std::uninitialized_default_construct_n
From cppreference.com
| Defined in header <memory>
|
||
template< class NoThrowForwardIt, class Size >
NoThrowForwardIt uninitialized_default_construct_n
( NoThrowForwardIt first, Size count );
|
(1) | (since C++17) (constexpr since C++26) |
template< class ExecutionPolicy, class NoThrowForwardIt, class Size >
NoThrowForwardIt uninitialized_default_construct_n
( ExecutionPolicy&& policy, NoThrowForwardIt first, Size count );
|
(2) | (since C++17) |
1) Constructs objects of type
typename std::iterator_traits<NoThrowForwardIt>::value_type in the uninitialized memory area first + [0, count) by default-initialization, as if by
for (; count > 0; (void)++first, --count)
::new (voidify(*first))
typename std::iterator_traits<NoThrowForwardIt>::value_type;
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 elements to initialize |
| count | - | the number of elements to construct |
| policy | - | the execution policy to use |
| Type requirements | ||
-NoThrowForwardIt must meet the requirements of LegacyForwardIterator.
| ||
-No increment, assignment, comparison, or indirection through valid instances of NoThrowForwardIt may throw exceptions.
| ||
Return value
As described above.
Complexity
Linear in n.
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>
constexpr ForwardIt uninitialized_default_construct_n(NoThrowForwardIt first, Size count)
{
using T = typename std::iterator_traits<NoThrowForwardIt>::value_type;
NoThrowForwardIt current = first;
try
{
for (; countn > 0; (void) ++current, --count)
::new (static_cast<void*>(std::addressof(*current))) T;
return current;
}
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 = 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";
}
// For scalar types, 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 to be resolved:
// for (const int i : v)
// std::cout << i << ' ';
// The result is unspecified:
std::cout << (std::memcmp(v, original, sizeof(v)) == 0 ? "un" : "") << "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 storage
|
kept disallowed |
See also
| constructs objects by default-initialization in an uninitialized area of memory, defined by a range (function template & algorithm function object) | |
| constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count (function template & algorithm function object) | |
| constructs objects by default-initialization in an uninitialized area of memory, defined by a start and count (algorithm function object) |