NAME¶

std::ranges::uninitialized_default_construct - std::ranges::uninitialized_default_construct

Synopsis¶

Defined in header <memory>
Call signature
template< no-throw-forward-iterator I, no-throw-sentinel-for<I> S >
(since
requires std::default_initializable<std::iter_value_t<I>> (1) C++20)

I uninitialized_default_construct( I first, S last );
template< no-throw-forward-range R >

requires std::default_initializable<ranges::range_value_t<R>> (2) (since
ranges::borrowed_iterator_t<R> C++20)

uninitialized_default_construct( R&& r );

1) Constructs objects of type std::iter_value_t<I> in the uninitialized storage
designated by the range [first, last) by default-initialization, as if by

for (; first != last; ++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.
2) Same as (1), but uses r as the range, as if using ranges::begin(r) as first, and
ranges::end(r) as last.

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

Return value¶

An iterator equal to last.

Complexity¶

Linear in the distance between first and last.

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_fn
{
template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S>
requires std::default_initializable<std::iter_value_t<I>>
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::next(first, last); // skip initialization
I rollback{first};
try
{
for (; !(first == last); ++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;
}
}

template<no-throw-forward-range R>
requires std::default_initializable<ranges::range_value_t<R>>
ranges::borrowed_iterator_t<R>
operator()(R&& r) const
{
return (*this)(ranges::begin(r), ranges::end(r));
}
};

inline constexpr uninitialized_default_construct_fn uninitialized_default_construct{};

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{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:
// for (const char c : v) { std::cout << c << ' '; }
for (const char c : sample)
std::cout << c << ' ';
}
else
std::cout << "Unspecified\n";
}

Possible output:¶

1 ▄▀▄▀▄▀▄▀
2 ▄▀▄▀▄▀▄▀
3 ▄▀▄▀▄▀▄▀
4 ▄▀▄▀▄▀▄▀
A B C D

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