Experimental Physics and Industrial Control System
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Benjamin Franksen wrote:
On Friday 15 July 2005 14:51, Marty Kraimer wrote:
The set of well defined types is:
enum NadType {
nadTypeUnknown,
nadTypeBoolean,
nadTypeOctet,
nadTypeInt16,
nadTypeUInt16,
nadTypeInt32,
nadTypeUInt32,
nadTypeInt64,
nadTypeUInt64,
nadTypeFloat32,
nadTypeFloat64,
nadTypeString,
nadTypeArray,
nadTypeMDArray,
nadTypePropertyCatalog,
// Following are usefull common types
nadTypeTimeStamp,
nadTypeEnum,
nadTypeMap
};
We can write down enumerations like that all day long and it won't bring
us any further. The real question is:
Q: How is a client supposed to find out how to store the data it wants,
so that no information is lost?
Knowing that something is an array obviously isn't enough, you need to
know how large it is and what the element type is. And then you must
recursively ask how to describe the element types, and so on.
We want to send /structured/ data over the network and we don't want to
have a fixed number of data types, that is what we have today in CA. We
need a way to describe the structure and send it along with the data
(yes, not each time, only whenever necessary). So why stick to this
useless enumeration of 'types', of which only some really are types and
the others are type /constructors/. A mere enumeration cannot specify
what the arguments to the type constructors are.
This does not invalidate the above set of enums. It just means that if
the type is nadTypeArray, it must be possible to ask for the element
type and number of elements. If the type is nadTypeMDArray, it must be
possible to ask for number dimensions, the element type and number of
elements. If the type is nadPropertyCatalog, then it must be possible to
ask for the number of fields and the type of each element.
Marty
- References:
- Network Accessable Types Marty Kraimer
- Re: Network Accessable Types Benjamin Franksen
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ANJ, 02 Feb 2012 |
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