NAME¶

std::uninitialized_fill_n - std::uninitialized_fill_n

Synopsis¶

Defined in header <memory>
template< class ForwardIt, class Size, class T >
ForwardIt uninitialized_fill_n( ForwardIt first, Size count, const (1)
T& value );
template< class ExecutionPolicy, class ForwardIt, class Size,
class T >

ForwardIt uninitialized_fill_n( ExecutionPolicy&& policy, (2) (since C++17)

ForwardIt first, Size count, const
T& value );

1) Copies the given value value to the first count elements in an uninitialized
memory area beginning at first as if by for (; n--; ++first)
::new (/* VOIDIFY */(*first))
typename std::iterator_traits<ForwardIt>::value_type(value);
where /* VOIDIFY */(e) is:

static_cast<void*>(&e) (until C++11)
static_cast<void*>(std::addressof(e)) (since C++11)

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

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until
C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (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¶

-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
No increment, assignment, comparison, or indirection through valid instances of
ForwardIt may throw exceptions.
Applying &* to a ForwardIt value must yield a pointer to its value type.
(until C++11)

Return value¶

Iterator to the element past the last element copied.

Complexity¶

Linear in count.

Exceptions¶

The overload with a template parameter named ExecutionPolicy reports errors as
follows:

* If execution of a function invoked as part of the algorithm throws an exception
and ExecutionPolicy is one of the standard policies, std::terminate is called.
For any other ExecutionPolicy, the behavior is implementation-defined.
* If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation¶

template<class ForwardIt, class Size, class T>
ForwardIt uninitialized_fill_n(ForwardIt first, Size count, const T& value)
{
using V = typename std::iterator_traits<ForwardIt>::value_type;
ForwardIt 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;
}
}

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
given T as the value type of ForwardIt,
LWG 866 C++98 if uses global replacement-
T::operator new exists, the program new instead
might be ill-formed
the location of the first element
LWG 1339 C++98 following returned
the filling range was not returned
LWG 2433 C++11 this algorithm might be hijacked by uses std::addressof
overloaded operator&
LWG 3870 C++20 this algorithm might create objects on kept disallowed
a const storage

See also¶

copies an object to an uninitialized area of memory,
uninitialized_fill defined by a range
(function template)
ranges::uninitialized_fill_n copies an object to an uninitialized area of memory,
(C++20) defined by a start and a count
(niebloid)