* New warning: unused values
Typed right-hand side symbols whose value are not used are reported.
- For instance
+ For instance:
+
+ exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
+ | exp "+" exp
+ ;
+
+ will trigger a warning about $$ and $5 in the first rule, and $3 in
+ the second ($1 is copied to $$ by the default rule). This example
+ most likely contains three errors, and should be rewritten as:
+
+ exp: exp "?" exp ":" exp { $$ = $1 ? $3 : $5; }
+ | exp "+" exp { $$ = $1 + $3; }
+ ;
+
+ However, if the original actions were really intended, the warnings
+ can be suppressed by letting Bison believe the values are used, e.g.:
+
+ exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
+ | exp "+" exp { $$ = $1; (void) $3; }
+ ;
+
+ If there are mid-rule actions, the warning is issued if no action
+ uses it. The following triggers no warning: $1 and $3 are used.
- exp: exp "?" exp ":" exp { $$ = $1 + $3; }
- | exp "+" exp
- ;
+ exp: exp { push ($1); } '+' exp { push ($3); sum (); };
- will trigger a warning about $5 of the first rule, and $3 in the
- second ($1 is copied to $$ by the default rule). To avoid this
- warning, let Bison believe the value is used, e.g.
+ Mid-rule actions that use $$ cause the corresponding value to be
+ set, therefore the following action must use it. The following rule
+ triggers a warning about $2.
- exp: exp "?" exp ":" exp { $$ = $1 + $3; $5; }
- | exp "+" exp { $$ = $1; $3; }
+ exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
- This helps catching lost values and memory leaks: if a value is
- ignored, its associated memory will never be reclaimed.
+ The warning is intended to help catching lost values and memory leaks.
+ If a value is ignored, its associated memory typically is not reclaimed.
* %destructor vs. YYABORT, YYACCEPT, and YYERROR.
Destructors are now called when user code invokes YYABORT, YYACCEPT,