Why is the C++ STL is so heavily based on template

Question

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 1

This question has many great answers. It should also be mentioned that templates supports an open design. With the current state of object oriented programming languages, one has to use the visitor pattern when dealing with such problems, and true OOP should support multiple dynamic binding. See Open Multi-Methods for C++, P. Pirkelbauer, et.al. for very intersting reading.

Another interesting point of templates are that they can be used on for runtime polymorphism as well. For example

template<class Value,class T>
Value euler_fwd(size_t N,double t_0,double t_end,Value y_0,const T& func)
    {
    auto dt=(t_end-t_0)/N;
    for(size_t k=0;k<N;++k)
        {y_0+=func(t_0 + k*dt,y_0)*dt;}
    return y_0;
    }

Notice that this function will also work if Value is a vector of some kind (not std::vector, which should be called std::dynamic_array to avoid confusion)

If func is small, this function will gain a lot from inlining. Example usage

auto result=euler_fwd(10000,0.0,1.0,1.0,[](double x,double y)
    {return y;});

In this case, you should know the exact answer (2.718...), but it is easy to construct a simple ODE without elementary solution (Hint: use a polynomial in y).

Now, you have a large expression in func, and you use the ODE solver in many places, so your executable gets polluted with template instantiations everywhere. What to do? First thing to notice is that a regular function pointer works. Then you want to add currying so you write an interface and an explicit instantiation

class OdeFunction
    {
    public:
        virtual double operator()(double t,double y) const=0;
    };

template
double euler_fwd(size_t N,double t_0,double t_end,double y_0,const OdeFunction& func);

But the above instantiation only works for double, why not write the interface as template:

template<class Value=double>
class OdeFunction
    {
    public:
        virtual Value operator()(double t,const Value& y) const=0;
    };

and specialize for some common value types:

template double euler_fwd(size_t N,double t_0,double t_end,double y_0,const OdeFunction<double>& func);

template vec4_t<double> euler_fwd(size_t N,double t_0,double t_end,vec4_t<double> y_0,const OdeFunction< vec4_t<double> >& func); // (Native AVX vector with four components)

template vec8_t<float> euler_fwd(size_t N,double t_0,double t_end,vec8_t<float> y_0,const OdeFunction< vec8_t<float> >& func); // (Native AVX vector with 8 components)

template Vector<double> euler_fwd(size_t N,double t_0,double t_end,Vector<double> y_0,const OdeFunction< Vector<double> >& func); // (A N-dimensional real vector, *not* `std::vector`, see above)

If the function had been designed around an interface first, then you would have been forced to inherit from that ABC. Now you have this option, as well as function pointer, lambda, or any other function object. The key here is that we must have operator()(), and we must be able to do use some arithmetic operators on its return type. Thus, the template machinery would break in this case if C++ did not have operator overloading.

Answer 2

The basic problem with

void MyFunc(ForwardIterator *I);

is how do you safely get the type of the thing the iterator returns? With templates, this is done for you at compile time.

Answer 3

The answer is found in this interview with Stepanov, the author of the STL:

Yes. STL is not object oriented. I think that object orientedness is almost as much of a hoax as Artificial Intelligence. I have yet to see an interesting piece of code that comes from these OO people.

Answer 4

The concept of separating interface from interface and being able to swap out the implementations is not intrinsic to Object-Oriented Programming. I believe it's an idea that was hatched in Component-Based Development like Microsoft COM. (See my answer on What is Component-Driven Development?) Growing up and learning C++, people were hyped out inheritance and polymorphism. It wasn't until 90s people started to say "Program to an 'interface', not an 'implementation'" and "Favor 'object composition' over 'class inheritance'." (both of which quoted from GoF by the way).

Then Java came along with built-in garbage collector and interface keyword, and all of a sudden it became practical to actually separate interface and implementation. Before you know it the idea became part of the OO. C++, templates, and STL predates all of this.

Answer 5

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.

I think you misunderstand the intended use of concepts by templates. Forward Iterator, for example, is a very well-defined concept. To find the expressions which must be valid in order for a class to be a Forward Iterator, and their semantics including computational complexity, you look at the standard or at http://www.sgi.com/tech/stl/ForwardIterator.html (you have to follow the links to Input, Output, and Trivial Iterator to see it all).

