The following code example shows a simple class bound to Io. The binding was tested to work, but it may not be fully correct.

The IoBindingTest class only stores a number, with the usual setter and getter accessors. Also, two instances of the class can be compared to each other. The class has the regular constructor, as well as a copy constructor, and a constructor that takes a number. The copy constructor is used in the binding when creating a clone.

// begin test class

class IoBindingTest;

class IoBindingTest
{
public:
 IoBindingTest() : num(0) {};
 IoBindingTest(int n) : num(n) {};
 IoBindingTest(IoBindingTest* ptr) : num(ptr->num) {};
 ~IoBindingTest() {};
 
 int GetNum(void) { return num; };
 void SetNum(int n) { num = n; };
 
 bool CompareWith(IoBindingTest* obj) { return num == obj->num; };

private:
 int num;
};

// end test class

The binding class creates a bunch of static class functions, which can be accessed from C code, such as Io's VM, for using the test class' similarly named instance methods. Also, the binding class has methods specific to the Io VM (proto, tag, rawClone, mark, free), and a final function to add the binding to the VM at runtime.

#include "IoVM.h"

class IoBindingTest_io
{
public:
 static IoObject* GetNum(IoObject *self, IoObject *locals, IoMessage *m);
 static IoObject* SetNum(IoObject *self, IoObject *locals, IoMessage *m);

 static IoObject* CompareWith(IoObject *self, IoObject *locals, IoMessage *m);

 static IoObject* proto(IoState* state);
 
 static IoTag* tag(IoState* state, const char* name);

 static IoObject* rawClone(IoObject* self);
 static IoObject* mark(IoObject* self);
 static IoObject* free(IoObject* self);
 
 static void addBinding(IoState* state);

};

IoTag *IoBindingTest_io::tag(IoState* state, const char* name)
{
 IoTag* tag = IoTag_newWithName_(name);
 tag->state = state;
 tag->cloneFunc = (TagCloneFunc*) rawClone;
 tag->markFunc = (TagMarkFunc*) mark;
 tag->freeFunc = (TagFreeFunc*) free;

 return tag;
}

The proto function is used for creating the initial prototype in the Io VM.

IoObject *IoBindingTest_io::proto(IoState* state)
{
 IoMethodTable methods[] = {
 {"GetNum", GetNum},
 {"SetNum", SetNum},
 {"CompareWith", CompareWith},
 {NULL, NULL}
 };
 IoObject* self = IoObject_new(state);
 self->tag = tag(state, "IoBindingTest");
 self->data = 0;
 IoObject_addMethodTable_(self, methods);
 return self;
}

The addBinding function is used to put the prototype of our object into the Io VM at runtime.

void IoBindingTest_io::addBinding(IoState* state)
{
 IoObject* self = proto(state);
 IoState_registerProtoWithFunc_(state, self, (IoStateProtoFunc*)proto);
 IoObject_setSlot_to_(state->lobby, IOSYMBOL("IoBindingTest"), self);
}

IoObject *IoBindingTest_io::rawClone(IoObject *self)
{
 IoObject *clone = IoObject_rawClonePrimitive(self);
 if (self->data)
  clone->data = new IoBindingTest(reinterpret_cast<IoBindingTest*>(self->data));
 else
  clone->data = new IoBindingTest;
 return clone;
}

The mark function is used by the garbage collector. If the C++ object has references to other objects in the Io VM, they must also be marked.

IoObject *IoBindingTest_io::mark(IoObject *self)
{
 return self;
}

free is pretty self-explanatory, this is where the C++ object should be freed/deleted, if appropriate.

IoObject *IoBindingTest_io::free(IoObject *self)
{
 if (self->data)
 {
  IoBindingTest* obj = reinterpret_cast<IoBindingTest*>(self->data);
  delete obj;
  self->data = NULL;
 }
 return self;
}

The following three functions are the actual bindings of the instance methods, so that they can be called from a script.

IoObject* IoBindingTest_io::GetNum(IoObject *self, IoObject *locals, IoMessage *m)
{
 IOASSERT(self->data, "No C++ object");
 IoBindingTest* obj = reinterpret_cast<IoBindingTest*>(self->data);

 return IoNumber_newWithDouble_(self->state, obj->GetNum());

};

IoObject* IoBindingTest_io::SetNum(IoObject *self, IoObject *locals, IoMessage *m)
{
 IOASSERT(self->data, "No C++ object");
 IoBindingTest* obj = reinterpret_cast<IoBindingTest*>(self->data);

 IOASSERT(IoMessage_argCount(m) == 1, "Wrong number of arguments");
 
 IoObject *arg1 = IoMessage_locals_numberArgAt_(m, locals, 0);

 obj->SetNum(IoNumber_asInt(arg1));

 return self;

};

IoObject* IoBindingTest_io::CompareWith(IoObject *self, IoObject *locals, IoMessage *m)
{
 IOASSERT(self->data, "No C++ object");
 IoBindingTest* obj = reinterpret_cast<IoBindingTest*>(self->data);

 IOASSERT(IoMessage_argCount(m) == 1, "Wrong number of arguments");
 
 IoObject *arg1 = IoMessage_locals_valueArgAt_(m, locals, 0);

 // make sure the object is tagged  
 IOASSERT(arg1->tag, "No tag in arg");

 // check the tag to make sure it is the right object class
 IOASSERT(strcmp(arg1->tag->name, "IoBindingTest") == 0, "arg not IoBindingTest object");

 // check for the actual existence of the C++ object
 IOASSERT(arg1->data, "No C++ object in arg");

 IoBindingTest* arg1obj = reinterpret_cast<IoBindingTest*>(arg1->data);

 // bool is simulated by returning self or nil
 if (obj->CompareWith(arg1obj))
  return self;
 else
  return IONIL(self);
};