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## Spirit FAQ

#### 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.

Boost.Mpl implements a macro `BOOST_MPL_ASSERT_MSG()` which essentially is a more powerfull version of static_assert. Unfortunately under certain circumstances using this macro may lead to the aformentioned 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.

#### 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: Spirit.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.

Spirit.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 Spirit.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 Spirit.Classic). It now consists out of 3 parts (sub-libraries): Spirit.Qi, Spirit.Karma, and Spirit.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

Spirit.Qi is the direct successor to Spirit.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 Spirit.Classic, the main concepts are similar, though.

Spirit.Karma is the counterpart to Spirit.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). Spirit.Karma is the Yang to Spirit.Qi's Yin, it's almost like a mirrored picture.

Spirit.Lex is (as the name implies) a library allowing to write lexical analyzers. These are either usable standalone or can be used as a frontend for Spirit.Qi parsers. If you know flex you shouldn't have problems understanding Spirit.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.

#### Why doesn't my symbol table work in a `no_case` directive?

In order to perform case-insensitive parsing (using `no_case`) with a symbol table (i.e. use a `symbols<Ch, T>` 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.

#### 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:

    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:

    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 Spirit.Karma, replacing 'skip parser' and `lexeme` by 'delimit generator' and `verbatim`. Similarily, corresponding error messages in Spirit.Karma reference `boost::function3` and the 3rd parameter (instead of the 4th).