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# Static and dynamic polymorphism

With a header only library, you can only do so much with static polymorphism alone. There are some situations where you have to handle dynamic polymorphism because of unavoidable runtime-based decision making. Although you can deal with the base types that remain static, behavior can vary greatly which derived type should be handling the situation based on runtime values.

This situation comes up in the `cpp-netlib` when we decide what kind of connection handler to use for a given HTTP URI – whether it’s plain HTTP or HTTPS. Although the HTTP semantics are the same for HTTP and HTTPS the implementation of the connection handler greatly varies on whether to use a plain TCP connection or an SSL-wrapped TCP connection.

The general pattern or technique is to combine tag-based dispatch with a strategy factory, all while not requiring any externally built libraries. Doing it in a header-only library requires a little creativity and additional layers of indirection that you otherwise will not need for a library with externally built static/dynamic libraries.

First we define the base type which we want to support dynamic behavior with. There’s nothing special with the base type, except that it supports the tag dispatch earlier defined and has a virtual destructor. In code it looks like this:

    template <class Tag>
    struct base {
        virtual void foo() = 0; // make this an abstract base
        virtual ~base() {
            // do the base destructor thing here.
        }
    };

We then define a set of derived types that specialize the implementation of the `foo` member function. To facilitate the dispatch of the correct type based on an input, we create a strategy factory function:

    template <class Tag>
    unique_ptr<base<Tag> > strategy(int input, Tag) {
        unique_ptr<base<Tag> > ptr;
        switch(input) {
            case 0: ptr.reset(new derived0()); break;
            case 1: ptr.reset(new derived1()); break;
            // ...
            default: ptr.reset(0); break;
        }
        return ptr;
    }

    unique_ptr<base<default_> > ptr =
        strategy(input, default_()); // input is a runtime value

The strategy factory can be a standalone function, or a static member of a factory class that is specialized by tag dispatch. This can be done like the following:

    template <class Tag>
    struct strategy;

    template <>
    struct strategy<default_> {
        static unique_ptr<base<default_> > create(int input) {
            unique_ptr<base<default_> > ptr;
            switch(input) {
                case 0: ptr.reset(new derived0()); break;
                case 1: ptr.reset(new derived1()); break;
                //...
                default: ptr.reset(0); break;
            }
            return ptr;
        }
    };

This approach allows the header-only libraries to define new dynamic types in subsequent versions of the library while keeping the static-dynamic bridge fluid. The only down-side to this is the possibility of derived type explosion in case there are a lot of different strategies or specializations available – this though is not unique to static-dynamic bridging, but is also a problem with pure object oriented programming with dynamic polymorphism.

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