std::ranges::uninitialized_fill_n
From cppreference.com
| Defined in header <memory>
|
||
| Call signature |
||
template< /*nothrow-forward-iterator*/ I, class T >
requires std::constructible_from<std::iter_value_t<I>, const T&>
I uninitialized_fill_n( I first, std::iter_difference_t<I> count,
const T& value );
|
(1) | (since C++20) (constexpr since C++26) |
template< /*execution-policy*/ Ep, /*nothrow-random-access-iterator*/ I,
class T = std::iter_value_t<I> >
requires std::constructible_from<std::iter_value_t<I>, const T&>
I uninitialized_fill_n( Ep&& exec, I first, std::iter_difference_t<I> count,
const T& value );
|
(2) | (since C++26) |
For the definition of /*execution-policy*/, see this page; for the definition of other exposition-only concepts, see this page.
1) Constructs elements in the destination range
first + [0, count) with the given value value as if by
return ranges::uninitialized_fill(std::counted_iterator(first, count),
std::default_sentinel, value).base();
If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
2) Same as (1), but executed according to
policy.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.
Parameters
| first | - | the beginning of the range of the elements to initialize |
| count | - | number of elements to construct |
| value | - | the value to construct the elements with |
| policy | - | the execution policy to use |
Return value
As described above.
Exceptions
Any exception thrown on construction of the elements in the destination range.
2) During the execution process:
- If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
- If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).
Notes
An implementation may improve the efficiency of the ranges::uninitialized_fill_n (by using e.g. ranges::fill_n) if the value type of the output range is TrivialType.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_parallel_algorithm |
202506L |
(C++26) | Parallel range algorithms |
__cpp_lib_raw_memory_algorithms |
202411L |
(C++26) | constexpr for specialized <memory> algorithms, (1)
|
Possible implementation
struct uninitialized_fill_n_fn
{
template</*nothrow-forward-range*/ I, class T>
requires std::constructible_from<std::iter_value_t<I>, const T&>
constexpr I operator()(I first, std::iter_difference_t<I> count,
const T& value) const
{
I rollback{first};
try
{
for (; count-- > 0; ++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;
}
}
};
inline constexpr uninitialized_fill_n_fn uninitialized_fill_n{};
|
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
See also
(C++20) |
copies an object to an uninitialized area of memory, defined by a range (algorithm function object) |
| copies an object to an uninitialized area of memory, defined by a start and a count (function template) |