Question

In the bison manual in section 2.1.2 Grammar Rules for rpcalc, it is written that:

In each action, the pseudo-variable $$ stands for the semantic value
for the grouping that the rule is going to construct. Assigning a
value to $$ is the main job of most actions

Does that mean $$ is used for holding the result from a rule? like:

exp exp '+'   { $$ = $1 + $2;      }

And what’s the typical usage of $$ after begin assigned to?

Answer 1

Yes, $$ is used to hold the result of the rule. After being assigned to, it typically becomes a $x in some higher-level (or lower precedence) rule.

Consider (for example) input like 2 * 3 + 4. Assuming you follow the normal precedence rules, you’d have an action something like: { $$ = $1 * $3; }. In this case, that would be used for the 2 * 3 part and, obviously enough, assign 6 to $$. Then you’d have your { $$ = $1 + $3; } to handle the addition. For this action, $1 would be given the value 6 that you assigned to $$ in the multiplication rule.