[/============================================================================== Copyright (C) 2001-2011 Joel de Guzman Copyright (C) 2001-2011 Hartmut Kaiser Copyright (C) 2009 Andreas Haberstroh? Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) ===============================================================================/] [section:indepth In Depth] [section:parsers_indepth Parsers in Depth] This section is not for the faint of heart. In here, are distilled the inner workings of __qi__ parsers, using real code from the __spirit__ library as examples. On the other hand, here is no reason to fear reading on, though. We tried to explain things step by step while highlighting the important insights. The `__parser_concept__` class is the base class for all parsers. [import ../../../../boost/spirit/home/qi/parser.hpp] [parser_base_parser] The `__parser_concept__` class does not really know how to parse anything but instead relies on the template parameter `Derived` to do the actual parsing. This technique is known as the "Curiously Recurring Template Pattern" in template meta-programming circles. This inheritance strategy gives us the power of polymorphism without the virtual function overhead. In essence this is a way to implement compile time polymorphism. The Derived parsers, `__primitive_parser_concept__`, `__unary_parser_concept__`, `__binary_parser_concept__` and `__nary_parser_concept__` provide the necessary facilities for parser detection, introspection, transformation and visitation. Derived parsers must support the following: [variablelist bool parse(f, l, context, skip, attr) [[`f`, `l`] [first/last iterator pair]] [[`context`] [enclosing rule context (can be unused_type)]] [[`skip`] [skipper (can be unused_type)]] [[`attr`] [attribute (can be unused_type)]] ] The /parse/ is the main parser entry point. /skipper/ can be an `unused_type`. It's a type used every where in __spirit__ to signify "don't-care". There is an overload for /skip/ for `unused_type` that is simply a no-op. That way, we do not have to write multiple parse functions for phrase and character level parsing. Here are the basic rules for parsing: * The parser returns `true` if successful, `false` otherwise. * If successful, `first` is incremented N number of times, where N is the number of characters parsed. N can be zero --an empty (epsilon) match. * If successful, the parsed attribute is assigned to /attr/ * If unsuccessful, `first` is reset to its position before entering the parser function. /attr/ is untouched. [variablelist void what(context) [[`context`] [enclosing rule context (can be `unused_type`)]] ] The /what/ function should be obvious. It provides some information about ["what] the parser is. It is used as a debugging aid, for example. [variablelist P::template attribute::type [[`P`] [a parser type]] [[`context`] [A context type (can be unused_type)]] ] The /attribute/ metafunction returns the expected attribute type of the parser. In some cases, this is context dependent. In this section, we will dissect two parser types: [variablelist Parsers [[`__primitive_parser_concept__`] [A parser for primitive data (e.g. integer parsing).]] [[`__unary_parser_concept__`] [A parser that has single subject (e.g. kleene star).]] ] [/------------------------------------------------------------------------------] [heading Primitive Parsers] For our dissection study, we will use a __spirit__ primitive, the `any_int_parser` in the boost::spirit::qi namespace. [import ../../../../boost/spirit/home/qi/numeric/int.hpp] [primitive_parsers_any_int_parser] The `any_int_parser` is derived from a `__primitive_parser_concept__`, which in turn derives from `parser`. Therefore, it supports the following requirements: * The `parse` member function * The `what` member function * The nested `attribute` metafunction /parse/ is the main entry point. For primitive parsers, our first thing to do is call: `` qi::skip(first, last, skipper); `` to do a pre-skip. After pre-skipping, the parser proceeds to do its thing. The actual parsing code is placed in `extract_int::call(first, last, attr);` This simple no-frills protocol is one of the reasons why __spirit__ is fast. If you know the internals of __classic__ and perhaps even wrote some parsers with it, this simple __spirit__ mechanism is a joy to work with. There are no scanners and all that crap. The /what/ function just tells us that it is an integer parser. Simple. The /attribute/ metafunction returns the T template parameter. We associate the `any_int_parser` to some placeholders for `short_`, `int_`, `long_` and `long_long` types. But, first, we enable these placeholders in namespace boost::spirit: [primitive_parsers_enable_short] [primitive_parsers_enable_int] [primitive_parsers_enable_long] [primitive_parsers_enable_long_long] Notice that `any_int_parser` is placed in the namespace boost::spirit::qi while these /enablers/ are in namespace boost::spirit. The reason is that these placeholders are shared by other __spirit__ /domains/. __qi__, the parser is one domain. __karma__, the generator is another domain. Other parser technologies may be developed and placed in yet another domain. Yet, all these can potentially share the same placeholders for interoperability. The interpretation of