ANTLR 4 : How to know the existence of subpart in rule

Question

I have this code :

varDeclaration
    :   type ID ('=' expression)? ';' 
    ;

So, not always ('=' expression) exist. But, sometimes, I want to process this part, but don't know it exist or not in this context. I'm using ANTLR 4 (and often using Listener), how can I know this.

Thanks :)

Answer 1

In your listener (exitVarDeclaration) or visitor (visitVarDeclaration) check whether ctx.expression() == null. If null, then ('=' expression) didn't exist. If non-null, then it did exist.