placement new to defer to a different constructor

Question

Is this safe? I\'m not using any virtual functions in my actual implementation, but I\'m tempted to believe that even if I was, it would still be safe.

clas         


        

Answer 1

The key problem here is that constructors are special - when you write a construct that calls a constructor (for example use new keyword to create an object) not only the constructor body is executed, instead the whole chain of objects is constructed first.

So when you use placement-new syntax to run another constructor first C++ automagically reruns all the base class object constructors and all the member variables constructors and only then the other constructor body is invoked. Sometimes you'll be okay, but many times you will run into unexpected behavior.