reuse parsed variable with boost karma - c++

Question

I have a code base which is quite equivalent to the code below.
I try to generate a text file with two times the content of a variable.
I feel that the answer is in semantic actions and _a and _val but cannot manage to get through even with the documentation.
How will you do to have :
"toto" in str
and output :
toto some stuff toto
i.e how to reuse a parsed variable in karma ?
struct data
{
std::string str;
};
BOOST_FUSION_ADAPT_STRUCT(
data,
(std::string, str)
)
template <typename Iterator>
struct data: karma::grammar<Iterator, data() >
{
data():data::base_type(start)
{
start = karma::string << karma::lit("some stuff") << karma::string; //Second string is in fact the first one
}
karma::rule<Iterator, data()> start;
};
Solution (according to posts below :)
#include <iostream>
#include <string>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/spirit/include/karma.hpp>
#include <boost/spirit/include/phoenix_core.hpp>
#include <boost/spirit/include/phoenix_operator.hpp>
#include <boost/spirit/include/phoenix_fusion.hpp>
#include <boost/spirit/include/phoenix_stl.hpp>
#include <boost/spirit/include/phoenix_object.hpp>
#include <boost/spirit/include/support_istream_iterator.hpp>
#include <boost/spirit/include/support_iso8859_1.hpp>
namespace ast
{
struct data
{
std::string str;
};
}
BOOST_FUSION_ADAPT_STRUCT(
ast::data,
(std::string, str)
)
namespace karma = boost::spirit::karma;
namespace parser
{
template <typename Iterator>
struct data: karma::grammar<Iterator, ast::data() >
{
data():data::base_type(start)
{
start =
karma::string[karma::_1 = boost::phoenix::at_c<0>(karma::_val)] <<
karma::lit("some stuff") <<
karma::string[karma::_1 = boost::phoenix::at_c<0>(karma::_val)]
;
}
karma::rule<Iterator, ast::data()> start;
};
}
main()
{
ast::data d;
d.str = "toto";
std::string generated;
typedef std::back_insert_iterator<std::string> iterator_type;
parser::data<iterator_type> d_p;
iterator_type sink(generated);
karma::generate(sink, d_p, d);
std::cout << generated << std::endl;
}

Answer 1

This should do the trick:
start = karma::string[karma::_1 = karma::_val]
<< karma::lit("some stuff")
<< karma::string[karma::_1 = karma::_val];