What is partially constructed object in C++?

When an object in a derived class is constructed, constructors are invoked from the root base class down to the leaf derived class. Until the leaf constructor finishes executing, the object is only partially constructed.

Let's say that some constructor adds the object to a container or makes it accessible in some other way. If other code then invokes a virtual function on the object, the resulting behavior is unpredictable if a derived class that has not yet been constructed overrides that function, because a base class version of the function will be invoked instead.

When the object is deleted, destructors are invoked from the leaf class up to the root base class. The same problem can arise here, only worse, because the virtual function might be pure virtual (i.e., unimplemented) when the destructor chain reaches some base class.

The safest practice is not to make an object available (by exporting a pointer or reference to it) until it has been fully constructed (edit: and to make it unavailable while it is being deleted). If you have a reason for violating this guideline, you need to be sure that doing so will not lead to problems.

EDIT: Another way to run into trouble is if a constructor invokes a virtual function in its own class hierachy. The virtual function that gets invoked is the one in the class whose constructor is currently running. This can be a problem because the object's data is still subject to change. It also means that a base class can't ask a derived class to do something by invoking a virtual function.

Also, when member initializers (those above the body of a constructor) are being executed, the object is still considered to be in the immediate base class, not the class being initialized.