Evan Lavelle wrote: > I'd like to pass a token class from Flex to Bison (ideally, a > tr1::shared_ptr - a reference counted smart pointer class). However, in > Bison itself I need to deal with a number of symbol types - tokens, AST > nodes, and so on. > > This means that I need to use a %union, but this is difficult because > the classes are too complex to put in a union. Using class pointers in > the union is difficult, because this messes up the reference counting, > and I have to do manual resource management.
Hmm = I tend to use a pointer-to-AST-node as main YYSTYPE, occasionally adding a char or integer field if needed (and it rarely is). Given that yylval is purely used to pass a token's value from flex to bison, reference counting seems fairly pointless (as long as you take care not to silently discard token values, but cases where flex constructs a value and bison ignores it completely should typically be rare). Of course, you could just make a class/struct MyTokenValue that has various shared_ptr members, and make YYSTYPE a shard_ptr to objects of that class - then there's no unions to worry about, and when a MyTokenValue object dies, its shared_ptr members should ensure that actual values die with it if needed. It will use more memory than a union, but will have the added flexibility of allowing multiple value types at once (e.g. perhaps letting a token have both a string value and an AST node value). > One fix might be to split up the Flex and Bison symbol types - is this > possible? In other words, Flex and Bison communicate using only one > type, and Bison has its own multiple types for terminals, non-terminals, > and so on (it seems strange that F/B share a single YYSTYPE - why is this?) YYSTYPE is the type for values of terminals and nonterminals alike; so since flex supplies the values of tokens, it makes sense that it returns a value of type YYSTYPE (note that terminals can have multiple value types too!). _______________________________________________ Help-bison@gnu.org http://lists.gnu.org/mailman/listinfo/help-bison
