How is is_standard_layout useful?

Question

From what I understand, standard layout allows three things:

• Empty base class optimization
• Backwards compatibility with C with certain pointer casts
• Use of offsetof

Now, included in the library is the is_standard_layout predicate metafunction, but I can't see much use for it in generic code as those C features I listed above seem extremely rare to need checking in generic code. The only thing I can think of is using it inside static_assert, but that is only to make code more robust and isn't required.

How is is_standard_layout useful? Are there any things which would be impossible without it, thus requiring it in the standard library?

Answer 1

General response

It is a way of validating assumptions. You wouldn't want to write code that assumes standard layout if that wasn't the case.

C++11 provides a bunch of utilities like this. They are particularly valuable for writing generic code (templates) where you would otherwise have to trust the client code to not make any mistakes.

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Notes specific to is_standard_layout

It looks to me like the (pseudo code) definition of is_pod would roughly be...

// note: applied recursively to all members
bool is_pod(T) { return is_standard_layout(T) && is_trivial(T); }

So, you need to know is_standard_layout in order to implement is_pod. Given that, we might as well expose is_standard_layout as a tool available to library developers. Also of note: if you have a use-case for is_pod, you might want to consider the possibility that is_standard_layout might actually be a better (more accurate) choice in that case, since POD is essentially a subset of standard layout.

I get the feeling that they added every conceivable variant of type evaluation, regardless of any obvious value, just in case someone might encounter a need sometime before the next standard comes out. I doubt if piling on these "extra" type properties adds a significant additional burden to compiler developers.

There is a nice discussion of standard layout here: Why is C++11's POD "standard layout" definition the way it is? There is also a lot of good detail at cppreference.com: Non-static data members