gcc: How to use __attribute((__may_alias__)) properly to avoid “derefencing type-punned pointer” warning

Question

I\'ve got some code that uses type-punning to avoid having to call a member \"object\"\'s constructor and destructor unless/until it\'s actually necessary to use the object.

Answer 1

What if you replace _isObjectConstructed with a pointer to the object:

class Lightweight
{
public:
   Lightweight() : object(NULL) {/* empty */}

   ~Lightweight()
   {
      // call object's destructor, only if we ever constructed it
      if (object) object->~T();
   }

   void MethodThatGetsCalledOften()
   {
      // Imagine some useful code here
   }

   void MethodThatGetsCalledRarely()
   {
      if (!object)
      {
         // demand-construct the heavy object, since we actually need to use it now
         object = new (_optionalObject._buf) T();
      }
      object->DoSomething();
   }

private:
   union {
      char _buf[sizeof(T)];
      unsigned long long _thisIsOnlyHereToForceEightByteAlignment;
   } _optionalObject;

   T *object;
};

Note, no GCC extension, only pure C++ code.

Using a T* instead of a bool won't even make Lightweight any bigger!