NAME¶

std::uninitialized_value_construct_n - std::uninitialized_value_construct_n

Synopsis¶

Defined in header <memory>
template< class ForwardIt, class Size >
ForwardIt uninitialized_value_construct_n( ForwardIt first, Size n (1) (since C++17)
);
template< class ExecutionPolicy, class ForwardIt, class Size >

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

ForwardIt first, Size n
);

1) Constructs n objects of type typename iterator_traits<ForwardIt>::value_type in
the uninitialized storage starting at first by value-initialization, as if by for (;
n > 0; (void) ++first, --n)
::new (static_cast<void*>(std::addressof(*first)))
typename std::iterator_traits<ForwardIt>::value_type();
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 elements to initialize
n - the number of elements to initialize
policy - the execution policy to use. See execution policy for
details.

Type requirements¶

-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
No increment, assignment, comparison, or indirection through valid instances of
ForwardIt may throw exceptions.

Return value¶

The end of the range of objects (i.e., std::next(first, n)).

Complexity¶

Linear in n.

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>
ForwardIt uninitialized_value_construct_n(ForwardIt first, Size n)
{
using T = typename std::iterator_traits<ForwardIt>::value_type;
ForwardIt current = first;
try
{
for (; n > 0 ; (void) ++current, --n)
::new (const_cast<void*>(static_cast<const volatile void*>(
std::addressof(*current)))) T();
return current;
}
catch (...)
{
std::destroy(first, current);
throw;
}
}

Example¶

// Run this code

#include <iostream>
#include <memory>
#include <string>

int main()
{
struct S { std::string m{"Default value"}; };

constexpr int n{3};
alignas(alignof(S)) unsigned char mem[n * sizeof(S)];

try
{
auto first{reinterpret_cast<S*>(mem)};
auto last = std::uninitialized_value_construct_n(first, n);

for (auto it{first}; it != last; ++it)
std::cout << it->m << '\n';

std::destroy(first, last);
}
catch (...)
{
std::cout << "Exception!\n";
}

// Notice that for "trivial types" the uninitialized_value_construct_n
// zero-initializes the given uninitialized memory area.
int v[]{1, 2, 3, 4};
for (const int i : v)
std::cout << i << ' ';
std::cout << '\n';
std::uninitialized_value_construct_n(std::begin(v), std::size(v));
for (const int i : v)
std::cout << i << ' ';
std::cout << '\n';
}

Output:¶

Default value
Default value
Default value
1 2 3 4
0 0 0 0

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 kept disallowed
storage

See also¶

constructs objects by value-initialization
uninitialized_value_construct in an uninitialized area of memory, defined
(C++17) by a range
(function template)
constructs objects by default-initialization
uninitialized_default_construct_n in an uninitialized area of memory, defined
(C++17) by a start and a count
(function template)
constructs objects by value-initialization
ranges::uninitialized_value_construct_n in an uninitialized area of memory, defined
(C++20) by a start and a count
(niebloid)