• Lars T. Kyllingstad (14/14) Apr 19 2014 Say I have two structs, defined like this:
• Andrej Mitrovic via Digitalmars-d-learn (9/13) Apr 19 2014 The best thing to do is add a static assert and then you can relax:
• Lars T. Kyllingstad (15/26) Apr 19 2014 That's what I've done, but it would be nice to know the code

"Lars T. Kyllingstad" <public kyllingen.net> writes:

Say I have two structs, defined like this:

     struct A { /* could contain whatever */ }

     struct B { A a; }

My question is, is it now guaranteed that A.sizeof==B.sizeof, 
regardless of how A is defined (member variable types, alignment, 
etc.)?  More to the point, say I have a function foo() which 
looks like this:

     extern(C) void foo(A* ptr, size_t len);

Is it now guaranteed that I can safely pass it a pointer to an 
array of Bs?  That is,

     auto arr = new B[10];
     foo(cast(A*) arr.ptr, arr.length);

Thanks,
Lars

Andrej Mitrovic via Digitalmars-d-learn writes:

On 4/19/14, Lars T. Kyllingstad via Digitalmars-d-learn
<digitalmars-d-learn puremagic.com> wrote:
 Say I have two structs, defined like this:

      struct A { /* could contain whatever */ }

      struct B { A a; }

 My question is, is it now guaranteed that A.sizeof==B.sizeof?

The best thing to do is add a static assert and then you can relax:

-----
struct A { }
struct B { A a; }
static assert(A.sizeof == B.sizeof);
-----

As for the ABI, I don't think I've ever seen it mentioned anywhere.

"Lars T. Kyllingstad" <public kyllingen.net> writes:

On Saturday, 19 April 2014 at 12:26:16 UTC, Andrej Mitrovic via 
Digitalmars-d-learn wrote:
 On 4/19/14, Lars T. Kyllingstad via Digitalmars-d-learn
 <digitalmars-d-learn puremagic.com> wrote:
 Say I have two structs, defined like this:

      struct A { /* could contain whatever */ }

      struct B { A a; }

 My question is, is it now guaranteed that A.sizeof==B.sizeof?

 The best thing to do is add a static assert and then you can 
 relax:

That's what I've done, but it would be nice to know the code 
won't break due to some combination of platform and/or compiler 
switches I didn't think to test.  Anyway, I've played around a 
bit, and found that a combination of struct and field alignment 
*can* break my assumption:

     align(1) struct A
     {
         char c;
         align(1) int i;
     }

     struct B { A a; }

Now, A.sizeof is 5, while B.sizeof is 8.  I'd have to add 
align(1) to the declaration of B to fix it.