std::uninitialized_default_construct_n() algorithm
- od C++17
// (1)
template< class ForwardIt, class Size >
ForwardIt uninitialized_default_construct_n( ForwardIt first, Size n );
// (2)
template< class ExecutionPolicy, class ForwardIt, class Size >
ForwardIt uninitialized_default_construct_n( ExecutionPolicy&& policy, ForwardIt first, Size n );
-
(1) Constructs
n
objects of type typenamestd::iterator_traits<ForwardIt>::value_type
in the uninitialized storage starting atfirst
bydefault-initialization
, as if by:for (; n > 0; (void) ++first, --n)
::new (static_cast<void*>(std::addressof(*first)))
typename std::iterator_traits<ForwardIt>::value_type;uwagaIf 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
.
These overloads participate in overload resolution only if
std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
(do C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
(od C++20) is true
.
Parameters
first | The beginning of the range of elements to initialize. |
n | The number of elements to construct. |
policy | The execution policy to use. See execution policy for details. |
Type requirements
ForwardIt | LegacyForwardIterator |
No increment, assignment, comparison, or indirection through valid instances of NoThrowForwardIt
may throw exceptions.
Return value
(none)
Complexity
Linear in n
.
Exceptions
The overloads with a template parameter named ExecutionPolicy
report 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 none otherExecutionPolicy
, the behavior is implementation-defined. - If the algorithm fails to allocate memory,
std::bad_alloc
is thrown.
Possible implementation
uninitialized_default_construct_n(1)
template<class ForwardIt, class Size>
ForwardIt uninitialized_default_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;
}
}
Examples
#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 = std::uninitialized_default_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_default_construct_n
// generally does not zero-initialize the given uninitialized memory area.
int v[]{1, 2, 3, 4};
const int original[]{1, 2, 3, 4};
std::uninitialized_default_construct_n(std::begin(v), std::size(v));
// An attempt to access v might be an 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 ? "un" : "") << "modified\n";
}
default value
default value
default value
unmodified
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