Permit objects to be shared between other objects. Define "template" objects to permit such sharing. For example, let's say that I want to write a program consisting of an object hierarchy where A is my top-level object, and it uses objects B and C and D. All of A, B, C, and D want to do general I/O using a keyboard/screen or serial terminal or whatnot. But it's critical that they all share the same I/O object (in the physical sense). At the same time, the flexibility to change from a serial interface to a keyboard/TV or keyboard/VGA interface, with only one or two lines of code, is important.