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From: sdm@cs.brown.edu (Scott Meyers)
Newsgroups: comp.lang.c++
Subject: Re: indirect message passing
Message-ID: <41462@brunix.UUCP>
Date: 31 May 90 18:20:19 GMT
References: <1990May31.131937.25923@aucs.uucp>
Sender: news@brunix.UUCP
Reply-To: sdm@cs.brown.edu (Scott Meyers)
Organization: Brown University Department of Computer Science
Lines: 44

In article <1990May31.131937.25923@aucs.uucp> 880716a@aucs.uucp (Dave Astels) writes:
>I am wondering if it is possible to pass messages in C++ indirectly.
>Let me explain:  say I have a class X which has a member function foo().
>Also, I have a variable y of type X.  Rather than invoking y.foo() directly
>I want to store the information for later use.  ie. I want to set up a
>callback (using X11 terminology).. to store the information that X::foo()
>is to be called for the object y, then do the call later (in response to
>some event, for example).  I have done this in a UI I developed in Turbo
>Pascal 5.5.  I have since bought Zortech C++, and wish to eventually
>rewrite the UI in C++.  The technique in TP was implemented using generic
>pointers (to the object and the method entry point), and some assembly to
>do the actual method invocation (supplying the Self/this parameter, etc).

Try this -- it works fine under cfront 2.0 (no assembly required):

    #include <iostream.h>

    class X {
    public:
      void foo() { cout << "Foo!\n"; }
    };

    main()
    {
      cout << "Declaring an X called y...\n";
      X y;

      cout << "Calling y.foo() directly...";
      y.foo();

      cout << "Declaring pointers...\n";
      X *py;
      py = &y;
      void (X::*pxmf)();
      pxmf = X::foo;

      cout << "Calling y.foo() indirectly...";
      (py->*pxmf)();
    }  

For details, see Lippman, p. 213ff.

Scott
sdm@cs.brown.edu