How much work should be done in a constructor?

You usually do not want the constructor to do any computation. Someone else using the code will not expect that it does more than basic set-up.

For a directory tree like you're talking about, the "elegant" solution is probably not to build a full tree when the object is constructed. Instead, build it on demand. Someone using your object may not really care what is in sub-directories, so start out by just having your constructor list the first level, and then if someone wants to descend into a specific directory, then build that portion of the tree when they request it.

To summarize:

• At a minimum, your constructor needs to get the object configured to the point that its invariants are true.

• Your choice of invariants may affect your clients.(Does the object promise to be ready for access at all times? Or only only in certain states?) A constructor that takes care of all of the set-up up-front may make life simpler for the class's clients.

• Long-running constructors are not inherently bad, but may be bad in some contexts.

• For systems involving a user-interaction, long-running methods of any type may lead to poor responsiveness, and should be avoided.

• Delaying computation until after the constructor may be an effective optimization; it may turn out to be unnecessary to perform all the work. This depends on the application, and shouldn't be determined prematurely.

• Overall, it depends.