Thursday, May 28, 2009

Another Note on Empty Structs

I mentioned this C++ Standard Violation by MSVC++ in my last post on empty structs:

- There is no padding (other than for alignment) between the last base class and the first class member or vft-pointer(s). *** NOTE: this is an over-aggressive empty-base-optimization that can break the C++ standard.

Let me explain:

The standard on empty struct inheritance specifies that separate structs of the same type should yield unique addresses. In the following code, the two addresses printed should be different in a standards compliant compiler (Intel or GCC). MSVC++ ends up printing the same address twice.

class a { public: void awork() {} };
class b : public a { public: int bdata; };
class c : public a { public: b cdata; };
c test;
a *pca=&(test);
a *pcb=&(test.cdata);
printf("Offset of c::a %d\n",size_t(pca));
printf("Offset of c.b::a %d\n",size_t(pcb));

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