Experimental Physics and
Industrial Control System

[email protected] (Jeff Hill) · Wed, 8 Mar 2000 10:24:20 -0700

Ralph,

My first comment is that I am fixing all bugs and deficiencies 
in the GDD code as they are discovered. This unfortunately has been a 
significant effort. In the long term we have prepared a vastly simpler and 
more efficient alternative to GDD which we intend to support in parallel to
GDD via a new base class for class "casPV".

> 
>  1) For easy-to-use and fast gdd assignments, the operator = is
>     overloaded in gddI.h to use memcpy().
> 
>     (I thought memcpy() was the default for operator =, but anyway.)
> 
>     When this operator is used to assign gdds that are container
>     members, the pointer to the next element is included in the memcpy()
>     and screws up the member lists of both containers.

This bug appears to still be in the version in CVS. Will you be committing 
a fix to the main trunk?

>     The frequently used gddApplicationTypeTable function to create
>     standard gdds returns a gdd*. So using operator [] will do regular
>     pointer arithmetics and not use the overloaded operator.
> 
>     Even if you use a gddContainer* (through a dangerous cast) the
>     overloaded operator [] takes an unsigned int as index argument
>     whereas the constants defined in gddApps.h are signed.

The version committed to CVS appears to have overloaded [] operator in class gdd
for both integer and unsigned indexes. These operators should also be in class 
gddContainer since it publicly derives from gdd. I created this patch last spring
but it appears to have been committed to CVS in August of last year. NOTE THAT
IN C++ THE OVERLOADED [] OPERATOR WILL BE USED ONLY IF YOU ARE DIRECTLY ACCESSING
THE OBJECT OR USING A REFERENCE TO THE OBJECT. THE BUILT-IN [] OPERATOR IS ALWAYS
USED BY C++ WHEN A POINTER IS ACCESSING THE OBJECT.

> 
>     New standard gddContainer objects (created using the AppTypeTable)
>     are always malloc'd as one contiguous block, so pointer arithmetics
>     will work. By chance.

In Jim's nomenclature a container GDD can be "flat". If it is flat the 
container is just a linear array of GDDs that can be indexed with the default
C++ [] operator. Last spring I found a nasty bug where internally GDD's "dbMapper.cpp"
was using the [] operator, but not checking to see if the container was flat. My fix
was to overloaded the [] operator so that the behavior is different when the GDD 
container is scattered around in memory and accessed as a linked list, and therefore not
"flat". The performance will of course be abysmal, but at least we can get reliable 
operation in all situations. This fix is committed to CVS.

> (... live from the coding dungeon)
>

Join the club. I have been stuck with the GDD dungeon since Jim left.

Jeff

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System