What is the impact of using a variant data type on performance, speed memory demands etc?

Question

This is another one of my "lets get this settled once and for all" threads.
I have avoided variant data types whenever possible to keep the performance of my apps up. From some observatsions i have made over the years, I am of the opinion that;
1) In-place operations can not be carried out on variants.
2) Passing a variant to a sub-VI (regardless of the terminal on the icon connector) are always copied.
I would like confirmation or correction of the above so we know more about this animal we call LabVIEW.
Thank you,
Ben
Ben Rayner
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Answer 1

A couple notes:
1. You can use the To/From Variant nodes for the Inplace Element Structure to operate inplace on the contained variant data. This should work just fine even if you have a big 2D array contained.
2. Variants are incredibly useful when you use them for Variant Attributes, rather than just storing data. The Variant Attribute implementation uses a binary tree to store the key value pairs, which provides quick insertion and lookup.
3. I don't work on the internals of LV, but I see no reason whatsoever that passing a variant into a subVI would always necessitate a copy. I could be wrong, but that sounds very odd to me.
However, I have generally started trending towards using flattened strings. Flattened strings are generally more efficient memory-wise, because they don't contain all the type descriptor information, just the raw data. (I think there's some exception when flattening LV Classes.) Variants always store the type descriptor, so even an empty variant can take up a bit of memory.
Variants received a big performance boost from 7.x to 8.x. They used to have a bad reputation, but I wouldn't be afraid to use them where applicable. I personally don't see a lot of benefits over flattened strings, though, except in the case of tools like the OpenG Variant INI files, which can dynamically parse out and construct variant types.
Jarrod S.
National Instruments