Joel de Guzman
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. More often than not, a parser constructs objects in a hierarchical manner that reflects the grammar: print(a + b); might be expressed as statement(plus_expression(symbol("a"), symbol("b"))) in some language. The latter objects are better be designed without default constructors, in order to guarantee that we never end up with, say, a meaningless symbol() or plus_expression(). When I write a parser by hand, that's straight forward: boost::optional< plus_expression > parse_plus_expression(input_tokens tokens) { ... } The function will return boost::none if the expression can't be parsed and no default plus_expressions will be necessary. 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? Jens