[iainmerrick]

I didn’t know that, thanks for the correction! (Edit: although now I’m not sure I was wrong after all; see my reply to the other comment)

I had the wrong idea because (if I understand right) C#’s generic collections seem to be a copy of Java’s (which in turn were inspired by C++’s STL). Do I have that one the right way round?

The C# collections use generally poorer name choices (e.g. IEnumerator, which does not actually enumerate the elements) so I assumed they wanted to avoid making it identical to Java.

[pvg]

Java's collections are not much like STL (and pre-date Java generics) and important implementation details of generics in C++, C# and Java differ very substantially.

[iainmerrick]

They are not very similar in implementation due to the nature of the languages. But the idea that it would be useful for the Java standard library to provide flexible, extensible, general-purpose containers (as opposed to ad-hoc stuff like Vector and Hashtable) was inspired by the STL.

[pvg]

I don't think that's accurate or that the general idea comes from STL. STL itself is a very particular kind of design.

[iainmerrick]

What’s a good example of a really flexible, composable containers library that predates STL?

I think STL was the first mainstream one. I guess some of the ideas came from Ada? But I don’t know if Ada had standardized containers.

[pjmlp]

Smalltalk and Eiffel collections.

Long before STL was a thing in C++.

Stepanov started STL original design in Ada, before switching to C++. Recent Ada standards have adopted collection classes as well.