[/==============================================================================
    Copyright (C) 2001-2010 Joel de Guzman
    Copyright (C) 2001-2010 Hartmut Kaiser

    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:faq Spirit FAQ]

[heading I'm getting multiple symbol definition errors while using Visual C++. Anything I could do about that?]

Do you see strange multiple symbol definition linker errors mentioning 
`boost::mpl::failed` and `boost::spirit::qi::rule`? Then this FAQ entry might 
be for you.

__mpl__ implements a macro `BOOST_MPL_ASSERT_MSG()` which essentially is a 
more powerful version of static_assert. Unfortunately under certain 
circumstances using this macro may lead to the aforementioned linker errors. 

__spirit__ allows you to define a preprocessor constant disabling the usage
of `BOOST_MPL_ASSERT_MSG()`, while switching to `BOOST_STATIC_ASSERT()` instead.
For that you need define BOOST_SPIRIT_DONT_USE_MPL_ASSERT_MSG=1. Do this by 
adding 

    -DBOOST_SPIRIT_DONT_USE_MPL_ASSERT_MSG=1

on the compiler command line or by inserting a 

    #define BOOST_SPIRIT_DONT_USE_MPL_ASSERT_MSG 1 

into your code before any Spirit headers get included.

Using this trick has no adverse effects on any of the functionality of 
__spirit__. The only change you might see while using this workaround is less
verbose error messages generated from static_assert.


[heading I'm very confused about the header hell in my boost/spirit directory. What's all this about?]

The boost/spirit directory currently holds two versions of the Spirit library: 
__classic__ (former V1.8.x) and SpiritV2. Both are completely independent 
and usually not used at the same time. Do not mix these two in the same grammar.

__classic__ evolved over years in a fairly complex directory structure:

    boost/spirit/actor
    boost/spirit/attribute
    boost/spirit/core
    boost/spirit/debug
    boost/spirit/dynamic
    boost/spirit/error_handling
    boost/spirit/iterator
    boost/spirit/meta
    boost/spirit/symbols
    boost/spirit/tree
    boost/spirit/utility

While introducing Spirit V2 we restructured the directory structure in order to 
accommodate two versions at the same time. All of __classic__ now lives in 
the directory

    boost/spirit/home/classic 

where the directories above contain forwarding headers to the new location 
allowing to maintain application compatibility. The forwarding headers issue a 
warning (starting with Boost V1.38) telling the user to change their include 
paths. Please expect the above directories/forwarding headers to go away soon.

This explains the need for the directory

    boost/spirit/include

which contains forwarding headers as well. But this time the headers won't go 
away. We encourage application writers to use only the includes contained in 
this directory. This allows us to restructure the directories underneath if 
needed without worrying application compatibility. Please use those files in 
your application only. If it turns out that some forwarding file is missing, 
please report this as a bug.

Spirit V2 is not about parsing only anymore (as __classic__). It now consists 
out of 3 parts (sub-libraries): __qi__, __karma__, and __lex__. The header 
files for those live in

    boost/spirit/home/qi
    boost/spirit/home/karma
    boost/spirit/home/lex

and have forwarding headers in

    boost/spirit/include

__qi__ is the direct successor to __classic__ as it implements a DSEL (domain 
specific embedded language) allowing to write parsers using the syntax of C++ 
itself (parsers in the sense turning a sequence of bytes into an internal data 
structure). It is not compatible with __classic__, the main concepts are 
similar, though.

__karma__ is the counterpart to __qi__. It implements a similar DSEL but for 
writing generators (i.e. the things turning internal data structures into a 
sequence of bytes, most of the time - strings). __karma__ is the Yang to 
__qi__'s Yin, it's almost like a mirrored picture.

__lex__ is (as the name implies) a library allowing to write lexical analyzers. 
These are either usable stand alone or can be used as a front end for __qi__ 
parsers. If you know flex you shouldn't have problems understanding __lex__. 
This library actually doesn't implement the lexer itself. All it does is to 
provide an interface to pre-existing lexical analyzers. Currently it's using 
Ben Hansons excellent __lexertl__ library (proposed for a Boost review, BTW) as 
its underlying workhorse.

Again, don't use any of the header files underneath the boost/spirit/home 
directory directly, always include files from the boost/spirit/include 
directory.

The last bit missing is __boost_phoenix__ (which currently still lives under the 
Spirit umbrella, but already has been accepted as a Boost library, so it will 
move away). __boost_phoenix__ is a library allowing to write functional style C++, 
which is interesting in itself, but as it initially has been developed for 
Spirit, it is nicely integrated and very useful when it comes to writing 
semantic actions. I think using the boost/spirit/include/phoenix_... headers 
will be safe in the future as well, as we will probably redirect to the 
Boost.Phoenix headers as soon as these are available.


[heading Why doesn't my symbol table work in a `no_case` directive?]

In order to perform case-insensitive parsing (using __qi_no_case__) with a  
symbol table (i.e. use a __qi_symbols__
parser in a `no_case` directive), that symbol table needs to be filled with
all-lowercase contents. Entries containing one or more uppercase characters
will not match any input.


[heading I'm getting a compilation error mentioning `boost::function` and/or
         `boost::function4`. What does this mean?]

If you are using Visual C++ and have an error like:

[pre
error C2664: \'bool boost::function4<R,T0,T1,T2,T3>::operator ()(T0,T1,T2,T3) const\' :
    cannot convert parameter 4 from '...' to '...'
]

or you are using GCC and have an error like:

[pre
error: no match for call to '(const boost::function<bool ()(...)>) (...)'
note: candidates are: ... boost::function4<R,T1,T2,T3,T4>::operator()(T0,T1,T2,T3) const [with ...\]
]

then this FAQ entry may help you.

The definition of a __rule__ or __grammar__ may contain a skip parser type. If
it does, it means that non-terminal can only be used with a skip parser of a
compatible type. The error above arises when this is not the case, i.e.:

* a non-terminal defined with a skip parser type is used without a skip parser;
  for example, a rule with a skip parser type is used inside a `lexeme`
  directive, or a grammar with a skip parser type is used in `parse` instead of
  `phrase_parse`,
* or a non-terminal is used with a skip parser of an incompatible type;
  for example, a rule defined with one skip parser type calls a second rule
  defined with another, incompatible skip parser type.

[note The same applies to __karma__, replacing 'skip parser' and `lexeme`
      by 'delimit generator' and `verbatim`. Similarily, corresponding error 
      messages in __karma__ reference `boost::function3` and the 3rd 
      parameter (instead of the 4th).]

[endsect]