When I was reading the sample code of std::construct_at at cppref, I noticed that std::destroy_at is called on a uninitialized buffer:
alignas(S) unsigned char storage[sizeof(S)]{};
S uninitialized = std::bit_cast<S>(storage);
std::destroy_at(&uninitialized);
I somehow believed that this should be UB operation because lifecycle of object is not started yet. Is calling std::destroy_at on an uninitialized buffer a valid operation by c++ standard?
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It’s wrong to call std::destroy_at on an unconstructed object – that would be UB.
But that’s not what’s happening here. It’s being called on the S object at uninitialized, not on storage. The latter seems to be a red herring – there’s no good reason for constructing uninitialized from random bytes rather than simply aggregate-initializing it.
A clearer example would omit the red-herring storage variable:
S uninitialized{0, 0.0f, 0.0};
std::destroy_at(&uninitialized);
There is a real bug in the example: we have
std::destroy_at(ptr);
and ptr points to uninitialized. So at end of scope, we have a second destruction of the same object, which is UB.
If that line is removed, the example becomes valid.
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