2 Comet

2.1 Basic COM from Erlang

In COM, there are interfaces. An interface is a handle to an object. Physically it consists of a pointer to an object with a method table.

Interfaces in COM are represented as a tuple in Erlang. This tuple should be considered oblique data.

There is a standard set of types in COM. These types can be converted to and from Erlang by the port program. (It is actually converted from the Erlang binary format.) Table 1 shows the types in Erlang and their corresponding COM type.

Erlang types and their corresponding COM type

integer	VT_I4, 32-bits integer
string	VT_STR, unicode string
atom	no type, however the two atomstrue and false are converted to VT_BOOL, the COM Boolean type
float	VT_R8, 64-bits floating point

However, there are fewer types in Erlang than in COM, so some types overlap. When calling a COM function, the parameters types must match exactly, since COM is strongly typed. Comet uses a special notation for the conversion of Erlang types to COM types, a tuple with an atom followed by the value, e.g. {vt_i2, 12}

2.2 Using the IDispatch interface

The IDispatch interface is a way for scripts to call a COM function. It is used by Visual Basic, JScript and other scripting language. It is sometimes referred to as the late-binding call interface.

This way to call COM objects shows off its VB heritage. An interface has methods and properties. A property is really two methods: put property and get property.

In the erl_com server, there are three functions for calling an IDispatch-interface.

calling an IDispatch interface

invoke(Interface, Method, Parameterlist)	Invokes a normal COM method. A list of out-parameters are returned, or, if there is a retval-parameter, it is returned.
property_put(Interface, Method, Parameterlist, Value)	Calls a COM method with the propput-attribute. An extra argument, after the Parameterlist, contains the property value to set. (Which really is just a parameter to the function.) If the property does not have parameters, the parameterlist might be omitted, but a value must always be provided.
property_get(Interface, Method, Parameterlist)	Calls a COM method with the propget-attribute. The value of the property is returned. If the property does not have parameters, the parameterlist might be omitted.