std::ranges::uninitialized_value_construct

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) (constexpr since C++26)
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) (constexpr since C++26)

1) Constructs objects of type std::iter_value_t<I> in the uninitialized memory area [first, last) by value-initialization, as if by

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

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.

2) Equivalent to ranges::uninitialized_value_construct(ranges::begin(r), ranges::end(r)).

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

Return value

As described above.

Complexity

Linear in the distance between first and last.

Exceptions

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

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 a CopyAssignable TrivialType.

Feature-test macro	Value	Std	Feature
`__cpp_lib_raw_memory_algorithms`	`202411L`	(C++26)	constexpr for specialized memory algorithms, (1,2)

Possible implementation

struct uninitialized_value_construct_fn
{
    template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S>
        requires std::value_initializable<std::iter_value_t<I>>
    constexpr I operator()(I first, S last) const
    {
        using ValueType = std::remove_reference_t<std::iter_reference_t<I>>;
        if constexpr (std::is_trivially_default_constructible_v<ValueType>)
            return ranges::fill(first, last, ValueType());
        I rollback{first};
        try
        {
            for (; !(first == last); ++first)
                ::new (static_cast<void*>(std::addressof(*first))) ValueType();
            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>>
    constexpr 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

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{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

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

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Diagnostics library
Memory management library
Metaprogramming library (C++11)
General utilities library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Strings library
Text processing library
Numerics library
Date and time library
Input/output library
Filesystem library (C++17)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

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)