[Boost-users] "nested" spirit grammars and semantic actions

From: Tim Robertson (timr_at_[hidden])
Date: 2004-10-25 15:52:22

Hi,

I'm looking for advice on ways to "nest" spirit grammars that use 
semantic actions.

In the past, I have written a spirit grammar that parses 2-dimensional 
array data from a file, and puts the data into an array object of my 
choosing.  To do this, the grammar defines a new functor (array_insert) 
and calls the functor whenever a new array element is parsed.  The 
functor puts the element into a reference to the destination object, 
held within the grammar:

struct array_grammar
{
    array_grammar(MyMatrix & m) :  _inserter(m) {}

    struct array_inserter
    {
       array_inserter(MyMatrix & m) : _m(m) {}

       // operator() defined here...

       MyMatrix & _m;
    }

   // ...the usual grammar components...

  array_inserter _inserter;
};

This works, as expected.  However, I now have to write a parser for a 
file containing a collection of 2D arrays, where each array is 
delimited by tags (say, "BEGIN" and "END").  I would like to push each 
array into a std::vector of MyMatrix objects.  Thus, it would be nice 
if I could re-use the array_grammar, and write a grammar which uses it 
to parse each array in the larger file:

struct multi_array_grammar
{
    multi_array_grammar(std::vector<MyMatrix> & v) : _inserter(v) {}

    struct array_inserter
    {
       array_inserter(std::vector<MyMatrix> & v) : _v(v) {}

      // ... operator(), etc.

      std::vector<MyMatrix> & _v;
    };

    template <typename ScannerT>
    struct definition
    {
       definition(multi_array_grammar const & self)
       {
        array_begin = str_p("BEGIN");
        array_end = str_p("END");

          // Problem:  how to create array_grammar with correct 
reference?
        array_grammar array(???);

        r =  array_begin >> +array >> array_end;
       }
    };

    array_inserter _inserter;
};

The problem is that I can't figure out how to instantiate an 
array_grammar (which requires a reference to the current MyMatrix 
object) in order to parse the individual arrays.

I have solved this problem by creating a functor in multi_array_grammar 
which calls the parse function for array_grammar directly, but this 
seems ugly to me.  Do any of you have a better way?

-Tim