std::ranges::uninitialized_fill
From cppreference.com
| Defined in header <memory>
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| Call signature |
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| template< no-throw-forward-iterator I, no-throw-sentinel-for<I> S, class T > requires std::constructible_from<std::iter_value_t<I>, const T&> |
(1) | (since C++20) |
| template< no-throw-forward-range R, class T > requires std::constructible_from<ranges::range_value_t<R>, const T&> |
(2) | (since C++20) |
1) Constructs N copies of the given value x in an uninitialized memory area, designated by the range
[first, last), where N is ranges::distance(first, last). The function has the effect equivalent to:
for (; first != last; ++first) ::new (static_cast<void*>(std::addressof(*first))) std::remove_reference_t<std::iter_reference_t<I>>(x); return first;
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 elements to initialize |
| r | - | the range of the elements to initialize |
| value | - | the value to construct the elements with |
[edit] Return value
An iterator equal to last.
[edit] Complexity
𝓞(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, e.g. by using ranges::fill, if the value type of the output range is TrivialType.
[edit] Possible implementation
struct uninitialized_fill_fn { template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S, class T> requires std::constructible_from<std::iter_value_t<I>, const T&> I operator()(I first, S last, const T& x) const { I rollback{first}; try { for (; !(first == last); ++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; } } template<no-throw-forward-range R, class T> requires std::constructible_from<ranges::range_value_t<R>, const T&> ranges::borrowed_iterator_t<R> operator()(R&& r, const T& x) const { return (*this)(ranges::begin(r), ranges::end(r), x); } }; inline constexpr uninitialized_fill_fn uninitialized_fill{}; |
[edit] Example
Run this code
#include <iostream> #include <memory> #include <string> int main() { constexpr int n{4}; alignas(alignof(std::string)) char out[n * sizeof(std::string)]; try { auto first{reinterpret_cast<std::string*>(out)}; auto last{first + n}; std::ranges::uninitialized_fill(first, last, "▄▀▄▀▄▀▄▀"); int count{1}; for (auto it{first}; it != last; ++it) std::cout << count++ << ' ' << *it << '\n'; std::ranges::destroy(first, last); } catch(...) { std::cout << "Exception!\n"; } }
Output:
1 ▄▀▄▀▄▀▄▀ 2 ▄▀▄▀▄▀▄▀ 3 ▄▀▄▀▄▀▄▀ 4 ▄▀▄▀▄▀▄▀
[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 start and a count (niebloid) |
| copies an object to an uninitialized area of memory, defined by a range (function template) |
