Virtual inheritance to get storage alignment

"Johannes Schaub (litb)" <>
Thu, 4 Mar 2010 14:08:32 CST
When someone asked a question about how to solve the problem that in a class
hierarchy some classes need a stricter alignment than others, and how to
write operator new in those cases, i came up with this solution using
virtual inheritance and the dominance rule:

// provides operators for any alignment >= 4 bytes
template<int Alignment>
struct DeAllocator;

template<int Alignment>
struct DeAllocator : virtual DeAllocator<Alignment/2> {
   void *operator new(size_t s) throw (std::bad_alloc) {
     std::cerr << "alignment: " << Alignment << "\n";
     return ::operator new(s);

   void operator delete(void *p) {
     ::operator delete(p);

struct DeAllocator<2> { };

// ........... Test .............
// different classes needing different alignments
struct Interface { };
struct A : Interface, virtual DeAllocator<16> { };
struct ASpecial : A, virtual DeAllocator<4> { };
struct B : Interface, virtual DeAllocator<8> { };

int main() {
   delete new A; // alignment: 16
   delete new ASpecial; // alignment: 16
   delete new B; // alignment: 8

What do you think of it? Is it a known idiom? Is it useful, or is the way
it's done wrong?

What worries me is how the size of these objects grow up from 1 byte to 4 or
8 bytes on GCC, just because we use virtual inheritance. But i think if we
already use virtual functions, the growth in size isn't too bad.

SO link to the question:

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