Unaligned access through reinterpret_cast

Question

I\'m in the middle of a discussion trying to figure out whether unaligned access is allowable in C++ through reinterpret_cast. I think not, but I\'m having tro

Answer 1

EDIT This answers the OP's original question, which was "is accessing a misaligned pointer safe". The OP has since edited their question to "is dereferencing a misaligned pointer safe", a far more practical and less interesting question.

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The round-trip cast result of the pointer value is unspecified under those circumstances. Under certain limited circumstances (involving alignment), converting a pointer to A to a pointer to B, and then back again, results in the original pointer, even if you didn't have a B in that location.

If the alignment requirements are not met, than that round trip -- the pointer-to-A to pointer-to-B to pointer-to-A results in a pointer with an unspecified value.

As there are invalid pointer values, dereferencing a pointer with an unspecified value can result in undefined behavior. It is no different than *(int*)0xDEADBEEF in a sense.

Simply storing that pointer is not, however, undefined behavior.

None of the above C++ quotes talk about actually using a pointer-to-A as a pointer-to-B. Using a pointer to the "wrong type" in all but a very limited number of circumstances is undefined behavior, period.

An example of this involves creating a std::aligned_storage_t. You can construct your T in that spot, and it will live their happily, even though it "actually" is an aligned_storage_t. (You may, however, have to use the pointer returned from the placement new for full standard compliance; I am uncertain. See strict aliasing.)

Sadly, the standard is a bit lacking in terms of what object lifetime is. It refers to it, but does not define it well enough last I checked. You can only use a T at a particular location while a T lives there, but what that means is not made clear in all circumstances.