std::ranges::uninitialized_default_construct_n

Header: <memory>

Constructs objects of type std::iter_value_t in the uninitialized memory area first+[0,count) by default-initialization, as if by return ranges::uninitialized_default_construct(std::counted_iterator(first, count),std::default_sentinel).base();

# Declarations

Call signature

template< no-throw-forward-iterator I >
requires std::default_initializable<std::iter_value_t<I>>
I uninitialized_default_construct_n( I first,
std::iter_difference_t<I> count );

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

# Parameters

first: the beginning of the range of elements to initialize
count: the number of elements to construct

# Return value

As described above.

# Notes

An implementation may skip the objects construction (without changing the observable effect) if no non-trivial default constructor is called while default-initializing a std::iter_value_t object, which can be detected by std::is_trivially_default_constructible.

# Example

#include <cstring>
#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 = std::ranges::uninitialized_default_construct_n(first, n);
 
        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_default_construct_n
    // generally does not zero-fill the given uninitialized memory area.
    constexpr int sample[]{1, 2, 3, 4, 5, 6};
    int v[]{1, 2, 3, 4, 5, 6};
    std::ranges::uninitialized_default_construct_n(std::begin(v), std::size(v));
    if (std::memcmp(v, sample, sizeof(v)) == 0)
    {
        // Maybe undefined behavior, pending CWG 1997:
        // for (const int i : v) { std::cout << i << ' '; }
        for (const int i : sample)
            std::cout << i << ' ';
    }
    else
        std::cout << "Unspecified!";
    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