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Qi flush_multi_pass parser


The Spirit.Qi flush_multi_pass parser is a primitive (pseudo) parser component allowing to clear the internal buffer of a multi_pass iterator. Clearing the buffer of a multi_pass might be beneficial for grammars where it is clear that no backtracking can occur. The general syntax for using the flush_multi_pass is:


which will call the clear_queue() member function if the current iterators are of the type multi_pass. This will cause any buffered data to be erased. This also will invalidate all other copies of multi_pass and they should not be used. If they are, an boost::illegal_backtracking exception will be thrown. For all other iterator types this is a no-op. The flush_multi_pass generates a parser component which always succeeds and which does not consume any input (very much like eps).

// forwards to <boost/spirit/repository/home/qi/primitive/flush_multi_pass.hpp>
#include <boost/spirit/repository/include/qi_flush_multi_pass.hpp>

The flush_multi_pass does not require any parameters.


The flush_multi_pass component exposes no attribute (the exposed attribute type is unused_type):

flush_multi_pass --> unused

The following example shows a simple use case of the flush_multi_pass parser.

We will illustrate its usage by generating different comment styles and a function prototype (for the full example code see here: flush_multi_pass.cpp)


In addition to the main header file needed to include the core components implemented in Spirit.Qi we add the header file needed for the new flush_multi_pass parser.

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/repository/include/qi_flush_multi_pass.hpp>

To make all the code below more readable we introduce the following namespaces.

namespace spirit = boost::spirit;
using boost::spirit::repository::flush_multi_pass;

Clearing the internal buffer

The example grammar recognizes the (simplified) preprocessor commands #define and #undef both of which are constraint to a single line. This makes it possible to delete all internal iterator buffers on each detected line break. This is safe as no backtracking will occur after any line end. The following code snippet shows the usage of flush_multi_pass for this purpose.

template <typename Iterator, typename Skipper>
struct preprocessor : spirit::qi::grammar<Iterator, Skipper>
    // This is a simplified preprocessor grammar recognizing
    //   #define MACRONAME something
    //   #undef MACRONAME
    // Its sole purpose is to show an example how to use the 
    // flush_multi_pass parser. At the end of each line no backtracking can
    // occur anymore so that it's safe to clear all internal buffers in the 
    // multi_pass.
    preprocessor() : preprocessor::base_type(file)
        using spirit::ascii::char_;
        using spirit::qi::eol;
        using spirit::qi::lit;

        file =

        line =  ( command |  *(char_ - eol) )
            >>  eol
            >>  flush_multi_pass

        command =
                "#define" >> *lit(' ') >> *(char_ - ' ') >> *lit(' ') >> *(char_ - eol)
            |   "#undef"  >> *lit(' ') >> *(char_ - eol)

    spirit::qi::rule<Iterator, Skipper> file, line, command;

[Note] Note

Using the flush_multi_pass parser component with iterators other than multi_pass is safe as it has no effect on the parsing.