When is %destructor invoked in BISON?

Question

When is %destructor invoked in BISON? I have the following bison code:

%union{
    char * sval; 
    Variable * vval; 
} 

%token VARIABLE 
%token Literal 
%type <vval> Expression VARIABLE 
%type <sval> Literal 

%destructor { delete $$; } <vval> 
%destructor { delete $$; } Literal 

where Variable is a class. I thought that after processing a line, all the Variable objects will be freed, but I can see no destructor invoked. And that will lead straightly to memory leak...

Edit: To be clear; I allocate a new Variable object for a new token, and this token is pushed to the BISON stack. I want to delete the Variable when it is popped by bison and discarded from the stack. I thought that %destructor serves that purpose, but I am not sure anymore..

Answer 1

From the Bison Manual:

Discarded symbols are the following:

• stacked symbols popped during the first phase of error recovery,
• incoming terminals during the second phase of error recovery,
• the current lookahead and the entire stack (except the current right-hand side symbols) when the parser returns immediately, and
• the start symbol, when the parser succeeds.

So if you don't hit an error, the %destructor will be called on the stack if you return immediately (call YYABORT or YYACCEPT), or it will call it on start symbol if parsing succeeds.

Answer 2

I figured out, that I should free() it after I perform the action, so for example

...
| String CONCAT String { $$ = concat($1,$3); free($1); free($3); }
...

That did the trick for me.