Difference between revisions of "cpp/memory/ranges/uninitialized fill n"
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m (→Exceptions: +) |
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{{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 | since=c++20 |1= | + | {{dcl|since=c++20|1= |
| − | template <no-throw-forward-range I, class T> | + | template< no-throw-forward-range I, class T > |
requires std::constructible_from<std::iter_value_t<I>, const T&> | requires std::constructible_from<std::iter_value_t<I>, const T&> | ||
I uninitialized_fill_n( I first, std::iter_difference_t<I> n, const T& x ); | I uninitialized_fill_n( I first, std::iter_difference_t<I> n, const T& x ); | ||
| Line 11: | Line 11: | ||
{{dcl end}} | {{dcl end}} | ||
| − | Constructs {{ | + | Constructs {{c|n}} copies of the given value {{c|x}} in an uninitialized memory area, designated by the range {{range|first|first + n}}, as if by |
{{source|1= | {{source|1= | ||
for (; n--; ++first) { | for (; n--; ++first) { | ||
| − | ::new | + | ::new (static_cast<void*>(std::addressof(*first))) |
| − | + | std::remove_reference_t<std::iter_reference_t<I>>(x); | |
| − | + | ||
} | } | ||
}} | }} | ||
| Line 26: | Line 25: | ||
===Parameters=== | ===Parameters=== | ||
{{par begin}} | {{par begin}} | ||
| − | {{par | first | the beginning of the range of the elements to initialize}} | + | {{par|first|the beginning of the range of the elements to initialize}} |
| − | {{par | n | number of elements to construct}} | + | {{par|n|number of elements to construct}} |
| − | {{par | x | the value to construct the elements with}} | + | {{par|x|the value to construct the elements with}} |
{{par end}} | {{par end}} | ||
| Line 41: | Line 40: | ||
===Notes=== | ===Notes=== | ||
| − | An implementation may improve the efficiency of the {{tt|ranges::uninitialized_fill_n}}, e.g. by using {{ | + | An implementation may improve the efficiency of the {{tt|ranges::uninitialized_fill_n}}, e.g. by using {{lc|ranges::fill_n}}, if the value type of the output range is {{named req|TrivialType}}. |
===Possible implementation=== | ===Possible implementation=== | ||
| − | {{eq fun | 1= | + | {{eq fun|1= |
| − | struct uninitialized_fill_n_fn { | + | struct uninitialized_fill_n_fn |
| − | template <no-throw-forward-range I, class T> | + | { |
| + | template<no-throw-forward-range I, class T> | ||
requires std::constructible_from<std::iter_value_t<I>, const T&> | requires std::constructible_from<std::iter_value_t<I>, const T&> | ||
| − | I operator()( I first, std::iter_difference_t<I> n, const T& x ) const { | + | I operator()(I first, std::iter_difference_t<I> n, const T& x) const |
| − | I rollback {first}; | + | { |
| − | try { | + | I rollback{first}; |
| + | try | ||
| + | { | ||
for (; n-- > 0; ++first) | for (; n-- > 0; ++first) | ||
ranges::construct_at(std::addressof(*first), x); | ranges::construct_at(std::addressof(*first), x); | ||
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 73: | Line 77: | ||
int main() | int main() | ||
{ | { | ||
| − | constexpr int n {3}; | + | constexpr int n{3}; |
alignas(alignof(std::string)) char out[n * sizeof(std::string)]; | alignas(alignof(std::string)) char out[n * sizeof(std::string)]; | ||
try | try | ||
{ | { | ||
| − | auto first {reinterpret_cast<std::string*>(out)}; | + | auto first{reinterpret_cast<std::string*>(out)}; |
auto last = std::ranges::uninitialized_fill_n(first, n, "cppreference"); | auto last = std::ranges::uninitialized_fill_n(first, n, "cppreference"); | ||
| − | for (auto it {first}; it != last; ++it) | + | for (auto it{first}; it != last; ++it) |
std::cout << *it << '\n'; | std::cout << *it << '\n'; | ||
| − | |||
std::ranges::destroy(first, last); | std::ranges::destroy(first, last); | ||
} | } | ||
| − | catch(...) | + | catch (...) |
{ | { | ||
std::cout << "Exception!\n"; | std::cout << "Exception!\n"; | ||
} | } | ||
} | } | ||
| − | | output= | + | |output= |
cppreference | cppreference | ||
cppreference | cppreference | ||
cppreference | cppreference | ||
}} | }} | ||
| + | |||
| + | ===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_fill}} | + | {{dsc inc|cpp/memory/ranges/dsc uninitialized_fill}} |
| − | {{dsc inc | cpp/memory/dsc uninitialized_fill_n}} | + | {{dsc inc|cpp/memory/dsc uninitialized_fill_n}} |
{{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 19:15, 25 July 2023
| Defined in header <memory>
|
||
| Call signature |
||
| template< no-throw-forward-range I, class T > requires std::constructible_from<std::iter_value_t<I>, const T&> |
(since C++20) | |
Constructs n copies of the given value x in an uninitialized memory area, designated by the range [first, first + n), as if by
for (; n--; ++first) { ::new (static_cast<void*>(std::addressof(*first))) std::remove_reference_t<std::iter_reference_t<I>>(x); }
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.
Contents |
[edit] Parameters
| first | - | the beginning of the range of the elements to initialize |
| n | - | number of elements to construct |
| x | - | the value to construct the elements with |
[edit] Return value
An iterator equal to first + n.
[edit] Complexity
Linear in n.
[edit] Exceptions
The exception thrown on construction of the elements in the destination range, if any.
[edit] Notes
An implementation may improve the efficiency of the ranges::uninitialized_fill_n, e.g. by using ranges::fill_n, if the value type of the output range is TrivialType.
[edit] Possible implementation
struct uninitialized_fill_n_fn { template<no-throw-forward-range I, class T> requires std::constructible_from<std::iter_value_t<I>, const T&> I operator()(I first, std::iter_difference_t<I> n, const T& x) const { I rollback{first}; try { for (; n-- > 0; ++first) ranges::construct_at(std::addressof(*first), x); return first; } catch (...) // rollback: destroy constructed elements { for (; rollback != first; ++rollback) ranges::destroy_at(std::addressof(*rollback)); throw; } } }; inline constexpr uninitialized_fill_n_fn uninitialized_fill_n{}; |
[edit] Example
Run this code
#include <iostream> #include <memory> #include <string> int main() { constexpr int n{3}; alignas(alignof(std::string)) char out[n * sizeof(std::string)]; try { auto first{reinterpret_cast<std::string*>(out)}; auto last = std::ranges::uninitialized_fill_n(first, n, "cppreference"); for (auto it{first}; it != last; ++it) std::cout << *it << '\n'; std::ranges::destroy(first, last); } catch (...) { std::cout << "Exception!\n"; } }
Output:
cppreference cppreference cppreference
[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
| (C++20) |
copies an object to an uninitialized area of memory, defined by a range (niebloid) |
| copies an object to an uninitialized area of memory, defined by a start and a count (function template) |