these placeholders is domain-specific. Now that we enabled the placeholders, we have to write generators for them. The make_xxx stuff (in boost::spirit::qi namespace): [primitive_parsers_make_int] This one above is our main generator. It's a simple function object with 2 (unused) arguments. These arguments are # The actual terminal value obtained by proto. In this case, either a short_, int_, long_ or long_long. We don't care about this. # Modifiers. We also don't care about this. This allows directives such as `no_case[p]` to pass information to inner parser nodes. We'll see how that works later. Now: [primitive_parsers_short_primitive] [primitive_parsers_int_primitive] [primitive_parsers_long_primitive] [primitive_parsers_long_long_primitive] These, specialize `qi:make_primitive` for specific tags. They all inherit from `make_int` which does the actual work. [heading Composite Parsers] Let me present the kleene star (also in namespace spirit::qi): [import ../../../../boost/spirit/home/qi/operator/kleene.hpp] [composite_parsers_kleene] Looks similar in form to its primitive cousin, the `int_parser`. And, again, it has the same basic ingredients required by `Derived`. * The nested attribute metafunction * The parse member function * The what member function kleene is a composite parser. It is a parser that composes another parser, its ["subject]. It is a `__unary_parser_concept__` and subclasses from it. Like `__primitive_parser_concept__`, `__unary_parser_concept__` derives from `parser`. unary_parser, has these expression requirements on Derived: * p.subject -> subject parser ( ['p] is a __unary_parser_concept__ parser.) * P::subject_type -> subject parser type ( ['P] is a __unary_parser_concept__ type.) /parse/ is the main parser entry point. Since this is not a primitive parser, we do not need to call `qi::skip(first, last, skipper)`. The ['subject], if it is a primitive, will do the pre-skip. If if it is another composite parser, it will eventually call a primitive parser somewhere down the line which will do the pre-skip. This makes it a lot more efficient than __classic__. __classic__ puts the skipping business into the so-called "scanner" which blindly attempts a pre-skip every time we increment the iterator. What is the /attribute/ of the kleene? In general, it is a `std::vector` where `T` is the attribute of the subject. There is a special case though. If `T` is an `unused_type`, then the attribute of kleene is also `unused_type`. `traits::build_std_vector` takes care of that minor detail. So, let's parse. First, we need to provide a local attribute of for the subject: `` typename traits::attribute_of::type val; `` `traits::attribute_of` simply calls the subject's `struct attribute` nested metafunction. /val/ starts out default initialized. This val is the one we'll pass to the subject's parse function. The kleene repeats indefinitely while the subject parser is successful. On each successful parse, we `push_back` the parsed attribute to the kleene's attribute, which is expected to be, at the very least, compatible with a `std::vector`. In other words, although we say that we want our attribute to be a `std::vector`, we try to be more lenient than that. The caller of kleene's parse may pass a different attribute type. For as long as it is also a conforming STL container with `push_back`, we are ok. Here is the kleene loop: `` while (subject.parse(first, last, context, skipper, val)) { // push the parsed value into our attribute traits::push_back(attr, val); traits::clear(val); } return true; `` Take note that we didn't call attr.push_back(val). Instead, we called a Spirit provided function: `` traits::push_back(attr, val); `` This is a recurring pattern. The reason why we do it this way is because attr [*can] be `unused_type`. `traits::push_back` takes care of that detail. The overload for unused_type is a no-op. Now, you can imagine why __spirit__ is fast! The parsers are so simple and the generated code is as efficient as a hand rolled loop. All these parser compositions and recursive parse invocations are extensively inlined by a modern C++ compiler. In the end, you get a tight loop when you use the kleene. No more excess baggage. If the attribute is unused, then there is no code generated for that. That's how __spirit__ is designed. The /what/ function simply wraps the output of the subject in a "kleene[" ... "]". Ok, now, like the `int_parser`, we have to hook our parser to the _qi_ engine. Here's how we do it: First, we enable the prefix star operator. In proto, it's called the "dereference": [composite_parsers_kleene_enable_] This is done in namespace `boost::spirit` like its friend, the `use_terminal` specialization for our `int_parser`. Obviously, we use /use_operator/ to enable the dereference for the qi::domain. Then, we need to write our generator (in namespace qi): [composite_parsers_kleene_generator] This essentially says; for all expressions of the form: `*p`, to build a kleene parser. Elements is a __fusion__ sequence. For the kleene, which is a unary operator, expect only one element in the sequence. That element is the subject of the kleene. We still don't care about the Modifiers. We'll see how the modifiers is all about when we get to deep directives. [endsect] [endsect]