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std::destroy_n(3) C++ Standard Libary std::destroy_n(3)

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)

2024.06.10 http://cppreference.com