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

I think the typedef is confusing GCC. These sorts of attributes seem to work best when applied directly to variable definitions.

This version of your class works for me (GCC 4.6.0):

template class Lightweight
{
private:
  //  typedef T __attribute((__may_alias__)) T_may_alias;

public:
  Lightweight() : _isObjectConstructed(false) {/* empty */}

  ~Lightweight()
  {
    // call object's destructor, only if we ever constructed it
    if (_isObjectConstructed) {
      T * __attribute__((__may_alias__)) p
        = (reinterpret_cast(_optionalObject._buf));
      p->~T();
    }
  }

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

  void MethodThatGetsCalledRarely()
  {
    T * __attribute__((__may_alias__)) p
      = (reinterpret_cast(_optionalObject._buf));
    if (_isObjectConstructed == false)
      {
        // demand-construct the heavy object, since we actually need to use it now

        (void) new (p) T();
        _isObjectConstructed = true;
      }
      p->DoSomething();
  }

  [etc.]