more spirit madness - parser-types (rules vs int_parser<>) and meta-programming techniques
C++BoostBoost SpiritTemplate Meta-ProgrammingC++ Problem Overview
The question is in bold at the bottom, the problem is also summarized by the distillation code fragment towards the end.
I am trying to unify my type system (the type system does to and from from type to string) into a single component(as defined by Lakos). I am using boost::array
, boost::variant
, and boost::mpl
, in order to achieve this. I want to have the parser and generator rules for my types unified in a variant. there is a undefined type, a int4(see below) type and a int8 type. The variant reads as variant<undefined, int4,int8>
.
int4 traits:
struct rbl_int4_parser_rule_definition
{
typedef boost::spirit::qi::rule<std::string::iterator, rbl_int4()> rule_type;
boost::spirit::qi::int_parser<rbl_int4> parser_int32_t;
rule_type rule;
rbl_int4_parser_rule_definition()
{
rule.name("rbl int4 rule");
rule = parser_int32_t;
}
};
template<>
struct rbl_type_parser_rule<rbl_int4>
{
typedef rbl_int4_parser_rule_definition string_parser;
};
the variant above starts out as undefined, and then I initialize the rules. I had a problem, which caused 50 pages of errors, and I have finally managed to track it down, Variant uses operator=
during assignment and a boost::spirit::qi::int_parser<>
cannot be assigned to another (operator=).
To contrast, I don't have a problem with my undefined type:
struct rbl_undefined_parser_rule_definition
{
typedef boost::spirit::qi::rule<std::string::iterator, void()> rule_type;
rule_type rule;
rbl_undefined_parser_rule_definition()
{
rule.name("undefined parse rule");
rule = boost::spirit::qi::eps;
}
};
template<>
struct rbl_type_parser_rule<rbl_undefined>
{
typedef rbl_undefined_parser_rule_definition string_parser;
};
Distillation of the problem:
#include <string>
#include <boost/spirit/include/qi.hpp>
#include <boost/variant.hpp>
#include <boost/cstdint.hpp>
typedef boost::spirit::qi::rule<std::string::iterator,void()> r1;
typedef boost::spirit::qi::rule<std::string::iterator,int()> r2;
typedef boost::variant<r1,r2> v;
int main()
{
/*
problematic
boost::spirit::qi::int_parser<int32_t> t2;
boost::spirit::qi::int_parser<int32_t> t1;
t1 = t2;
*/
//unproblematic
r1 r1_;
r2 r2_;
r1_ = r2_;
v v_;
// THIS is what I need to do.
v_ = r2();
}
There is a semantic gap between concrete parsers and rules. My brain is smoking at the moment so I am not going to think about pramatism, My question is, how do I solve this problem ? I can think of three approaches to solve the problem.
one: Static function members:
struct rbl_int4_parser_rule_definition
{
typedef boost::spirit::qi::rule<std::string::iterator, rbl_int4()> rule_type;
//boost::spirit::qi::int_parser<rbl_int4> parser_int32_t;
rule_type rule;
rbl_int4_parser_rule_definition()
{
static boost::spirit::qi::int_parser<rbl_int4> parser_int32_t;
rule.name("rbl int4 rule");
rule = parser_int32_t;
}
};
I guess approach one prevents thread safe code ? ?
two: The integral parser is wrapped in a shared_ptr. There are two reasons I'm bothering with TMP for the typing system: 1 efficiency, 2 centralizing concerns into components. using pointers defeats the first reason.
three: operator= is defined as a no-op. variant guarantees that the lhs
is default constructed before assignment.
Edit: I am thinking option 3 makes the most sense (operator= is a no-op). Once the rule container is created it will not change, and I am only assigning to force a type's rule trait into its offset.
C++ Solutions
Solution 1 - C++
I'm not so sure I get the full extent of the question, but here are a few hints
-
The line commented with
// THIS is what I need to do.
compiles fine with me (problem solved? I'm guessing you actually meant assigning a parser, not a rule?) -
Initialization of function-local
static
has been defined to be thread safe in the latest standard (C++11). Check your compiler support for C++0x threading. (If the initializer throws, a pass of the initialization statement will try to initialize again, by the way). -
rules
alias()
As described in http://boost-spirit.com/home/articles/doc-addendum/faq/#aliases
You can create 'logical copies' of rules without having to actually value-copy the proto expression. As the FAQ says, this is mainly to allow lazy-binding
-
The Nabialek Trick might be precisely what you need, basically it lazily selects a parser for subsequent parsing
one = id; two = id >> ',' >> id; keyword.add ("one", &one) ("two", &two) ; start = *(keyword[_a = _1] >> lazy(*_a));
In your context, I could see
keyword
defined asqi::symbols<char, qi::rule<Iterator>*> keyword;
doing all the work with attributes from semantic actions. Alternatively,
qi::symbols<char, qi::rule<Iterator, std::variant<std::string,int>() >*> keyword;
-
Bring the rules under the same type (like shown in the previous line, basically)
This is the part where I'm getting confused: You say you want to unify your type system. There might not be a need for strongtyped parsers (distinct attribute signatures).
typedef boost::variant<std::string,int> unified_type; typedef qi::rule<std::string::iterator, unified_type() > unified_rule; unified_rule rstring = +(qi::char_ - '.'); unified_rule rint = qi::int_; unified_rule combine = rstring | rint;