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Casting the first member of a struct (and back)

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Hi, Gurus,

i'm trying to clarify my understanding of how exactly the casting rules work for the first member of a C struct. For example...

struct Base {
  int foo;
  ...
};

struct Subclass {
  Base base;
  ...
};

(struct typedefs elided)

Subclass sub = { {...}, ... };
Base * b = (Base*)⊂

It is my understanding (please correct me if i'm wrong) that that is completely kosher in C because C guarantees (IIUC) that the offset of the first member in the struct is 0.

What i'm not clear on is if the reverse of that cast is legal:

Subclass * original = (Subclass*)b;

(insofar as the coder knows that the conversion is legal, of course, e.g. by checking a type-id indicator stored in the base type.)

My intuition says, "if the offset is 0, the reverse cast must be just as legal because the address is the same," but i am experienced enough to know that intuition is only a part of the story when it comes to details like this :/. Additionally, i seem to remember reading that C++ does _not_ guaranty that the addresses are the same for such casts.

Can one of you provide me a bit of enlightenment on this topic?

:-?
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Casting the first member of a struct (and back) stephan beal <sgbeal@googlemail.com> - 2013-08-07 01:06 -0500
  Re: Casting the first member of a struct (and back) Francis Glassborow <francis.glassborow@btinternet.com> - 2013-08-07 13:51 -0500
    Re: Casting the first member of a struct (and back) Keith Thompson <kst-u@mib.org> - 2013-09-02 04:07 -0500
  Re: Casting the first member of a struct (and back) James Kuyper <jameskuyper@verizon.net> - 2013-08-07 13:51 -0500
  Re: Casting the first member of a struct (and back) Jasen Betts <jasen@xnet.co.nz> - 2013-08-07 13:51 -0500

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