Manual

Chapter 10: C++ Language Interface 101
10 C++ Language Interface
10.1 C++ Parsers
10.1.1 C++ Bison Interface
The C++ parser LALR(1) skeleton is named ‘lalr1.cc’. To select it, you may either pass the
option ‘--skeleton=lalr1.cc’ to Bison, or include the directive ‘%skeleton "lalr1.cc"’
in the grammar preamble. When run, bison will create several entities in the ‘yy’ namespace.
Use the ‘%name-prefix’ directive to change the namespace name, see Section 3.7.11
[Decl Summary], page 59. The various classes are generated in the following files:
‘position.hh’
‘location.hh’
The definition of the classes position and location, used for location tracking.
See Section 10.1.3 [C++ Location Values], page 102.
‘stack.hh’
‘file.hh’
‘file.cc’
An auxiliary class stack used by the parser.
(Assuming the extension of the input file was ‘.yy’.) The declaration and implementation
of the C++ parser class. The basename and extension of these two
files follow the same rules as with regular C parsers (see Chapter 9 [Invocation],
page 97).
The header is mandatory; you must either pass ‘-d’/‘--defines’ to bison, or
use the ‘%defines’ directive.
All these files are documented using Doxygen; run doxygen for a complete and accurate
documentation.
10.1.2 C++ Semantic Values
The %union directive works as for C, see Section 3.7.4 [The Collection of Value Types],
page 55. In particular it produces a genuine union 1 , which have a few specific features in
C++.
− The type YYSTYPE is defined but its use is discouraged: rather you should refer to the
parser’s encapsulated type yy::parser::semantic_type.
− Non POD (Plain Old Data) types cannot be used. C++ forbids any instance of classes
with constructors in unions: only pointers to such objects are allowed.
Because objects have to be stored via pointers, memory is not reclaimed automatically:
using the %destructor directive is the only means to avoid leaks. See Section 3.7.7 [Freeing
Discarded Symbols], page 57.
1 In the future techniques to allow complex types within pseudo-unions (similar to Boost variants) might
be implemented to alleviate these issues.