std::destroy_n

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

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
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
  • 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

(C++17)
 destroys a range of objects
(function template)
(C++17)
 destroys an object at a given address
(function template)
(C++20)
 destroys a number of objects in a range
(algorithm function object)
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