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From: don@zardoz.coral.com (Don Dewar)
Newsgroups: gnu.g++.help,comp.lang.c++
Subject: Combination of C and C++
Message-ID: <9102211435.AA29323@zardoz.coral.com>
Date: 21 Feb 91 14:35:05 GMT
References: <1991Feb12.151007.29831@uni-paderborn.de>
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) Resent-Date: 12 Feb 91 15:10:07 GMT
) Date: 12 Feb 91 15:10:07 GMT
) From: uunet!.uni-paderborn.de!eggberd  (Bernd Eggert)
) Sender: uunet!ai.mit.edu!gnulists
) Organization: Uni-GH Paderborn, West Germany
) Subject: Combination of C and C++
) To: help-g++@prep.ai.mit.edu
) 
) 
) This message is posted for someone who cannot post to the net himself.
) 
) Please respond to the net or to axel@cadlab.cadlab.de !
) 
) -------------------------------------------------------------
) I have a question concerning the combination of C and C++.
) 
) 
) Assume the following situation:
) 
) You want to write a library in C++. People working with your library should be
) able to use it either from C++ or from C.
) 
) In order to make functions callable from C you use the 'extern "C"' directive.
) With this directive you can write ordinary C functions 'around' the C++
) objects. These functions can be called from a C program.
) 
) The problem occurs when you try to link the C program with the C++ library.
) The normal linker does not care for constructors or destructors and therefore
) static objects will not be initialized.
) 
) Is there an elegant and portable way to solve this problem? We are interested
) in solutions for AT&T CC and g++.
) 
) Below is an example which works if you use the C++-linker (but not with
) a normal linker)
) 
) Thanks for any help.
) 
) -------------------------------------------------------------------------------
) File 'hello.C':
) -------------------------------------------------------------------------------
) 
) /* this is a C++ class to be used by a C program */
) 
) #include <stream.h>
) #include <string.h>
) #include <stdio.h>
) 
) 
) class hello
) {
)     private:
)       char name[20];
) 
)     public:
)       hello(char *n)
)       {
)         strcpy(name,n);
)       }
) 
)       void print()
)       {
)         cout << "Hello " << name << "\n"; /* cout requires initialization */
)         printf("Hello %s\n",name);        /* printf is no problem */
)       }
) };
) 
) 
) /* these functions enable the non-C++-user to use the class 'hello': */
) 
) extern "C"
) {
) 
) void *create_person(char *n)
) {
)     return((void *)new hello(n));
) }
) 
) void print_person(void *p)
) {
)     ((hello *)p)->print();
) }
) 
) }
) 
) -------------------------------------------------------------------------------
) File 'chello.c'
) -------------------------------------------------------------------------------
) 
) /* this is a C program dealing with a C++ object */
) 
) 
) void *create_person(char *n);
) void print_person(void *p);
) 
) 
) main()
) {
)     void *p;
) 
)     __main(); /* this is necessary for initializing static objects in g++ */
) 
)     p = create_person("Henry");
) 
)     print_person(p);
) }
) 
) -------------------------------------------------------------------------------
) File 'makefile':
) -------------------------------------------------------------------------------
) 
) chello: chello.o hello.o
)         g++ -o chello chello.o hello.o
) 
) chello.o: chello.c
)         gcc -c chello.c
) 
) hello.o: hello.C
)         g++ -c hello.C
) 
) 
) -------------------------------------------------------------------------------
) Axel Meckenstock                            CADLAB
)                                             University of Paderborn
) phone : +49 5251 284-120                    Siemens Nixdorf Information Systems
) fax   : +49 5251 284-140                    Bahnhofstr. 32
) email : axel@cadlab.cadlab.de               D-4790 Paderborn
) -------------------------------------------------------------------------------
) 
) 
) 
) 

In the case of g++, you must use the g++ linker.  I don't find that
particularly unportable, just a little inconvenient for those persons
that want to link with you library and don't have the linker.  In the
case of CFront, I don't think you have a problem.  CFront just creates
"C" code that is compiled and should act just like a normal "C"
program with name mangling.

I would point out, however, that distributing C++ binary libraries can
be a problem in general for a couple of reasons.  If the user wants to
add to the library, they may want to add a new virtual function at a
root class, which throws off the virtual tables.  The user must use
the C++ compiler used to generate the libraries if they are linking in
more C++ code that calls the library, because the name mangling scheme
between the two compilers is likely to be different.

I just thought I might pass that on to you.  Good luck!


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