Re: [Boost-users] Library Interface Design

Jens Theisen wrote:

Joel de Guzman writes:

...

One-phase construction definitely!

I'd like to ask you a question about your parser framework that is related to this. I probably get most of the following wrong, so please be patient.

Well, the proper mailing list is: https://lists.sourceforge.net/lists/listinfo/spirit-general Anyway...

With spirit, there appear to be two approaches:

1) semantic actions, which store away the parsed expression in some way

and

2) closures, which implicitly "return" their first member (member1).

I don't like 1 for most purposes, because it's too imperative for my taste. I need to keep track of what my actions did and, if something fails to parse at a point and backtracking occurs, need to manually revert the changes. This seems error prone to me.

2 looks better, but, and this is the connection to the OP, it appears that the returned value, as all closure members, must be default constructible. In particular, I risk returning such default constructed values when I failed to assign to them.

It's very likely that I missed something, as I didn't seriously tried using spirit. Can you shed some light on this?

Yeah. You did miss something. The closure members can be initialized. See "Initializing closure variables" section in http://tinyurl.com/85hbq "Sometimes, we need to initialize our closure variables upon entering a non-terminal (rule, subrule or grammar). Closure enabled non-terminals, by default, default-construct variables upon entering the parse member function. If this is not desirable, we can pass constructor arguments to the non-terminal."... Regards, -- Joel de Guzman http://www.boost-consulting.com http://spirit.sf.net