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From: hargrove@asc.slb.com (Jim Hargrove)
Newsgroups: comp.lang.c++
Subject: Re: call C++ from C
Message-ID: <1991Jun29.142749.26044@asc.slb.com>
Date: 29 Jun 91 14:27:49 GMT
References: <359@soleil.jupiter.UUCP> <20463@alice.att.com> <1991Jun28.153143.5476@ccu.umanitoba.ca>
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In-Reply-To: dab@ubitrex.mb.ca's message of 28 Jun 91 15:31:43 GMT
Nntp-Posting-Host: bart

>>>>> dab@ubitrex.mb.ca (Danny Boulet) writes:
The listing below shows one way of calling a C++ function from a C
program. There is one slight enhancement that we can make. Instead of
propagating yet another void *, we can define a C typedef to use in
pointing to a C++ object. In the case below, where the class is named
fred, we define

    typdef  struct  fred    *   CFred_t;

As the struct fred is not defined in the C scope this is an
"incomplete" type definition in ANSI C. Only pointers can be defined
in this way.

Then the definition of the function freddoit becomes

    int
    freddoit(CFred_t    myFred,
             int        x);

This means that the compiler can distinguish between pointers to freds
and pointers to other objects. 

We should avoid void * whenever possible.

dab> In article <20463@alice.att.com> ark@alice.UUCP () writes:
>In article <359@soleil.jupiter.UUCP> rh@jupiter.UUCP (#Rachid Himmi) writes:
>
>> I'm looking for the way to call a C++ module from a C program through
>> the declaration of a class and the call of its function members.

dab> In your C++ code, write C callable functions that invoke the C++ members.
dab> For example:

dab> ---------------------- C++ code ------------------------

dab> class fred {

dab> public:
dab> 	.
dab> 	.
dab> 	.
dab>     int doit(int x);
dab> 	.
dab> 	.
dab> 	.
dab> };

dab> extern "C" int
dab> freddoit(void *ptr,int x)
dab> {
dab>     fred *p = (fred *)ptr;
dab>     return( p->doit(x) );
dab> }

dab> ----------------------------------------------------------

dab> Call "freddoit" from C with a pointer to an instance of "fred" as the first
dab> parameter and (in this example) an appropriate value for "x" as the
dab> second parameter:

dab> ---------------------- C code ----------------------------

dab>     w = freddoit(fredptr,124);

dab> ----------------------------------------------------------

dab> I've no idea if this is supposed to work but it does for me.

>
>Turn your C program into a C++ program and it's easy.
>Otherwise, it's a matter between you and your implementer(s)
>and it's not guaranteed to work no matter what you do.
>-- 
>				--Andrew Koenig
>				  ark@europa.att.com
-- 

        -- jwh