CODEWITHSUNDEEP
c++memorylanguage-lawyermemory-alignmentobject-lifetime
plasmacel
plasmacel

Reputation: 8530

Creating/destroying an object in a static memory section

Is the static memory section alignas(alignof(T)) char bytes[sizeof(T)] suitable to hold an instance of T during its lifetime by calling std::construct_at(bytes, ...) / std::destroy_at(bytes)? My instincts say, yeah, the alignment and size requirements are guaranteed, so after construction (since there are also trivially constructible types, I prefer to call it initialization) reinterpret_cast<T*>(bytes) is a valid pointer to a completely valid instance of type T.

Am I missing something?

PS: I could also write std::aligned_storage_t<sizeof(T), alignof(T)> bytes - in that case its memory would be referenced as &bytes or std::addressof(bytes).

Upvotes: 1

Views: 105

Answers (1)

eerorika
eerorika

Reputation: 238321

Is the static memory section alignas(alignof(T)) char bytes[sizeof(T)] suitable to hold an instance of T during its lifetime by calling std::construct_at(bytes, ...) / std::destroy_at(bytes)?

Yes.

reinterpret_cast<T*>(bytes) is a valid pointer to a completely valid instance of type T.

Am I missing something?

You would still need to launder a pointer to reused memory:

T* ptr1 = std::construct_at(reinterpret_cast<T*>(bytes), ...); // valid
T* ptr2 = std::launder(reinterpret_cast<T*>(bytes)); // valid
std::destroy_at(ptr2); // valid, just like std::destroy_at(ptr1)

Upvotes: 3

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