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.
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 |
Return value
An iterator equal to last.
Complexity
𝓞(N).
Exceptions
The exception thrown on construction of the elements in the destination range, if any.
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.
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{}; |
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 ▄▀▄▀▄▀▄▀
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 |
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) |