std::uninitialized_fill_n
From cppreference.com
| Defined in header <memory>
|
||
template< class NoThrowForwardIt, class Size, class T >
NoThrowForwardIt uninitialized_fill_n( NoThrowForwardIt first,
Size count, const T& value );
|
(1) | (constexpr since C++26) |
template< class ExecutionPolicy,
class NoThrowForwardIt, class Size, class T >
NoThrowForwardIt uninitialized_fill_n( ExecutionPolicy&& policy,
NoThrowForwardIt first,
Size count, const T& value );
|
(2) | (since C++17) |
1) Constructs the first
count elements in the destination range beginning at first with the given value value as if by
for (; count--; ++first)
::new (voidify(*first))
typename std::iterator_traits<NoThrowForwardIt>::value_type(value);
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 executed according to
policy. This overload participates in overload resolution only if the value of the following expression is
true:
|
|
(until C++20) |
|
|
(since C++20) |
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 |
| Type requirements | ||
-NoThrowForwardIt must meet the requirements of LegacyForwardIterator.
| ||
-No increment, assignment, comparison, or indirection through valid instances of NoThrowForwardIt may throw exceptions. Applying &* to a NoThrowForwardIt value must yield a pointer to its value type.(until C++11)
| ||
Return value
As described above.
Exceptions
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
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_raw_memory_algorithms |
202411L |
(C++26) | constexpr for specialized <memory> algorithms, (1)
|
Possible implementation
template<class NoThrowForwardIt, class Size, class T>
constexpr NoThrowForwardIt uninitialized_fill_n(NoThrowForwardIt first,
Size count, const T& value)
{
using V = typename std::iterator_traits<NoThrowForwardIt>::value_type;
NoThrowForwardIt current = first;
try
{
for (; count > 0; ++current, (void) --count)
::new (static_cast<void*>(std::addressof(*current))) V(value);
return current;
}
catch (...)
{
for (; first != current; ++first)
first->~V();
throw;
}
return current;
}
|
Example
Run this code
#include <algorithm>
#include <iostream>
#include <memory>
#include <string>
#include <tuple>
int main()
{
std::string* p;
std::size_t sz;
std::tie(p, sz) = std::get_temporary_buffer<std::string>(4);
std::uninitialized_fill_n(p, sz, "Example");
for (std::string* i = p; i != p + sz; ++i)
{
std::cout << *i << '\n';
i->~basic_string<char>();
}
std::return_temporary_buffer(p);
}
Output:
Example
Example
Example
Example
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 866 | C++98 | given T as the value type of NoThrowForwardIt, ifT::operator new exists, the program might be ill-formed
|
uses global placement new instead
|
| LWG 1339 | C++98 | the location of the first element following the filling range was not returned |
returned |
| LWG 2433 | C++11 | this algorithm might be hijacked by overloaded operator&
|
uses std::addressof |
See also
| copies an object to an uninitialized area of memory, defined by a range (function template & algorithm function object) | |
(C++20) |
|
(C++20) |
copies an object to an uninitialized area of memory, defined by a start and a count (algorithm function object) |