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From: steve@taumet.com (Stephen Clamage)
Newsgroups: comp.lang.c++
Subject: Re: calling C++ from C
Message-ID: <673@taumet.com>
Date: 16 Apr 91 15:39:27 GMT
References: <1991Apr15.154120.13484@murdoch.acc.Virginia.EDU>
Organization: Taumetric Corporation, San Diego
Lines: 51

pts@faraday.clas.Virginia.EDU (Paul T. Shannon) writes:

>Though many books explain how to call C functions from C++, I'm 
>having difficulty figuring out how to do the opposite.  

>I have a large C program, built from several modules of Ansi C 
>code.  I want to add two C++ modules to this, and call the member 
>functions from the C program.

It is interesting that this question comes up three times in today's
news.  The answer is that the same mechanism serves both requirements.
You may declare and define a C++ function as
	extern "C" ...
and call it from a C program, as long as
1.  The function is not a member of any class.
2.  No more than one overloaded version of the function is so declared.
3.  The function declaration requires only types visible to the C program.

Requirement 3 can be relaxed a bit by using pointers to C++ class types,
if no object of the type is needed by the C program.  That is, a C++
function can pass a pointer to a class object to a C function, which can
pass on that pointer to another C++ function.  In the C file, you
just use the incomplete declaration
	struct SomeClass;
and then use pointers to struct SomeClass.  You cannot safely provide
"equivalent" C declarations of C++ classes, since you cannot portably
predict how the C++ compiler will lay out the class members.

You can relax Requirements 1 and 2 by providing "wrapper functions" in
C++.  Example:

C++ file:
=========
	class SomeClass { ... int f1(int); ... };
	extern "C" int call_f1(SomeClass* p, int i) // wrapper for C
	{
	    return p->f1(i);
	}

C file:
========
	struct SomeClass;			/* defined in C++ files */
	int call_f1(struct SomeClass*, int);	/* defined in C++ files */

	int myfunc(struct SomeClass* p)
	{
	    ... call_f1(p, 3) ...
	}
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com