Difference between revisions of "cpp/memory/ranges/uninitialized default construct"
From cppreference.com
m (→Parameters: I-S pair) |
m (→Example: ~) |
||
| (3 intermediate revisions by 2 users not shown) | |||
| Line 2: | Line 2: | ||
{{cpp/memory/navbar}} | {{cpp/memory/navbar}} | ||
{{dcl begin}} | {{dcl begin}} | ||
| − | {{dcl header | memory}} | + | {{dcl header|memory}} |
| − | {{dcl h | Call signature}} | + | {{dcl h|Call signature}} |
| − | {{dcl | num=1 | since=c++20 |1= | + | {{dcl|num=1|since=c++20|1= |
| − | template <no-throw-forward-iterator I, no-throw-sentinel-for<I> S> | + | template< no-throw-forward-iterator I, no-throw-sentinel-for<I> S > |
requires std::default_initializable<std::iter_value_t<I>> | requires std::default_initializable<std::iter_value_t<I>> | ||
I uninitialized_default_construct( I first, S last ); | I uninitialized_default_construct( I first, S last ); | ||
}} | }} | ||
| − | {{dcl | num=2 | since=c++20 |1= | + | {{dcl|num=2|since=c++20|1= |
| − | template <no-throw-forward-range R> | + | template< no-throw-forward-range R > |
requires std::default_initializable<ranges::range_value_t<R>> | requires std::default_initializable<ranges::range_value_t<R>> | ||
ranges::borrowed_iterator_t<R> | ranges::borrowed_iterator_t<R> | ||
| Line 17: | Line 17: | ||
{{dcl end}} | {{dcl end}} | ||
| − | @1@ Constructs objects of type {{c|std::iter_value_t<I>}} in the uninitialized storage designated by the range {{ | + | @1@ Constructs objects of type {{c|std::iter_value_t<I>}} in the uninitialized storage designated by the range {{range|first|last}} by [[cpp/language/default_initialization|default-initialization]], as if by |
{{source|1= | {{source|1= | ||
for (; first != last; ++first) | 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. | + | @@ If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order. |
| − | @2@ Same as {{v|1}}, but uses {{ | + | @2@ Same as {{v|1}}, but uses {{c|r}} as the range, as if using {{c|ranges::begin(r)}} as {{c|first}}, and {{c|ranges::end(r)}} as {{c|last}}. |
{{cpp/ranges/niebloid}} | {{cpp/ranges/niebloid}} | ||
| Line 32: | Line 32: | ||
===Parameters=== | ===Parameters=== | ||
{{par begin}} | {{par begin}} | ||
| − | {{par | first, last | iterator-sentinel pair denoting the range of the elements to initialize}} | + | {{par|first, last|iterator-sentinel pair denoting the range of the elements to initialize}} |
| − | {{par | r | the range of the elements to initialize}} | + | {{par|r|the range of the elements to initialize}} |
{{par end}} | {{par end}} | ||
===Return value=== | ===Return value=== | ||
| − | An iterator equal to {{ | + | An iterator equal to {{c|last}}. |
===Complexity=== | ===Complexity=== | ||
| − | Linear in the distance between {{ | + | Linear in the distance between {{c|first}} and {{c|last}}. |
===Exceptions=== | ===Exceptions=== | ||
| Line 49: | Line 49: | ||
===Possible implementation=== | ===Possible implementation=== | ||
| − | {{eq fun | 1= | + | {{eq fun|1= |
| − | struct uninitialized_default_construct_fn { | + | struct uninitialized_default_construct_fn |
| − | template <no-throw-forward-iterator I, no-throw-sentinel-for<I> S> | + | { |
| + | template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S> | ||
requires std::default_initializable<std::iter_value_t<I>> | requires std::default_initializable<std::iter_value_t<I>> | ||
| − | I operator()( I first, S last ) const { | + | I operator()(I first, S last) const |
| + | { | ||
using ValueType = std::remove_reference_t<std::iter_reference_t<I>>; | using ValueType = std::remove_reference_t<std::iter_reference_t<I>>; | ||
if constexpr (std::is_trivially_default_constructible_v<ValueType>) | if constexpr (std::is_trivially_default_constructible_v<ValueType>) | ||
return ranges::next(first, last); // skip initialization | return ranges::next(first, last); // skip initialization | ||
| − | I rollback {first}; | + | I rollback{first}; |
| − | try { | + | try |
| + | { | ||
for (; !(first == last); ++first) | for (; !(first == last); ++first) | ||
::new (const_cast<void*>(static_cast<const volatile void*> | ::new (const_cast<void*>(static_cast<const volatile void*> | ||
| − | + | (std::addressof(*first)))) ValueType; | |
return first; | return first; | ||
| − | } catch (...) | + | } |
| + | catch (...) // rollback: destroy constructed elements | ||
| + | { | ||
for (; rollback != first; ++rollback) | for (; rollback != first; ++rollback) | ||
ranges::destroy_at(std::addressof(*rollback)); | ranges::destroy_at(std::addressof(*rollback)); | ||
| Line 70: | Line 75: | ||
} | } | ||
| − | template <no-throw-forward-range R> | + | template<no-throw-forward-range R> |
requires std::default_initializable<ranges::range_value_t<R>> | requires std::default_initializable<ranges::range_value_t<R>> | ||
ranges::borrowed_iterator_t<R> | ranges::borrowed_iterator_t<R> | ||
| − | operator()( R&& r ) const { | + | operator()(R&& r) const |
| + | { | ||
return (*this)(ranges::begin(r), ranges::end(r)); | return (*this)(ranges::begin(r), ranges::end(r)); | ||
} | } | ||
| Line 82: | Line 88: | ||
===Example=== | ===Example=== | ||
