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<h4 class="subsubsection"> 4.3.1.1 How it Works </h4>
<p>This section contains technical details about <small>SCF</small>. If you're not
interested in details, you can skip directly to the next section which
explains how <small>SCF</small> should be used from the end-user's point of view.
</p>
<p><small>SCF</small> is somewhat similar to <small>COM</small>. This is because historically
Crystal Space initially used <small>COM</small> for inter-module communication. Thanks
to Dan Ogles for the initial implementation of a cross-platform <small>COM</small>
library; his implementation was a good test-bed for this <small>SCF</small>
implementation.
</p>
<p>The differences between <small>COM</small> and <small>SCF</small> are due to the increased need for
an easier-to-use and more lightweight mechanism than <small>COM</small>. The <small>COM</small>
implementation caused some problems (because of historic bugs in <small>EGCS</small>
1.1.x and incompatibilities with platforms to which Crystal Space has ported).
Also it has some problems due to the native Windows types used in <small>COM</small>;
this experience was taken into account while developing <small>SCF</small>.
</p>
<p>The main paradigm of <small>SCF</small>, as in <small>COM</small>/<small>CORBA</small>/Java/etc. is the
<em>interface</em>. You define an <em>interface</em>, that is, a set of abstract
methods that you want to access within some object. <small>SCF</small> interfaces are
plain C++ structs (or classes, it doesn't matter much except that if you're
using classes to which you will have to add the <samp>‘public:’</samp> keyword).
Theoretically, interfaces can contain member variables and inline methods
(though not non-virtual methods or constructors), <em>but</em> this practice is
not encouraged because later someone may want to add a <small>COM</small>, <small>CORBA</small>,
etc. layer between the client and the actual object, and this will not be
possible if the interfaces contains variables, inline methods, or non-virtual
methods. If you don't plan to use <small>COM</small> or <small>CORBA</small> later, then you can
use variables and inline functions as much as you like in your own code, but
their use in Crystal Space code is highly discouraged.
</p>
<p>Here is an example of an interface in a file named <tt>‘idog.h’</tt>:
</p>
<table><tr><td> </td><td><pre class="example">struct iDog
{
virtual bool IsAlive() = 0;
virtual char const* GetName() = 0;
virtual void SetName (char const*) = 0;
virtual void Shout(int Volume) = 0;
virtual void Run(int Speed, float Direction) = 0;
virtual bool GetChildren(iObjVector* oBrood) = 0;
};
</pre></td></tr></table>
<p>Note the last method that gets a pointer of <samp>‘iObjVector’</samp> type.
<samp>‘iObjVector’</samp> is yet another interface. We could pass a pointer to a
<samp>‘csObjVector’</samp> (the implementation of that interface) as well, but this
will mean both modules (caller and callee) should have same idea about what a
<samp>‘csObjVector’</samp> is, and if it happened that you compiled the shared library
with a slightly different version of <samp>‘csObjVector’</samp> (that, say, had one
member variable fewer) you will end up with a <small>SIGSEGV</small> crash. By instead
specifying the abstract <samp>‘iObjVector’</samp>, there is no need to link the
<samp>‘csObjVector’</samp> class into both the caller and the callee.
</p>
<p>Now let's write a particular <em>implementation</em> of the above interface.
</p>
<table><tr><td> </td><td><pre class="example">#include "idog.h"
class MyDog : public iDog
{
private:
// private member functions & variables ...
csString Name;
public:
virtual bool IsAlive();
virtual char const* GetName();
virtual void SetName(char const*);
virtual void Shout(int Volume);
virtual void Run(int Speed, float Direction);
virtual bool GetChildren(iObjVector* oBrood);
... public member functions & variables ...
};
bool MyDog::IsAlive()
{
return true;
};
char const* MyDog::GetName()
{
return Name;
}
void MyDog::SetName(char const* NewName)
{
Name = NewName;
}
// And so on ...
</pre></td></tr></table>
<p>Now, we put the actual implementation into a separate module (i.e. into a
shared library), and include within the client just the interface file
<tt>‘idog.h’</tt>. Since the client does not have any idea how we should build
an object of the <samp>‘MyDog’</samp> class, we also provide a function that will
return a newly-allocated object of that class. This is called the
<em>class factory</em> (in fact, a class factory is a bit more than just this,
but more about this later). Here is how to do it:
</p>
<table><tr><td> </td><td><pre class="example">static iDog* MyDog_Create()
{
return new MyDog();
}
</pre></td></tr></table>
<p>Okay, we did it. Now back to the client. To work with an object that
implements the <samp>‘iDog’</samp> interface we need to load the shared library, get
a pointer to the <code>MyDog_Create()</code> function, call it and get a new
<samp>‘MyDog’</samp> object. Further we work with this pointer as if it were pointing
to an <samp>‘iDog’</samp> object:
</p>
<table><tr><td> </td><td><pre class="example">csLibraryHandle handle = csLoadLibrary("./libdog.so");
iDog (*iDog_Create)() = csGetLibrarySymbol(handle, "MyDog_Create");
iDog* dog = iDog_Create();
printf("Doggy's name is %s\n", dog->GetName());
dog->Shout(100);
...
</pre></td></tr></table>
<p>Of course, you don't have to do all this stuff manually with the <small>SCF</small>
implementation. The <small>SCF</small> library provides a number of useful templates and
functions which hide these ugly implementation details from end-user.
</p>
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