What is the difference between message-passing and method-invocation?

Question

Is there a difference between message-passing and method-invocation, or can they be considered equivalent? This is probably specific to the language; many languages don\'t suppo

Answer 1

IIRC, they've been formally proven to be equivalent. It doesn't take a whole lot of thinking to at least indicate that they should be. About all it takes is ignoring, for a moment, the direct equivalence of the called address with an actual spot in memory, and consider it simply as a number. From this viewpoint, the number is simply an abstract identifier that uniquely identifies a particular type of functionality you wish to invoke.

Even when you are invoking functions in the same machine, there's no real requirement that the called address directly specify the physical (or even virtual) address of the called function. For example, although almost nobody ever really uses them, Intel protected mode task gates allow a call to be made directly to the task gate itself. In this case, only the segment part of the address is treated as an actual address -- i.e., any call to a task gate segment ends up invoking the same address, regardless of the specified offset. If so desired, the processing code can examine the specified offset, and use it to decide upon an individual method to be invoked -- but the relationship between the specified offset and the address of the invoked function can be entirely arbitrary.

A member function call is simply a type of message passing that provides (or at least facilitates) an optimization under the common circumstance that the client and server of the service in question share a common address space. The 1:1 correspondence between the abstract service identifier and the address at which the provider of that service reside allows a trivial, exceptionally fast, mapping from one to the other.

At the same time, make no mistake about it: the fact that something looks like a member function call doesn't prevent it from actually executing on another machine or asynchronously, or (frequently) both. The typical mechanism to accomplish this is proxy function that translates the "virtual message" of a member function call into a "real message" that can (for example) be transmitted over a network as needed (e.g., Microsoft's DCOM, and CORBA both do this quite routinely).