Why is the C++ STL is so heavily based on templates? (and not on *interfaces*)
I mean, aside from its obligating name (the Standard Template Library)...
C++ initially intended to present OOP concepts into C. That is: you could tell what a specific entity could and couldn't do (regardless of how it does it) based on its class and class hierarchy. Some compositions of abilities are more difficult to describe in this manner due to the problematics of multiple inheritance, and the fact that C++ supports the concept of interfaces in a somewhat clumsy way (compared to java, etc), but it's there (and could be improved).
And then templates came into play, along with the STL. The STL seemed to take the classical OOP concepts and flush them down the drain, using templates instead.
There should be a distinction between cases when templates are used to generalize types where the types themeselves are irrelevant for the operation of the template (containers, for examples). Having a vector<int> makes perfect sense.
However, in many other cases (iterators and algorithms), templated types are supposed to follow a "concept" (Input Iterator, Forward Iterator, etc...) where the actual details of the concept are defined entirely by the implementation of the template function/class, and not by the class of the type used with the template, which is a somewhat anti-usage of OOP.
For example, you can tell the function:
void MyFunc(ForwardIterator<...> *I);
Update: As it was unclear in the original question, ForwardIterator is ok to be templated itself to allow any ForwardIterator type. The contrary is having ForwardIterator as a concept.
expects a Forward Iterator only by looking at its definition, where you'd need either to look at the implementation or the documentation for:
template <typename Type> void MyFunc(Type *I);
Two claims I can make in favor of using templates: compiled code can be made more efficient, by tailor-compiling the template for each used type, instead of using vtables. And the fact that templates can be used with native types.
However, I am looking for a more profound reason why abandoning classical OOP in favor of templating for the STL? (Assuming you read that far :P)
Answer
The short answer is "because C++ has moved on". Yes, back in the late 70's, Stroustrup intended to create an upgraded C with OOP capabilities, but that is a long time ago. By the time the language was standardized in 1998, it was no longer an OOP language. It was a multi-paradigm language. It certainly had some support for OOP code, but it also had a turing-complete template language overlaid, it allowed compile-time metaprogramming, and people had discovered generic programming. Suddenly, OOP just didn't seem all that important. Not when we can write simpler, more concise and more efficient code by using techniques available through templates and generic programming.
OOP is not the holy grail. It's a cute idea, and it was quite an improvement over procedural languages back in the 70's when it was invented. But it's honestly not all it's cracked up to be. In many cases it is clumsy and verbose and it doesn't really promote reusable code or modularity.
That is why the C++ community is today far more interested in generic programming, and why everyone are finally starting to realize that functional programming is quite clever as well. OOP on its own just isn't a pretty sight.
Try drawing a dependency graph of a hypothetical "OOP-ified" STL. How many classes would have to know about each others? There would be a lot of dependencies. Would you be able to include just the vector header, without also getting iterator or even iostream pulled in? The STL makes this easy. A vector knows about the iterator type it defines, and that's all. The STL algorithms know nothing. They don't even need to include an iterator header, even though they all accept iterators as parameters. Which is more modular then?
The STL may not follow the rules of OOP as Java defines it, but doesn't it achieve the goals of OOP? Doesn't it achieve reusability, low coupling, modularity and encapsulation?
And doesn't it achieve these goals better than an OOP-ified version would?
As for why the STL was adopted into the language, several things happened that led to the STL.
First, templates were added to C++. They were added for much the same reason that generics were added to .NET. It seemed a good idea to be able to write stuff like "containers of a type T" without throwing away type safety. Of course, the implementation they settled on was quite a lot more complex and powerful.
Then people discovered that the template mechanism they had added was even more powerful than expected. And someone started experimenting with using templates to write a more generic library. One inspired by functional programming, and one which used all the new capabilities of C++.
He presented it to the C++ language committee, who took quite a while to grow used to it because it looked so strange and different, but ultimately realized that it worked better than the traditional OOP equivalents they'd have to include otherwise. So they made a few adjustments to it, and adopted it into the standard library.
It wasn't an ideological choice, it wasn't a political choice of "do we want to be OOP or not", but a very pragmatic one. They evaluated the library, and saw that it worked very well.
In any case, both of the reasons you mention for favoring the STL are absolutely essential.
The C++ standard library has to be efficient. If it is less efficient than, say, the equivalent hand-rolled C code, then people would not use it. That would lower productivity, increase the likelihood of bugs, and overall just be a bad idea.
And the STL has to work with primitive types, because primitive types are all you have in C, and they're a major part of both languages. If the STL did not work with native arrays, it would be useless.
Your question has a strong assumption that OOP is "best". I'm curious to hear why. You ask why they "abandoned classical OOP". I'm wondering why they should have stuck with it. Which advantages would it have had?