Manual

104 Bison 2.3
10.2.1 Calc++ — C++ Calculator
Of course the grammar is dedicated to arithmetics, a single expression, possibly preceded
by variable assignments. An environment containing possibly predefined variables such as
one and two, is exchanged with the parser. An example of valid input follows.
three := 3
seven := one + two * three
seven * seven
10.2.2 Calc++ Parsing Driver
To support a pure interface with the parser (and the scanner) the technique of the “parsing
context” is convenient: a structure containing all the data to exchange. Since, in addition
to simply launch the parsing, there are several auxiliary tasks to execute (open the file for
parsing, instantiate the parser etc.), we recommend transforming the simple parsing context
structure into a fully blown parsing driver class.
The declaration of this driver class, ‘calc++-driver.hh’, is as follows. The first part
includes the CPP guard and imports the required standard library components, and the
declaration of the parser class.
#ifndef CALCXX_DRIVER_HH
# define CALCXX_DRIVER_HH
# include
# include
# include "calc++-parser.hh"
Then comes the declaration of the scanning function. Flex expects the signature of yylex
to be defined in the macro YY_DECL, and the C++ parser expects it to be declared. We can
factor both as follows.
// Announce to Flex the prototype we want for lexing function, ...
# define YY_DECL \
yy::calcxx_parser::token_type \
yylex (yy::calcxx_parser::semantic_type* yylval, \
yy::calcxx_parser::location_type* yylloc, \
calcxx_driver& driver)
// ... and declare it for the parser’s sake.
YY_DECL;
The calcxx_driver class is then declared with its most obvious members.
// Conducting the whole scanning and parsing of Calc++.
class calcxx_driver
{
public:
calcxx_driver ();
virtual ~calcxx_driver ();
std::map variables;
int result;