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spec: document that Alignof, Offsetof, and Sizeof do not evaluate their arguments #12946

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mdempsky opened this issue Oct 15, 2015 · 2 comments

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@mdempsky
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commented Oct 15, 2015

At least in cmd/compile, unsafe.Alignof, unsafe.Offsetof, and unsafe.Sizeof do not evaluate their arguments: http://play.golang.org/p/4QE3mVrFaS

Although unsurprising and consistent with C/C++, the Go spec doesn't mention this. It does say

Calls to Alignof, Offsetof, and Sizeof are compile-time constant expressions of type uintptr.

but I don't think that's necessarily mutually exclusive with evaluating the argument expression.

@ianlancetaylor ianlancetaylor added this to the Unplanned milestone Oct 15, 2015

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commented Oct 16, 2015

CL https://golang.org/cl/16004 mentions this issue.

@tzneal

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commented Aug 11, 2018

I like the current behavior, as it allows the following to work which lets you get sizes of arbitrary types given their names:

unsafe.Sizeof(*(*uint8)(nil))

See https://play.golang.org/p/MeFfDzoGpxo, my reading of "compile-time constant expressions of" implied that is wouldn't evaluate the expression, leading me to the above construct. I'm using this in some generated code now.

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