NetNews Usenet Archive 1992 #27/NN_1992_27.iso/spool/comp/lang/cplus/16518

Internet Message Format | 1992-11-18 | 2.5 KB

Path: sparky!uunet!know!cass.ma02.bull.com!catfish.az05.bull.com!elm!aking
From: [e-mail] (Andy King)
Newsgroups: comp.lang.c++
Subject: Implementing a 'message' class
Message-ID: <[e-mail]>
Date: 18 Nov 92 16:32:32 GMT
Sender: [e-mail] (placeholder for future)
Reply-To: [e-mail] (Andy King)
Organization: Bull HN Information Systems Inc., Phoenix Product Division
Lines: 51

In C, it is relatively simple to create a facility for displaying
messages which contain arbitrary data and data types.  For example,
I usually define a library function with a prototype similar to:

    void display_message(char* format, ...);

and then use variable argument processing to create a message
string.  This message can then be displayed to the user (in a
Motif dialog or Windows MessageBox, for example) and also written
to a log file.

I'd like to do the equivalent using the '<<' operator in C++, but
I can't figure out how to implement the operator function.  I
envision the use of this class as follows:

    display_class display("logfile"); // an instance of the message class

    display << "Error opening " << file_name << " Status: " << errno;

The '<<' operator in 'display_class' would put this message into a
dialog box and write it to "logfile".  It would also display the
dialog and wait for the user to click on the "OK" button.

Obviously, I can derive the 'display_class' from 'ofstream', and
send all messages to a file.  But I need to 'capture' the stream
and manipulate it before it goes anywhere.  Another solution would be
to use 'strstream' as the base class for 'display_class', and to
implement a member function which does the actual dispatching of the
message.  For example, after every message, the program would call:

    display.out();

which would take the most recent message and send it to the screen
and to a log file.  But this seems as though it shouldn't be
necessary.  Of course, I could overload 'operator<<' for every type,
but this is laborious, and would preclude the use of an existing
'ostream' based '<<' for user-defined types.

Can this be done?  How is the 'endl' specifier implemented in the
'ostream' class?  Would it be possible to implement a similar facility,
requiring that the programmer terminate a message with a class-defined
specifier, thus activating the output processing?

I'd really appreciate any suggestions.  I'll post a summary, if
appropriate.

Thanks,

Andy

[e-mail]