std::ranges::uninitialized_value_construct

Header: <memory>

Constructs objects of type std::iter_value_t 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>(); return first;

# Declarations

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 );

(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 );

(since C++20) (constexpr since C++26)

# 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.

# 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.

# 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';
}

# Defect reports

DR	Applied to	Behavior as published	Correct behavior
LWG 3870	C++20	this algorithm might create objects on a const storage	kept disallowed