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copied!<p>Short answer: offsetof is a feature that is only in the C++ standard for legacy C compatibility. Therefore it is basically restricted to the stuff than can be done in C. C++ supports only what it must for C compatibility.</p> <p>As offsetof is basically a hack (implemented as macro) that relies on the simple memory-model supporting C, it would take a lot of freedom away from C++ compiler implementors how to organize class instance layout.</p> <p>The effect is that offsetof will often work (depending on source code and compiler used) in C++ even where not backed by the standard - except where it doesn't. So you should be very careful with offsetof usage in C++, especially since I do not know a single compiler that will generate a warning for non-POD use...</p> <p><b>Edit</b>: As you asked for example, the following might clarify the problem:</p> <pre><code>#include <iostream> using namespace std; struct A { int a; }; struct B : public virtual A { int b; }; struct C : public virtual A { int c; }; struct D : public B, public C { int d; }; #define offset_d(i,f) (long(&(i)->f) - long(i)) #define offset_s(t,f) offset_d((t*)1000, f) #define dyn(inst,field) {\ cout << "Dynamic offset of " #field " in " #inst ": "; \ cout << offset_d(&i##inst, field) << endl; } #define stat(type,field) {\ cout << "Static offset of " #field " in " #type ": "; \ cout.flush(); \ cout << offset_s(type, field) << endl; } int main() { A iA; B iB; C iC; D iD; dyn(A, a); dyn(B, a); dyn(C, a); dyn(D, a); stat(A, a); stat(B, a); stat(C, a); stat(D, a); return 0; } </code></pre> <p>This will crash when trying to locate the field <code>a</code> inside type <code>B</code> statically, while it works when an instance is available. This is because of the virtual inheritance, where the location of the base class is stored into a lookup table.</p> <p>While this is a contrived example, an implementation could use a lookup table also to find the public, protected and private sections of a class instance. Or make the lookup completely dynamic (use a hash table for fields), etc.</p> <p>The standard just leaves all possibilities open by restricting offsetof to POD (IOW: no way to use a hash table for POD structs... :)</p> <p>Just another note: I had to reimplement offsetof (here: offset_s) for this example as GCC actually errors out when I call offsetof for a field of a virtual base class.</p>

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