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std::ranges::uninitialized_default_construct() algorithm

// (1)
I uninitialized_default_construct( I first, S last );

// (2)
ranges::borrowed_iterator_t<R>
uninitialized_default_construct( R&& r );

The type of arguments are generic and have following constraints:

  • I - no-throw-forward-iterator
  • S - no-throw-sentinel-for<I>
  • R - no-throw-forward-range

Additionally, each overload has the following constraints:

  • (1) std::default_initializable<std::iter_value_t<I>>
  • (2) std::default_initializable<ranges::range_value_t<R>>

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>>;
uwaga

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.

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.

Possible implementation

uninitialized_default_construct(1)
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{};

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.

Examples

Main.cpp
#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 etalon[]{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, etalon, sizeof(v)) == 0)
{
// Maybe undefined behavior, pending CWG 1997:
// for (const int i : v) { std::cout << i << ' '; }
for (const int i : etalon)
std::cout << i << ' ';
}
else
std::cout << "Unspecified!";
std::cout << '\n';
}
Output
1 █▓▒░ █▓▒░
2 █▓▒░ █▓▒░
3 █▓▒░ █▓▒░
4 █▓▒░ █▓▒░
1 2 3 4 5 6
This article originates from this CppReference page. It was likely altered for improvements or editors' preference. Click "Edit this page" to see all changes made to this document.
Hover to see the original license.

std::ranges::uninitialized_default_construct() algorithm

// (1)
I uninitialized_default_construct( I first, S last );

// (2)
ranges::borrowed_iterator_t<R>
uninitialized_default_construct( R&& r );

The type of arguments are generic and have following constraints:

  • I - no-throw-forward-iterator
  • S - no-throw-sentinel-for<I>
  • R - no-throw-forward-range

Additionally, each overload has the following constraints:

  • (1) std::default_initializable<std::iter_value_t<I>>
  • (2) std::default_initializable<ranges::range_value_t<R>>

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>>;
uwaga

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.

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.

Possible implementation

uninitialized_default_construct(1)
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{};

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.

Examples

Main.cpp
#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 etalon[]{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, etalon, sizeof(v)) == 0)
{
// Maybe undefined behavior, pending CWG 1997:
// for (const int i : v) { std::cout << i << ' '; }
for (const int i : etalon)
std::cout << i << ' ';
}
else
std::cout << "Unspecified!";
std::cout << '\n';
}
Output
1 █▓▒░ █▓▒░
2 █▓▒░ █▓▒░
3 █▓▒░ █▓▒░
4 █▓▒░ █▓▒░
1 2 3 4 5 6
This article originates from this CppReference page. It was likely altered for improvements or editors' preference. Click "Edit this page" to see all changes made to this document.
Hover to see the original license.