OOP / language design question

[Duncan Booth]

Alex Martelli wrote:

>> What I think I'm trying to get at is that I believe that most
>> situations where someone actually tries to do something in the base
>> initialiser which requires calling a virtual method are probably also
>> cases where the initialiser is doing too much: i.e. separating the 
>> construction/initialisation from actually doing something is usually
>> a good idea.
> 
> But why should that be?  Template Method is perhaps the MOST generally
> useful design pattern -- why would it be any less useful in
> initialization than elsewhere?!
> 
Because it is error prone?

Any method which is called from the constructor/initialiser has to operate 
correctly on an object which at that point is not fully 
constructed/initialised. So instead of having to write a method on a Foo 
object, your template method has to operate on a partial Foo. The danger is 
that you haven't clearly defined the partial Foo interface sufficiently and 
the method tries to use other parts of the object which haven't yet been 
set up. That situation gets worse when you have a class hierarchy as the 
subclass needs to know that it has to do complete its own initialisation 
before constructing the base class instead of afterwards, and if you are 
going to document that requirement, why not do it properly and split the 
construction in two?

That's why I would go for the 2-phase construction: after the first phase 
you have an object which is fully initialised, just not yet 
used/connected/running. For example httplib.HTTPConnection does this: you 
construct the object with a host and port, but the actual connection is 
triggered by a separate object.
I would suggest your example of a database connection belongs in that 
category: it should have an initial unconnected idle state and a separate 
connection.

I think your example of a composite window building subwindows is the sort 
of use case I was asking for: it does sound tempting to construct the 
window and all its subwindows together. I'm happy to concede on that one.