NAME¶

std::uninitialized_default_construct - std::uninitialized_default_construct

Synopsis¶

Defined in header <memory>
template< class ForwardIt >
void uninitialized_default_construct( ForwardIt first, ForwardIt (1) (since C++17)
last );
template< class ExecutionPolicy, class ForwardIt >

void uninitialized_default_construct( ExecutionPolicy&& policy, (2) (since C++17)

ForwardIt first, ForwardIt
last );

1) Constructs objects of type typename std::iterator_traits<ForwardIt>::value_type
in the uninitialized storage designated by the range [first, last) by
default-initialization, as if by for (; first != last; ++first)
::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, last - the range of the 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¶

(none)

Complexity¶

Linear in the distance between first and last.

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>
void uninitialized_default_construct(ForwardIt first, ForwardIt last)
{
using Value = typename std::iterator_traits<ForwardIt>::value_type;
ForwardIt current = first;
try
{
for (; current != last; ++current)
{
::new (const_cast<void*>(static_cast<const volatile void*>(
std::addressof(*current)))) Value;
}
}
catch (...)
{
std::destroy(first, current);
throw;
}
}

Example¶

// Run this code

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

struct S
{
std::string m{"Default value"};
};

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

try
{
auto first{reinterpret_cast<S*>(mem)};
auto last{first + n};

std::uninitialized_default_construct(first, last);

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_default_construct
// generally does not zero-fill the given uninitialized memory area.
int v[]{1, 2, 3, 4};
const int original[]{1, 2, 3, 4};
std::uninitialized_default_construct(std::begin(v), std::end(v));

// Maybe undefined behavior, pending CWG 1997.
// for (const int i : v)
// std::cout << i << ' ';

// The result is unspecified.
std::cout <<
(std::memcmp(v, original, sizeof(v)) == 0 ? "Unmodified\n" : "Modified\n");
}

Possible output:¶

Default value
Default value
Default value
Unmodified

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