std::ranges::uninitialized_fill_n() algorithm
- od C++20
- Simplified
- Detailed
// (1)
I uninitialized_fill_n( I first, std::iter_difference_t<I> n, const T& x );
The type of arguments are generic and have following constraints:
I
-no-throw-forward-range
T
- (none)
Additionally, each overload has the following constraints:
- (1)
std::constructible_from<std::iter_value_t<I>, const T& >
// (1)
template< no-throw-forward-range 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> n, const T& x );
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.
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. |
Return value
An iterator equal to first + n
.
Complexity
Linear in n
.
Exceptions
The exception thrown on construction of the elements in the destination range, if any.
Possible implementation
uninitialized_fill_n(1)
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{};
Examples
#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";
}
}
cppreference
cppreference
cppreference
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