std::ranges::destroy_n

Defined in header <memory>

Call signature

Defined in header `<memory>`
Call signature
template< /nothrow-input-iterator/ I > requires std::destructible<std::iter_value_t<I>> constexpr I destroy_n( I first, std::iter_difference_t<I> count ) noexcept;	(1)	(since C++20)
template< /execution-policy/ Ep, /nothrow-random-access-iterator/ I > requires std::destructible<std::iter_value_t<I>> I destroy_n( Ep&& policy, I first, std::iter_difference_t<I> count );	(2)	(since C++26)

template< /*nothrow-input-iterator*/ I >
    requires std::destructible<std::iter_value_t<I>>
constexpr I destroy_n( I first, std::iter_difference_t<I> count ) noexcept;

(1)

(since C++20)

template< /*execution-policy*/ Ep, /*nothrow-random-access-iterator*/ I >
    requires std::destructible<std::iter_value_t<I>>
I destroy_n( Ep&& policy, I first, std::iter_difference_t<I> count );

(2)

(since C++26)

For the definition of /*execution-policy*/, see this page; for the definition of other exposition-only concepts, see this page.

1) Destroys elements in the target range first + [0, count) as if by

return std::ranges::destroy(std::counted_iterator(first, count),
                            std::default_sentinel).base();

2) Same as (1), but executed according to policy.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

Explicit template argument lists cannot be specified when calling any of them.
None of them are visible to argument-dependent lookup.
When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Parameters

first	-	the beginning of the range of elements to destroy
count	-	the number of elements to destroy
policy	-	the execution policy to use

Return value

As described above.

Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_parallel_algorithm`	`202506L`	(C++26)	Parallel range algorithms

Possible implementation

struct destroy_n_fn
{
    template</*nothrow-input-iterator*/ I>
        requires std::destructible<std::iter_value_t<I>>
    constexpr I operator()(I first, std::iter_difference_t<I> n) const noexcept
    {
        for (; n != 0; (void)++first, --n)
            std::ranges::destroy_at(std::addressof(*first));
        return first;
    }
};

inline constexpr destroy_n_fn destroy_n{};

Example

Demonstrates how to use ranges::destroy_n to destroy a contiguous sequence of elements.

#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) * 4];
    
    for (int i = 0; i < 4; ++i)
        new (buffer + sizeof(Tracer) * i) Tracer{i}; // manually construct objects

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

    std::ranges::destroy_n(ptr, 4);
}

Output:

0 destructed
1 destructed
2 destructed
3 destructed