std::ranges::uninitialized_fill
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ranges::uninitialized_fill
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Defined in header
<memory>
Call signature
template< no-throw-forward-iterator I, no-throw-sentinel-for<I> S,
(1)
(since C++20) class T >
requires std::constructible_from <std::iter_value_t <I>, const T&>
(constexpr since C++26)
template< no-throw-forward-range R, class T >
(2)
(since C++20) requires std::constructible_from <ranges::range_value_t <R>,
const T&>
ranges::borrowed_iterator_t <R> uninitialized_fill( R&& r,
(constexpr since C++26)
1) Copies value to an uninitialized memory area
[first, last) as if by
for (; first != last; ++first)
::new (voidify (*first)) std::remove_reference_t <std::iter_reference_t <I>>(value);
return first;
If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
2) Equivalent to return ranges::uninitialized_fill(ranges::begin (r), ranges::end (r), value);.
The function-like entities described on this page are algorithm function objects (informally known as 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.
Contents
[edit] Parameters
first, last
-
the iterator-sentinel pair defining 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
As described above.
[edit] Complexity
Linear in the size of the uninitialized memory area.
[edit] Exceptions
Any exception thrown on construction of the elements in the destination range.
[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.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_raw_memory_algorithms |
202411L |
(C++26) | constexpr for specialized memory algorithms, (1,2) |
[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&> constexpr I operator()(I first, S last, const T& value) const { I rollback{first}; try { for (; !(first == last); ++first) ranges::construct_at (std::addressof (*first), value); 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&> constexpr ranges::borrowed_iterator_t <R> operator()(R&& r, const T& value) const { return (*this)(ranges::begin (r), ranges::end (r), value); } }; 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)
(algorithm function object)[edit]