TITLE: Auto type conversion selects surprising constructor

PROBLEM: James M. Coggins (coggins@cs.unc.edu)


I have some classes that need to work together closely so I created
constructors for each of them that take an object of each of the
other classes.  We started seeing some very strange behavior during
test runs that we could not explain from our code.

It turned out that C++ was doing automatic type conversions using
a different path through our class relationships than I wanted.
Simplified particulars are as follows:

Class Base
{
   Base (Base&)
   Base (Other&)
}

class Derived1: public Base
{
   Derived1 (Derived1&)
   Derived1 (Derived2&)
   Derived1 (Other&)
}

class Derived2: public Base
{
   Derived2 (Derived1&)
   Derived2 (Derived2&)
   Derived2 (Other&)
}

Class Other
{
   Other (Derived1&)
   Other (Derived2&)
   Other (Other&)
}

Now when I implemented constructors Derived1::Derived1(Derived2&) and
Derived2::Derived2(Derived1&) I referred most of the initialization
processing to the base class as in

      Derived1::Derived1(Derived2& d) : (d)

figuring that since d was in fact a kind of Base object that the
Base::Base(Base&) constructor would be selected.  No such luck.
My object d was converted to class Other so that it matched the
Base::Base(Other&) constructor.  Since Other had different properties
from the Derived classes (that's why Other is a different class!) this
did not work as expected, leading to some interesting hours of debugging.


[ Note that it is generally not a good idea to pollute a class with
  knowledge of descendent classes, violating principles of encapsulation.
  - adc ]