Re: [Boost-users] Library Interface Design

From: Thorsten Ottosen (thorsten.ottosen_at_[hidden])
Date: 2006-09-13 07:02:59

Hi Scott,

Scott Meyers wrote:
> I have a question about library interface design in general with a 
> strong interest in its application to C++, so I hope the moderators will 
> view this at on-topic for a list devoted to the users' view of a set of 
> widely used C++ libraries.  In its most naked form, the question is 
> this:  how important is it that class constructors ensure that their 
> objects are in a "truly" initialized state?
> 
> I've always been a big fan of the "don't let users create objects they 
> can't really use" philosophy.  So, in general, I don't like the idea of 
> "empty" objects or two-phase construction.  It's too easy for people to 
> make mistakes. So, for example, if I were to design an interface for an 
> event log that required a user-specified ostream on which to log events, 
> I'd declare the constructor taking an ostream parameter (possibly with a 
> default, but that's a separate question, so don't worry about it):
> 
>    EventLog::EventLog(std::ostream& logstream); // an ostream must be
>                                                 // specified

One-phase construction that establishes the class invariant is the one 
of the two main benefits of OO programming (the other being polymorphism).

So I rate it as very important.

As for testability, then things like the strategy pattern helps a lot.
In your example above, the argument is passed by refernce and so you 
could perhaps use a dummy class derived from std::ostream.

For tightly coupled code (and otherwise), DbC is indispensible.

Michael Feathers describes a range of techniques for writing test
for legacy code in "Working effectively with legacy code".

cheers

Thorsten