Do polymorphism or conditionals promote better design?

Question

I recently stumbled across this entry in the google testing blog about guidelines for writing more testable code. I was in agreement with the author until this point:

Answer 1

This is mainly to do with encapsulation of knowledge. Let's start with a really obvious example - toString(). This is Java, but easily transfers to C++. Suppose you want to print a human friendly version of an object for debugging purposes. You could do:

switch(obj.type): {
case 1: cout << "Type 1" << obj.foo <<...; break;   
case 2: cout << "Type 2" << ...

This would however clearly be silly. Why should one method somewhere know how to print everything. It will often be better for the object itself to know how to print itself, eg:

cout << object.toString();

That way the toString() can access member fields without needing casts. They can be tested independently. They can be changed easily.

You could argue however, that how an object prints shouldn't be associated with an object, it should be associated with the print method. In this case, another design pattern comes in helpful, which is the Visitor pattern, used to fake Double Dispatch. Describing it fully is too long for this answer, but you can read a good description here.