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From: gwu@tixdemo.tcs.com (George Wu)
Newsgroups: comp.lang.c++
Subject: Re: Problems maintaining header files
Message-ID: <1584@tcs.tcs.com>
Date: 23 Jan 91 00:36:58 GMT
References: <15917@reed.UUCP>
Sender: root@tcs.com
Reply-To: gwu@tixdemo.tcs.com (George Wu)
Organization: Teknekron Communications Systems
Lines: 46

In article <15917@reed.UUCP>, minar@reed.bitnet (Nelson
Minar,L08,x640,7776519) writes:
|> 
|> In C, I barely managed to keep all my header files straight and organized.
|> C++ just compounds my problems.
|> 
|> main.C is what you would expect.  In the process of doing its thing,
|> it creates an instance of LineWindow and an instance of Turtle.
|> Therefore, it includes the files lineWindow.h and turtle.h
|> 
|> lineWindow.C has to include lineWindow.h, obviously, and turtle.C includes
|> turtle.h.
|> 
|> The problem is, one of the member variables in class ScreenTurtle is a
|> reference to an instance of LineWindow.  Therefore, the header turtle.h
|> must also include lineWindow.h for the class declaration.
|> 
|> So, when one goes to compile main.C, it includes lineWindow.h and then
|> later includes turtle.h which again includes lineWindow.h.  Unless I take
|> precautions (either a '#pragma once' or a series of '#ifndef .. #define')
|> the header is included twice, and class LineWindow is declared twice, and
|> the compiler gets annoyed.

     If ScreenTurtle has a member variable *pointing* to an instance of a
LineWindow, then it doesn't need to include lineWindow.h.  You can just add

class LineWindow;

to lineWindow.h.  It's sort of like a function extern declaration for
externally defined classes.  You also might be able to do this with a
reference, but I'm not sure.  Try it, and if it doesn't work, switch to a
pointer.  (Yeah, I know, some people prefer references to explicit pointers.)

|> Are constructs like '#pragma once' just entirely common in C++ headers? I
|> find them ugly..

     We've never used a #pragma, and we've got hundreds of thousands of
lines of code.  We do use the #ifndef X, #define X, /* declarations */,
#endif to surround the contents of all header files, though.

							George

----
George J Wu                           | gwu@tcs.com or uunet!tcs!gwu
Software Engineer                     | 2121 Allston Way, Berkeley, CA, 94704
Teknekron Communications Systems, Inc.| (415) 649-3752