Static Polymorphism

Using compile time techniques to grant PolymorphicBehaviour? at CompileTime , as opposed to RunTime.

A CeePlusPlus example follows:

 template <class impl>
 class base : public impl { 
 public: void op() { impl::op(); }};

struct impl { void op() { /* work here */ }}; base<impl>().op();

Typically StaticPolymorphism surfaces as implementation in the form of the CuriouslyRecurringTemplatePattern?.

If you wish to define base as in terms of an abstract base class, and hold a collection, with differing implementations:

 #include <iostream>
 #include <vector>

class virtual_base { public: virtual void dynamic_op() = 0; };

template <class impl> class base : public impl, public virtual_base { public: void dynamic_op() { impl::op(); }};

struct impl_1 { void op() { std::cout << "impl_1" << std::endl; }}; struct impl_2 { void op() { std::cout << "impl_2" << std::endl; }}; typedef std::vector<std::auto_ptr<virtual_base> > bv;

void main() { bv vector_; vector_.push_back(new base<impl_1>()); vector_.push_back(new base<impl_2>()); for (bv::iterator item_(vector_.begin()); item_ != vector_.end(); (*item_++)->dynamic_op()){} }

However, std::vector<std::auto_ptr<T> > isn't a good thing. And main() should return int...


Equivalent code in OcamlLanguage:

 type base = unit -> unit
 let impl1 : base = fun () -> print_endline "impl_1"
 let impl2 : base = fun () -> print_endline "impl_2"

let _tmain () = let c_vector = [impl1, impl2] in List.iter (fun c_current -> c_current ()) c_vector

If you need many statically polymorphic methods on "base", you have to model it as a record / object instead of a function. -- PanuKalliokoski

Can you give an example?
A C Example:

The Microsoft Windows API uses compile switches to select between 8 bit ASCII and wide characters. This was done by naming the basic interface calls something like FooA and FooW and then use a compile switch and a #define statement to map Foo to either FooA or FooW.

An attempt to recreate the initial example with this technique.

// Possible Method Choices

#if defined(USE_A)
   #define op opA
#endif

#if defined(USE_B)
   #define op opB
#endif

#if defined(USE_C)
   #define op opC
#endif


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