Przejdź do głównej zawartości

std::ranges::uninitialized_value_construct() algorithm

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

// (2)
ranges::borrowed_iterator_t<R>
uninitialized_value_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 value-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 the distance between first and last.

Exceptions

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

Possible implementation

uninitialized_value_construct(1)
struct uninitialized_value_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 T = std::remove_reference_t<std::iter_reference_t<I>>;
if constexpr (std::is_trivial_v<T> && std::is_copy_assignable_v<T>)
return ranges::fill_n(first, n, T());
I rollback{first};
try
{
for (; n-- > 0; ++first)
::new (const_cast<void*>(static_cast<const volatile void*>
(std::addressof(*first)))) T();
return first;
}
catch (...) // rollback: destroy constructed elements
{
for (; rollback != first; ++rollback)
ranges::destroy_at(std::addressof(*rollback));
throw;
}
}
};

inline constexpr uninitialized_value_construct_n_fn uninitialized_value_construct_n{};

Notes

An implementation may improve the efficiency of the ranges::uninitialized_value_construct, e.g. by using ranges::fill, if the value type of the range is TrivialType and CopyAssignable.

Examples

Main.cpp
#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_value_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_value_construct_n
// zero-initializes the given uninitialized memory area.
int v[]{1, 2, 3, 4, 5, 6, 7, 8};
std::cout << ' ';
for (const int i : v)
std::cout << i << ' ';
std::cout << "\n ";
std::ranges::uninitialized_value_construct_n(std::begin(v), std::size(v));
for (const int i : v)
std::cout << i << ' ';
std::cout << '\n';
}
Output
1 █▓▒░ █▓▒░ █▓▒░ 
2 █▓▒░ █▓▒░ █▓▒░
3 █▓▒░ █▓▒░ █▓▒░
4 █▓▒░ █▓▒░ █▓▒░
1 2 3 4 5 6 7 8
0 0 0 0 0 0 0 0
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_value_construct() algorithm

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

// (2)
ranges::borrowed_iterator_t<R>
uninitialized_value_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 value-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 the distance between first and last.

Exceptions

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

Possible implementation

uninitialized_value_construct(1)
struct uninitialized_value_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 T = std::remove_reference_t<std::iter_reference_t<I>>;
if constexpr (std::is_trivial_v<T> && std::is_copy_assignable_v<T>)
return ranges::fill_n(first, n, T());
I rollback{first};
try
{
for (; n-- > 0; ++first)
::new (const_cast<void*>(static_cast<const volatile void*>
(std::addressof(*first)))) T();
return first;
}
catch (...) // rollback: destroy constructed elements
{
for (; rollback != first; ++rollback)
ranges::destroy_at(std::addressof(*rollback));
throw;
}
}
};

inline constexpr uninitialized_value_construct_n_fn uninitialized_value_construct_n{};

Notes

An implementation may improve the efficiency of the ranges::uninitialized_value_construct, e.g. by using ranges::fill, if the value type of the range is TrivialType and CopyAssignable.

Examples

Main.cpp
#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_value_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_value_construct_n
// zero-initializes the given uninitialized memory area.
int v[]{1, 2, 3, 4, 5, 6, 7, 8};
std::cout << ' ';
for (const int i : v)
std::cout << i << ' ';
std::cout << "\n ";
std::ranges::uninitialized_value_construct_n(std::begin(v), std::size(v));
for (const int i : v)
std::cout << i << ' ';
std::cout << '\n';
}
Output
1 █▓▒░ █▓▒░ █▓▒░ 
2 █▓▒░ █▓▒░ █▓▒░
3 █▓▒░ █▓▒░ █▓▒░
4 █▓▒░ █▓▒░ █▓▒░
1 2 3 4 5 6 7 8
0 0 0 0 0 0 0 0
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.