That document is a perfectly good interface, and "the actual details of the concept" are defined right there. They are not defined by the implementations of Forward Iterators, and neither are they defined by the algorithms which use Forward Iterators.

The differences in how interfaces are handled between STL and Java are three-fold:

1) STL defines valid expressions using the object, whereas Java defines methods which must be callable on the object. Of course a valid expression might be a method (member function) call, but it doesn't have to be.

2) Java interfaces are runtime objects, whereas STL concepts are not visible at runtime even with RTTI.

3) If you fail to make valid the required valid expressions for an STL concept, you get an unspecified compilation error when you instantiate some template with the type. If you fail to implement a required method of a Java interface, you get a specific compilation error saying so.

This third part is if you like a kind of (compile-time) "duck typing": interfaces can be implicit. In Java, interfaces are somewhat explicit: a class "is" Iterable if and only if it says it implements Iterable. The compiler can check that the signatures of its methods are all present and correct, but the semantics are still implicit (i.e. they're either documented or not, but only more code (unit tests) can tell you whether the implementation is correct).

In C++, like in Python, both semantics and syntax are implicit, although in C++ (and in Python if you get the strong-typing preprocessor) you do get some help from the compiler. If a programmer requires Java-like explicit declaration of interfaces by the implementing class, then the standard approach is to use type traits (and multiple inheritance can prevent this being too verbose). What's lacking, compared with Java, is a single template which I can instantiate with my type, and which will compile if and only if all the required expressions are valid for my type. This would tell me whether I've implemented all the required bits, "before I use it". That's a convenience, but it's not the core of OOP (and it still doesn't test semantics, and code to test semantics would naturally also test the validity of the expressions in question).

STL may or may not be sufficiently OO for your taste, but it certainly separates interface cleanly from implementation. It does lack Java's ability to do reflection over interfaces, and it reports breaches of interface requirements differently.

you can tell the function ... expects a Forward Iterator only by looking at its definition, where you'd need either to look at the implementation or the documentation for ...

Personally I think that implicit types are a strength, when used appropriately. The algorithm says what it does with its template parameters, and the implementer makes sure those things work: it's exactly the common denominator of what "interfaces" should do. Furthermore with STL, you're unlikely to be using, say, std::copy based on finding its forward declaration in a header file. Programmers should be working out what a function takes based on its documentation, not just on the function signature. This is true in C++, Python, or Java. There are limitations on what can be achieved with typing in any language, and trying to use typing to do something it doesn't do (check semantics) would be an error.

That said, STL algorithms usually name their template parameters in a way which makes it clear what concept is required. However this is to provide useful extra information in the first line of the documentation, not to make forward declarations more informative. There are more things you need to know than can be encapsulated in the types of the parameters, so you have to read the docs. (For example in algorithms which take an input range and an output iterator, chances are the output iterator needs enough "space" for a certain number of outputs based on the size of the input range and maybe the values therein. Try strongly typing that.)

Here's Bjarne on explicitly-declared interfaces: http://www.artima.com/cppsource/cpp0xP.html

In generics, an argument must be of a class derived from an interface (the C++ equivalent to interface is abstract class) specified in the definition of the generic. That means that all generic argument types must fit into a hierarchy. That imposes unnecessary constraints on designs requires unreasonable foresight on the part of developers. For example, if you write a generic and I define a class, people can't use my class as an argument to your generic unless I knew about the interface you specified and had derived my class from it. That's rigid.

Looking at it the other way around, with duck typing you can implement an interface without knowing that the interface exists. Or someone can write an interface deliberately such that your class implements it, having consulted your docs to see that they don't ask for anything you don't already do. That's flexible.

Answer 6

"OOP to me means only messaging, local retention and protection and hiding of state-process, and extreme late-binding of all things. It can be done in Smalltalk and in LISP. There are possibly other systems in which this is possible, but I'm not aware of them." - Alan Kay, creator of Smalltalk.

C++, Java, and most other languages are all pretty far from classical OOP. That said, arguing for ideologies is not terribly productive. C++ is not pure in any sense, so it implements functionality that seems to make pragmatic sense at the time.