NAME¶

std::ranges::uninitialized_default_construct_n - std::ranges::uninitialized_default_construct_n

Synopsis¶

Defined in header <memory>
Call signature
template< no-throw-forward-iterator I >

requires std::default_initializable<std::iter_value_t<I>> (since C++20)

I uninitialized_default_construct_n( I first,
std::iter_difference_t<I> n );

Constructs n objects of type std::iter_value_t<I> in the uninitialized memory area
starting at first by default-initialization, as if by

for (; n-- > 0; ++first)
::new (static_cast<void*>(std::addressof(*first)))
std::remove_reference_t<std::iter_reference_t<I>>;

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

The function-like entities described on this page are 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.

In practice, they may be implemented as function objects, or with special compiler
extensions.

Parameters¶

first - the beginning of the range of elements to initialize
n - the number of elements to construct

Return value¶

The end of the range of objects (i.e., ranges::next(first, n)).

Complexity¶

Linear in n.

Exceptions¶

The exception thrown on construction of the elements in the destination range, if
any.

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<I> object, which can be detected by
std::is_trivially_default_constructible_v.

Possible implementation¶

struct uninitialized_default_construct_n_fn {
template<no-throw-forward-iterator I>
requires std::default_initializable<std::iter_value_t<I>>
I operator()(I first, std::iter_difference_t<I> n) const
{
using ValueType = std::remove_reference_t<std::iter_reference_t<I>>;
if constexpr (std::is_trivially_default_constructible_v<ValueType>)
return ranges::next(first, n); // skip initialization
I rollback{first};
try
{
for (; n-- > 0; ++first)
::new (const_cast<void*>(static_cast<const volatile void*>
(std::addressof(*first)))) ValueType;
return first;
}
catch (...) // rollback: destroy constructed elements
{
for (; rollback != first; ++rollback)
ranges::destroy_at(std::addressof(*rollback));
throw;
}
} };

inline constexpr uninitialized_default_construct_n_fn uninitialized_default_construct_n{};

Example¶

// Run this code

#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";
}

// Notice that for "trivial types" the 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';
}

Possible output:¶

1 █▓▒░ █▓▒░
2 █▓▒░ █▓▒░
3 █▓▒░ █▓▒░
4 █▓▒░ █▓▒░
1 2 3 4 5 6

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 kept disallowed
storage

See also¶

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