| − | {{example|code= | + | {{example |
| + | |code= | ||
#include <cstring> | #include <cstring> | ||
#include <iostream> | #include <iostream> | ||
| Line 90: | Line 97: | ||
int main() | int main() | ||
{ | { | ||
| − | struct S { std::string m{ "▄▀▄▀▄▀▄▀" }; }; | + | struct S { std::string m{"▄▀▄▀▄▀▄▀"}; }; |
| − | constexpr int n {4}; | + | constexpr int n{4}; |
alignas(alignof(S)) char out[n * sizeof(S)]; | alignas(alignof(S)) char out[n * sizeof(S)]; | ||
try | try | ||
{ | { | ||
| − | auto first {reinterpret_cast<S*>(out)}; | + | auto first{reinterpret_cast<S*>(out)}; |
| − | auto last {first + n}; | + | auto last{first + n}; |
std::ranges::uninitialized_default_construct(first, last); | std::ranges::uninitialized_default_construct(first, last); | ||
| − | auto count {1}; | + | auto count{1}; |
| − | for (auto it {first}; it != last; ++it) | + | for (auto it{first}; it != last; ++it) |
std::cout << count++ << ' ' << it->m << '\n'; | std::cout << count++ << ' ' << it->m << '\n'; | ||
| − | |||
std::ranges::destroy(first, last); | std::ranges::destroy(first, last); | ||
} | } | ||
| − | catch(...) { std::cout << "Exception!\n"; } | + | catch (...) { std::cout << "Exception!\n"; } |
// Notice that for "trivial types" the uninitialized_default_construct | // Notice that for "trivial types" the uninitialized_default_construct | ||
// generally does not zero-fill the given uninitialized memory area. | // generally does not zero-fill the given uninitialized memory area. | ||
| − | constexpr char | + | constexpr char sample[]{'A', 'B', 'C', 'D', '\n'}; |
| − | char v[] { 'A', 'B', 'C', 'D', '\n' }; | + | char v[]{'A', 'B', 'C', 'D', '\n'}; |
std::ranges::uninitialized_default_construct(std::begin(v), std::end(v)); | std::ranges::uninitialized_default_construct(std::begin(v), std::end(v)); | ||
| − | if (std::memcmp(v, | + | if (std::memcmp(v, sample, sizeof(v)) == 0) |
| + | { | ||
std::cout << " "; | std::cout << " "; | ||
// Maybe undefined behavior, pending CWG 1997: | // Maybe undefined behavior, pending CWG 1997: | ||
// for (const char c : v) { std::cout << c << ' '; } | // for (const char c : v) { std::cout << c << ' '; } | ||
| − | for (const char c : | + | for (const char c : sample) |
| − | + | std::cout << c << ' '; | |
| − | + | ||
} | } | ||
| + | else | ||
| + | std::cout << "Unspecified\n"; | ||
} | } | ||
| − | | p=true | + | |p=true |
| − | | output= | + | |output= |
1 ▄▀▄▀▄▀▄▀ | 1 ▄▀▄▀▄▀▄▀ | ||
2 ▄▀▄▀▄▀▄▀ | 2 ▄▀▄▀▄▀▄▀ | ||
| Line 133: | Line 141: | ||
A B C D | A B C D | ||
}} | }} | ||
| + | |||
| + | ===Defect reports=== | ||
| + | {{dr list begin}} | ||
| + | {{dr list item|wg=lwg|dr=3870|std=C++20|before=this algorithm might create objects on a {{c/core|const}} storage|after=kept disallowed}} | ||
| + | {{dr list end}} | ||
===See also=== | ===See also=== | ||
{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc inc | cpp/memory/ranges/dsc uninitialized_default_construct_n}} | + | {{dsc inc|cpp/memory/ranges/dsc uninitialized_default_construct_n}} |
| − | {{dsc inc | cpp/memory/ranges/dsc uninitialized_value_construct}} | + | {{dsc inc|cpp/memory/ranges/dsc uninitialized_value_construct}} |
| − | {{dsc inc | cpp/memory/ranges/dsc uninitialized_value_construct_n}} | + | {{dsc inc|cpp/memory/ranges/dsc uninitialized_value_construct_n}} |
| − | {{dsc inc | cpp/memory/dsc uninitialized_default_construct}} | + | {{dsc inc|cpp/memory/dsc uninitialized_default_construct}} |
{{dsc end}} | {{dsc end}} | ||
{{langlinks|de|es|fr|it|ja|pt|ru|zh}} | {{langlinks|de|es|fr|it|ja|pt|ru|zh}} | ||
Latest revision as of 05:35, 19 January 2024
| Defined in header <memory>
|
||
| Call signature |
||
| template< no-throw-forward-iterator I, no-throw-sentinel-for<I> S > requires std::default_initializable<std::iter_value_t<I>> |
(1) | (since C++20) |
| template< no-throw-forward-range R > requires std::default_initializable<ranges::range_value_t<R>> |
(2) | (since C++20) |
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.
Contents |
[edit] Parameters
| first, last | - | iterator-sentinel pair denoting the range of the elements to initialize |
| r | - | the range of the elements to initialize |
[edit] Return value
An iterator equal to last.
[edit] Complexity
Linear in the distance between first and last.
[edit] Exceptions
The exception thrown on construction of the elements in the destination range, if any.
[edit] 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.
[edit] 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{}; |
[edit] 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
[edit] 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 storage | kept disallowed |
[edit] See also
| constructs objects by default-initialization in an uninitialized area of memory, defined by a start and count (niebloid) | |
| constructs objects by value-initialization in an uninitialized area of memory, defined by a range (niebloid) | |
| constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count (niebloid) | |
| constructs objects by default-initialization in an uninitialized area of memory, defined by a range (function template) |
