Optimal way to implement an attribute map

I’d like to get opinions on the good ways to implement an attribute map that I can save to a ROOT file. Effectively I have a bunch of objects in my files, and I want to store alongside them a generic map of attributes of potentially arbitrary type, so I could use this to attach e.g. string or int attributes to any object in my file.

So one idea is try using a TMap, with key equal to the object, and value equal to a TList. Inside the TList I could utilise TParameter for numerical attributes, TNamed for string attributes.

My concern is perhaps this is inefficient? Lots of TObjects, I believe there are eventually limits on the number of TObjects you can have in a given process (16M?). Also the TList doesnt ensure each object it contains has a unique name …

Are there any other thoughts here? Remember I need to be able to save this attribute map to the file alongside the objects that it is providing attributes for.


There is no limit on the number of TObjects. (There used to be a limit on the number of TObjects that were referenced by a TRef but that has been lifted too a very long time ago).

The only limit you might encounter is the 1Gb limit for each I/O transaction/operation/TKey.


Ok thanks for the comment there Philippe.

I wanted to share some experience from experimenting over the past couple of days, because it still isn’t entirely clear what the best implementation is.

I started out with a TMap, with keys being the objects themselves and values being a TList of the attributes to associate to the object. Initially I found this limited me to the attribute names effectively being the same as the attributes themselves: I couldn’t attribute one object to two different objects under different attribute names. A hacky workaround was to use a TObjArray as a containing object, since I can name the TObjArray how I please, and the single element in the array is interpreted as the attribute value. I really wanted something like a TRef but I cannot give a TRef a name of my choosing. Have I missed an obvious existing class here?

This made me think then, should I just be using another TMap for the attributes, rather than a TList? I could then use TObjStrings as the keys here.

Are there some good pros and cons other using a TMap rather than a TList of the container type of my attributes?

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