www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.bugs - [Issue 851] New: strange bug when a delegate access a member variable.

reply d-bugmail puremagic.com writes:

           Summary: strange bug when a delegate access a member variable.
           Product: D
           Version: 1.00
          Platform: PC
               URL: http://dsource.org/projects/qonkd/browser/trunk/bug
        OS/Version: Linux
            Status: NEW
          Severity: blocker
          Priority: P1
         Component: DMD
        AssignedTo: bugzilla digitalmars.com
        ReportedBy: rodolfo.borges gmail.com

I was implementing a menu system for my game, when I found this bug.
The menu item object is created with a delegate for the action to be taken.
When I pass a member function as the delegate, and it tries to access a member
variable, it crashes.

The funny thing is (that helped me think it can be a dmd bug), if the calling
part is inside a loop or function, it crashes, but if it's inside the main, it
works ok.  Check the version(crash) part in main.d to see what I'm talking

The menu.d file is unchanged, but I tried and made main.d the minimum possible
to reproduce the bug.  I hope it's clear enough.


Jan 16 2007
next sibling parent d-bugmail puremagic.com writes:

I'd just like to add that this bug has been around since at least dmd v0.167.
Sorry I didn't reported it before.

Jan 17 2007
prev sibling parent d-bugmail puremagic.com writes:

lio lunesu.com changed:

           What    |Removed                     |Added
             Status|NEW                         |RESOLVED
         Resolution|                            |INVALID

This is not a bug.

This issue gets posted on the newsgroups at least once per month, and I'd
suggest you search the groups for more detailed information. It's not a bug:
nested functions are only valid inside the function where they are declared.
This is because those function expect the stack to be exactly the way it was in
that function. In your example, do_start is only valid inside the constructor.
You can create delegates to nested function, but these delegates are not valid
outside the function. When these delegates are invoked, the stack might be
different, which means the this-pointer cannot be found. 

As I said: the stack might be different. This is why it might work if the stack
is left intact. If you do something that changes the stack (like an extra
function call, or declare some variables) the stack will have changed and the
delegates will no longer work.

To solve your code, move the do_start to the class. Then, it will no longer
depend on the stack, but it gets the "this" pointer directly (instead of a
stack pointer) and you can safely create and return delegates to it.

Feb 12 2007