NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/comp/lang/cplus/18381

Text File | 1992-12-23 | 1.8 KB | 50 lines

Newsgroups: comp.lang.c++
Path: sparky!uunet!microsoft!wingnut!samk
From: [e-mail] (Sam Kho)
Subject: Re: Access non-static data members in Motif callback functions.
Message-ID: <[e-mail]>
Date: 23 Dec 92 23:23:05 GMT
Organization: Microsoft Corporation
References: <[e-mail]>
Distribution: global
Lines: 38

In article <[e-mail]> [e-mail] (James Ng) writes:
>
>We use X11/R4 and Motif 1.1 for our GUI development. Our C++ compiler is
>Object Center C++ (formerly called Saber C++). When we try to use a
>non-static member function as a callback, sometimes nonstatic data members
>get corrupted inside the callback function. Static data members seem to
>maintain their integrity. Does anyone have similar experience or even
>solution to solve the above problem? Any suggestion will be appreciated.
>Suggestion can be mailed to [e-mail] or posted if you prefer.
>
>James Ng

static data members r like global variables.  "this" is irrelevant.  non-
static data members r like fields in a struct, where "this" is the pointer
to the struct.  in a non-static member function, "this" is assumed to be
the first hidden parameter, i.e.:

    class myclass
    {
    public:
        void nonstatic    (int x);
    };

is like:

    void myclass_nonstatic(myclass *this, int x);

now when Xt calls the callback, the first argument passed is the widget.
my guess is u'r using the widget as "this"!  i'm surprised u'r even able
to get non-corrupted values (probably that offset in the widget just happens
to have the right value).

to verify, cast this into a widget inside the non-static member function,
and do something with it (e.g. if it's a callback to a PushButton, change
the label).

  - sam -