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    <p>Having two objects at the same address would mean that there would be no way to distinguish between these two objects when referencing them with pointers. For example, in the following code:</p> <pre><code>EmptyClass o1; EmptyClass o2; EmptyClass * po = &amp;o; po-&gt;foo(); </code></pre> <p>Should the foo method be called on <code>o1</code> or <code>o2</code>?</p> <p>It could be argued that since these objects have no data and no virtual methods (otherwise they would have a non-zero size), it doesn't matter on which instance the method is invoked. However, this becomes more important when we want to test if two objects are equal (i.e. if they are the same):</p> <pre><code>template &lt; typename T &gt; bool isSame( T const &amp; t1, T const &amp; t2 ) { return &amp;t1 == &amp;t2; } EmptyClass o1; // one object and... EmptyClass o2; // ...a distinct object... assert( ! isSame( o1, o2 ) ); // ...should not be one and same object! </code></pre> <p>For a more concrete example, let's assume that I want to associate some objects (I could say entities) with some values, let's say in an associative container:</p> <pre><code>Person you; Person me; // You and I are two different persons // (unless I have some dissociative identity disorder!) // Person is a class with entity semantics (there is only one 'me', I can't make // a copy of myself like I would do with integers or strings) std::map&lt; Person *, std::string &gt; personToName; personToName[&amp;you] = "Andrew_Lvov"; personToName[&amp;me] = "Luc Touraille"; // Oh, bother! The program confused us to be the same person, so now you and I // have the same name! </code></pre> <p>So yes, it all boils down to object identity: if objects were allowed to be empty, they could be deprived of their identity, which is simply not allowed by the language (thankfully).</p>
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    1. COYeah, that's a great explanation, thank you. One additional problem which actually arises: what if you have an array of empty objects. You can't select array member, since the array's size is 0.
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    2. CO@Andrew_Lvov: Well, I thought about adding that remark, but in fact I'm not sure this is an issue. Indeed, if I have the array `EmptyClass array[10]`, the code `array[3]` translates more or less to `*(array + 3 * sizeof( EmptyClass ))`; since `sizeof( EmptyClass ) == 0`, we end up accessing the object at the address `array`, which is indeed the fourth object of the array (as well as the first, second, third, etc.).
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