NAME¶

std::destroy_n - std::destroy_n

Synopsis¶

Defined in header <memory>
template< class ForwardIt, class Size > (since C++17)
ForwardIt destroy_n( ForwardIt first, Size n ); (until C++20)
template< class ForwardIt, class Size > (since C++20)
constexpr ForwardIt destroy_n( ForwardIt first, Size n ); (1)
template< class ExecutionPolicy, class ForwardIt, class Size >
ForwardIt destroy_n( ExecutionPolicy&& policy, ForwardIt (2) (since C++17)
first, Size n );

1) Destroys the n objects in the range starting at first, as if by

for (; n > 0; (void) ++first, --n)
std::destroy_at(std::addressof(*first));

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 destroy
n - the number of elements to destroy
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 that has been destroyed (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>
constexpr // since C++20
ForwardIt destroy_n(ForwardIt first, Size n)
{
for (; n > 0; (void) ++first, --n)
std::destroy_at(std::addressof(*first));
return first;
}

Example¶

The following example demonstrates how to use destroy_n to destroy a contiguous
sequence of elements.

// Run this code

#include <iostream>
#include <memory>
#include <new>

struct Tracer
{
int value;
~Tracer() { std::cout << value << " destructed\n"; }
};

int main()
{
alignas(Tracer) unsigned char buffer[sizeof(Tracer) * 8];

for (int i = 0; i < 8; ++i)
new(buffer + sizeof(Tracer) * i) Tracer{i}; //manually construct objects

auto ptr = std::launder(reinterpret_cast<Tracer*>(buffer));

std::destroy_n(ptr, 8);
}

Output:¶

0 destructed
1 destructed
2 destructed
3 destructed
4 destructed
5 destructed
6 destructed
7 destructed

See also¶

destroy destroys a range of objects
(C++17) (function template)
destroy_at destroys an object at a given address
(C++17) (function template)
ranges::destroy_n destroys a number of objects in a range
(C++20) (niebloid)