2012/1/24 Andrey Semashev <andrey.semashev@gmail.com> [...]
A few more questions, if I may.
1. I would like to cache some data in the actor at the point of AST construction. For instance, I'd like to cache the attribute name (in the actual code it's not really a string but rather an adapter constructible from a string; I'd like to avoid its construction at the expression evaluation time) and, possibly, a default value for the attribute. Is this possible? The attribute keyword should still be a POD (or at least, be safely constructible at the namespace scope), so the cached data can only be provided only by a function call.
2. I would like the keyword have additional methods, in particular, for default attribute value provision. Extending my initial syntax, this would be:
BOOST_LOG_REGISTER_ATTRIBUTE("Severity", severity, my_severity)
attribute_values_view attrs;
severity(attrs); // returns optional<my_severity> severity.or_default(info)(attrs); // returns my_severity, which is info by default
As I understand, the only way I can do that, except for reimplementing phoenix::actor, is to derive my actor from phoenix::actor. This makes the attribute keyword non-POD in C++03 but is it still safe to be constructed in the namespace scope, provided that the keyword does not have its own data members?
I'm not sure as well, that's what I always want to ask Eric: http://www.boost.org/doc/libs/1_48_0/doc/html/proto/appendices.html#boost_pr... Anyway, I played a trick while not knowing if it's available. The idea is: struct Pod {...you could have some data...}; struct Derived : Pod {}; // non-POD in C++03 Pod p_base = {...some data...}; // statically initialized Derived& p = static_cast<Derived&>(p_base); // statically initialized, not? Also, what should be the template argument
for the phoenix::actor specialization? Here's the code example to illustrate what I'm talking about:
[...] Again, I wrote the sample code (a rewrite from my last sample). I store the attribute name, this way, you don't need a new type for each keyword (which could be of the same value_type). Oh, BTW, I hope I can have severity(attrs) return optional<my_severity const&> instead of optional<my_severity> in case that my_severity is heavy (not this case, I know), but then the comparison ops are lost :( HTH