Access to an array of OSStructure - why this way ?

[marten]

Today I came across a nice, hidden "feature" when accessing an array of instances of subclasses of OSStructure and I do not understand, why the access was done this way in VA.

When first working with this data structure I wrote the following code:

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osBlobArray := OSBlob calloc: 10
(osBlobArray at: 0) cbSize: 12.


The intention was, that in the first element of this array the structure member cbSize should be set to 12. But this code failed ....

After reading the source code in the upper class I found out, that it should be written like:

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osBlobArray := OSBlob calloc: 10
osBlobArray at: 0 put: ((osBlobArray at: 0) cbSize: 12).


Why all that copying ? What was the intention behind that ? Or is there a better way to do it ?

Marten

[wembley]

Marten -

Yes, this is the most straightforward way of accessing/updating a member of an array of OSStructures. However, you can use something like the following to perform the update directly in OS memory:

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|osBlobArray element |

osBlobArray := OSBlob calloc: 10.
element := 7.
(osBlobArray pointerAt: osBlobArray class fixedSize * element type: osBlobArray class) cbSize: 12.


Regards, John

[TheIconoclast]

You can always go the direct route and just muck with things directly:

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| osBlobArray element cbSizeOffset |
osBlobArray := OSBlob calloc: 10.
element := 7.
cbSizeOffset := 0.
osBlobArray uint32At: (cbSizeOffset + osBlobArray class fixedSize * element) put: 12.


You can then get access that value with

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(osBlobArray at: element) cbSize

Sure it defeats the purpose of using a high-level language, but I'm guessing it could lead to some wonderfully obfuscated code.

----------------------
Brian O'Connell
Simulation Domain
Joint Analysis System (JAS)

[wembley]

Brian -

I find almost all code involved with OSPtr and its subclasses to be wonderfully obfuscated. We are always open to suggestions for how to remedy this situation.