[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.