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    <p>Under the as-if rule:</p> <pre><code>struct A { EmptyThing x; int y; }; A a; assert((void*)&amp;(a.x) != (void*)&amp;(a.y)); </code></pre> <p>The assert must not be triggered. So I don't see any benefit in secretly making <code>x</code> have size 0, when you'd just need to add padding to the structure anyway.</p> <p>I suppose in theory a compiler could track whether pointers might be taken to the members, and make the optimization only if they definitely aren't. This would have limited use, since there'd be two different versions of the struct with different layouts: one for the optimized case and one for general code.</p> <p>But for example if you create an instance of A on the stack, and do something with it that is entirely inlined (or otherwise visible to the optimizer), yes, parts of the struct could be completely omitted. This isn't specific to empty objects, though - an empty object is just a special case of an object whose storage isn't accessed, and therefore could in some situations never be allocated at all.</p>
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    1. POEmpty Data Member Optimization: would it be possible?
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    1. USSteve Jessop
    1. USSteve Jessop
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    1. COyes, this address identity thing is what really annoys me here. While I can guess it could have use, an object whose address is not taken amounts to an object that is not used (since invoking any method on it requires its address) and I don't see how a C++ compiler could realize an object is unused apart from all in-lined classes. Do you know why this non-zero is used for ? (ie if it's a remnant of C or still has used in more modern style)
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      1. USMatthieu M.
    2. CO@Matthieu: well, I suppose that non-zero size of complete objects is so you can do `sizeof(array)/sizeof(*array)` and not divide by zero. I can't think of a strong rationale for non-zero size of member subobjects - if you need some member objects with different addresses, you (the programmer) could just make sure they're of type `char` rather than `EmptyThing`. I guess there's somewhere in the object/memory model that would need special-case handling, and it's just simpler to assume that member subobjects are distinct and non-overlapping.
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      1. USSteve Jessop
    3. COI've updated my question somewhat. I am not requiring a non-zero size of complete objects (though that too could be nice, allowing heaps of other optimizations, and `sizeof` is compile-time code too, so no worry of a divide by zero exception at runtime anyway). I don't understand why if we can have EBO (that is a zero-size base class) we cannot have EDMO (that is a zero-size data member).
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