CODEWITHSUNDEEP
bisonbisonc++
Guy Sadoun
Guy Sadoun

Reputation: 535

bison - can I pass non-trivial YYSTYPE by reference?

for example, I have this declaration in .ypp:

%{
#include <iostream>
#include <stdlib.h>
#include "structs.h"
#include "func.h"
#define YYSTYPE Types
%}
%union Types{
   int Integer;
   bool Boolean;
   Expression Exp;
 };

and in "structs.h" I have:

typedef union{
string str;
int integer;
bool boolean;
} Value;

struct Expression {
int lineNum;
Value val;
};

in "func.h" I have this func:

int binop(Types& a, Types& b); //assume all file needed included

can I pass inside the .ypp the $$ arg:

Exp : Exp Add Exp {$$.integer = binop($1, $2);}

Upvotes: 0

Views: 72

Answers (1)

rici
rici

Reputation: 241791

You can certainly pass a semantic value by reference, but you can't assume that the reference will remain valid beyond the execution of the semantic action. So don't keep a persistent reference to a semantic value.

Semantic values are stored on the parser stack, and parser stack slots are reused at the parser's convenience (since it is really a stack). In particular, the $1...$n slots are popped immediately after the semantic action is executed, while $$ is a temporary which is pushed (copied) onto the stack after $1...$n are popped.

Even if the value were not popped right away -- as would be the case for a mid-rule semantic action -- the parser stack may be reallocated (thereby invalidating all references) in order to accommodate a push.

Upvotes: 2

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