Experimental Physics and
Industrial Control System

Benjamin Franksen <[email protected]> · Thu, 11 Aug 2005 15:24:14 +0200

On Tuesday 09 August 2005 14:57, Kay-Uwe Kasemir wrote:
> Do you (and Ben) think that an interface like the DataDescriptor
> with getType(), getBitsize(), .. would work?
> Or is it necessary to get a
>    struct { Type type, int bitsize, ... }
> as you suggested earlier?

It may have advantages to use an abstract data type (for the type 
descriptor) instead of a concrete one. However, it should be possible 
for generic user code to store the type descriptor along with the data.

Your EpicsDataDescriptor is a pure interface. That means generic user 
code must always retrieve all the information bit by bit and construct 
its own variant of type descriptor, which I think is not very 
convenient. I would recommend to use a non-virtual class, here, that 
also contains copy-constructor and operator=, so that it can be passed 
(and stored) by value.

Another point: I think that calling inappropriate methods of 
EpicsDataDescriptor should throw an exception. For instance, calling 
getNumDims() on a type descriptor that does not represent an array 
throws an exception.

An alternative would be to use the visitor pattern. A C++ interface 
could look like this (sketch):

class TypeDescriptor {
    ...
	void traverse(TypeDescriptorVisitor *visitor);
}

class TypeDescriptorVisitor {
    virtual void on_Octet();
    virtual void on_Bool();
    virtual void on_Integer(bool signed, unsigned bitSize);
    virtual void on_Floating(unsigned bitSize);
    virtual void on_String();
    virtual void on_Timestamp();
    virtual void on_Enum(unsigned count, EpicsString *names);
    virtual void on_Array(unsigned dimCount, unsigned *sizes, 
TypeDescriptor &elemType);
    virtual void on_Structure(propertyCatalog &catalog);
    virtual void on_Any();
}

where the user code has to create an implementation of this interface 
and give DataAccess a reference to an object of this class if it wants 
type information.

In case you wonder what the on_Any() method is for: Note that the 
methods in class TypeDescriptorVisitor are /not/ pure virtual. This is 
because for convenient use they should have default definitions (i.e. 
implementations in the base class). The default is to call on_Any and 
the latter does nothing. Of course, you can override on_Any() to 
provide another default.

Example: Testing whether a type is an integer

// user code:
struct IsInteger : public TypeDescriptorVisitor {
    bool isInteger;
    virtual void on_Any() { isInteger = false; }
    virtual void on_Integer(bool, unsigned) { isInteger = true; }
}

Ben

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System