# std::destroy_n

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< cpp‎ | memory

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

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