Fatal GDD deficiencies

[Ralph.Lange@mail.bessy.de]

If you don't use the GDD library, relax.

I have found two GDD features that were not quite obvious to me and
might cause serious trouble.

Both qualities show up when using gddContainer objects. These containers
are used to represent the Channel Access complex DBR types. They
consist of a container object (derived from gdd) and a linked list of
member gdds.

 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.

|   Do not use operator = for gdds that are located inside containers. |
|   Use the target gdd's put() method instead, which only copies the   |
|   data. It is less efficient, but works.                             |
    
 2) gddContainer has an overloaded operator [] that is designed to make
    the container members available as if the container was an array of
    gdds. 

    This "addressing mode" for container members looks very efficient,
    but is isn't, as it creates an iterator and walks through the
    container list to find the addressed member. (This fact is mentioned
    in the header file, but not in the docs.)

    If you nevertheless try to use this operator, be careful: you don't.

    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.

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

|   If you set up a gddContainer yourself or change/insert/remove      |
|   member gdds, using operator [] will screw up your container.       |
|   Using an iterator in a loop and switching on the member's appType  |
|   seems the only safe way to access member gdds.                     |

Ralph     (... live from the coding dungeon)