std::ranges::uninitialized_default_construct

Header: <memory>

Constructs objects of type std::iter_value_t in the uninitialized memory area [first,last) by default-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_default_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_default_construct( R&& r );

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

# Parameters

first, last: iterator-sentinel pair denoting the range of the elements to initialize
r: the range of the elements to initialize

# 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{first + n};
 
        std::ranges::uninitialized_default_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_default_construct
    // generally does not zero-fill the given uninitialized memory area.
    constexpr char sample[]{'A', 'B', 'C', 'D', '\n'};
    char v[]{'A', 'B', 'C', 'D', '\n'};
    std::ranges::uninitialized_default_construct(std::begin(v), std::end(v));
    if (std::memcmp(v, sample, sizeof(v)) == 0)
    {
        std::cout << "  ";
        // Maybe undefined behavior, pending CWG 1997 to be resolved:
        // for (const char c : v) { std::cout << c << ' '; }
        for (const char c : sample)
            std::cout << c << ' ';
    }
    else
        std::cout << "Unspecified\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