how to get a double from an NSString

FROM : Bob Sabiston
DATE : Mon May 26 21:13:49 2008

Hello,

  I'm trying to get an NSTableView to be editable, and I am having 
trouble getting a numerical value from the supplied object.  The 
setObjectValue method (?) supplies an argument called aData for the 
specific row and column.  That aData is an NSObject, and an NSString I 
think.  How do I convert that to a double floating point number?  I 
need to set a value in an array that is being passed from a Carbon app.

I've tried

double val = [aData doubleValue];

but that doesn't work.  I have little experience with Cocoa and I 
wrote this code years ago.

Thanks for any help,
Bob


- (void)tableView:(NSTableView *)aTableView setObjectValue:(id)aData
    forTableColumn:(NSTableColumn *)aCol
           row:(int)rowIndex
{
   double val = [aData doubleValue];