution-policy*/, see this page; for the definition of other exposition-only concepts, see this page.

1) Constructs elements in the destination range [first, last) by value-initialization as if by

for(;first!=last;++first) ::new(voidify(*first)) std::remove_reference_t<std::iter_reference_t<I>>(); returnfirst;

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 destination range.

3,4) Same as (1,2), but executed according to policy.

The function-like entities described on this page are algorithm function objects (informally known as 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.

Parameters

first, last	-	the iterator-sentinel pair defining the range of elements to value-initialize
r	-	the `range` of the elements to value-initialize
policy	-	the execution policy to use

Return value

As described above.

Exceptions

Any exception thrown on construction of the elements in the destination range.

3,4) During the execution process:

If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

Notes

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

Feature-test macro	Value	Std	Feature
`__cpp_lib_parallel_algorithm`	`202506L`	(C++26)	Parallel range algorithms
`__cpp_lib_raw_memory_algorithms`	`202411L`	(C++26)	constexpr for specialized `<memory>` algorithms, (1,2)

Possible implementation

struct uninitialized_value_construct_fn
{
    template</*nothrow-forward-iterator*/ I, /*nothrow-sentinel-for*/<I> S>
        requires std::value_initializable<std::iter_value_t<I>>
    constexpr I operator()(I first, S last) const
    {
        using value_type = std::remove_reference_t<std::iter_reference_t<I>>;
        if constexpr (std::is_trivially_default_constructible_v<value_type>)
            return ranges::fill(first, last, ValueType());
        I rollback{first};
        try
        {
            for (; !(first == last); ++first)
                ::new (static_cast<void*>(std::addressof(*first))) value_type();
            return first;
        }
        catch (...) // rollback: destroy constructed elements
        {
            for (; rollback != first; ++rollback)
                ranges::destroy_at(std::addressof(*rollback));
            throw;
        }
    }
    
    template</*nothrow-forward-range*/ R>
        requires std::default_initializable<ranges::range_value_t<R>>
    constexpr ranges::borrowed_iterator_t<R> operator()(R&& r) const
    {
        return (*this)(ranges::begin(r),
                       ranges::next(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";
    }
    
    // For scalar types, 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

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