integrating c code and c++ code

integrating c code and c++ code

[qhwang]

Hi folks,

Does anyone have experience of integrating c code together with c++ code? I
know it may be unusual but I need to do it. The case is, I need to integrate
some well defined c++ wavelet classes into my c application, but I don't
have enough time to rewrite the wavelet classes in plain c. So how can I do
it? Any help or further pointer will be greatly appreciated. Many thanks.

QingHua Wang

RE: integrating c code and c++ code

[Joseph D. Wagner]

Just combine the two.  C++ is a subset of C which means everything that
worked under C will continue to work under C++.

Sure, it will result in sloppy code, a weird mix of C and C++ syntax, but it
will compile and work fine.

Or at least it should 8-)

Joseph Wagner

-----Original Message-----
From: linux-c-programming-owner@vger.kernel.org
[mailto:linux-c-programming-owner@vger.kernel.org] On Behalf Of qhwang
Sent: Thursday, December 26, 2002 9:23 AM
To: linux-c-programming@vger.kernel.org
Subject: integrating c code and c++ code

Hi folks,

Does anyone have experience of integrating c code together with c++ code? I
know it may be unusual but I need to do it. The case is, I need to integrate
some well defined c++ wavelet classes into my c application, but I don't
have enough time to rewrite the wavelet classes in plain c. So how can I do
it? Any help or further pointer will be greatly appreciated. Many thanks.

QingHua Wang

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Re: supersets (Was: RE: integrating c code and c++ code)

[Glynn Clements]

Joseph D. Wagner wrote:

> Just combine the two.  C++ is a subset of C

C++ definitely isn't a subset of C. It's almost a superset of C (which
is presumably what you meant), but not quite. E.g.:

	struct foo {
		int dummy;
	};
	
	typedef int foo;

is perfectly legal in C but not in C++.

Also, it's quite possible to write a program which is valid in both C
and C++, but does something different in each[1].

For a language L2 to truly be a superset of language L1, every program
which is valid in L1 must be valid in L2 and have exactly the same
semantics in both.

[1] Actually, the same is technically true of C variants which support
C++ "//" comments compared to ANSI C; e.g.:

	a = b//*comment*/c
		;

is equivalent to:

	a = b/c;

in ANSI C, but recognising C++ comments changes the interpretation to:

	a = b;

Admittedly this is something of a "corner case", which a programmer
would probably avoid. But not all C code is generated by humans. E.g. 
compilers for high-level languages often use C as a portable assembly
language (i.e. they generate C then spawn a C compiler rather than
directly generating machine code for the target platform).

-- 
Glynn Clements <glynn.clements@virgin.net>

Re: integrating c code and c++ code

[Glynn Clements]

[-- Attachment #1: message body and .signature --]
[-- Type: text/plain, Size: 760 bytes --]


qhwang wrote:

> Does anyone have experience of integrating c code together with c++ code? I
> know it may be unusual but I need to do it.

It isn't at all unusual to integrate C++ with C. However, calling C
from C++ is a lot easier than calling C++ from C.

> The case is, I need to integrate
> some well defined c++ wavelet classes into my c application, but I don't
> have enough time to rewrite the wavelet classes in plain c. So how can I do
> it? Any help or further pointer will be greatly appreciated. Many thanks.

You can't call C++ methods from C, only plain functions. If you want
to call methods, you must write wrapper functions; such functions must
use C linkage.

A simple example is attached.

-- 
Glynn Clements <glynn.clements@virgin.net>


[-- Attachment #2: private.h --]
[-- Type: text/plain, Size: 58 bytes --]

/* private.h */

class foo
{
public:
	void bar(void);
};


[-- Attachment #3: public.h --]
[-- Type: text/plain, Size: 143 bytes --]

/* public.h */

#ifdef __cplusplus
extern "C" {
#endif

extern void *new_foo(void);
extern void foo_bar(void *);

#ifdef __cplusplus
}
#endif


[-- Attachment #4: foo.cpp --]
[-- Type: text/plain, Size: 236 bytes --]

/* foo.cpp */

#include <iostream.h>
#include "private.h"
#include "public.h"

void foo::bar(void)
{
	cout << "hello, world" << endl;
}

void foo_bar(void *f)
{
	((foo *) f)->bar();
}

void *new_foo(void)
{
	return (void *) new foo;
}


[-- Attachment #5: main.c --]
[-- Type: text/plain, Size: 101 bytes --]

/* main.c */

#include "public.h"

int main(void)
{
	void *f = new_foo();
	foo_bar(f);
	return 0;
}

Re: integrating c code and c++ code

[Jamie Risk]

It's also useful to surround your c code with "extern c { ... code ... }"
when compiling c files with a cpp compiler. Doing so will avoid confusion
when calling C routines compiled by a cpp compiler from C routines compiled
by a C compiler (name mangling issues).
More useful still try:

#ifdef __c_plus_plus /* Check this macro defnition, it's a close variant of
this */
extern c
{
#endif
int myfunc(int a);

int myfunc(int a)
{
  ...
}

#ifdef __c_plus_plus
}
#endif




"Glynn Clements" <glynn.clements@virgin.net> wrote in message
news:15883.27533.81795.913324@cerise.nosuchdomain.co.uk...
>
> qhwang wrote:
>
> > Does anyone have experience of integrating c code together with c++
code? I
> > know it may be unusual but I need to do it.
>
> It isn't at all unusual to integrate C++ with C. However, calling C
> from C++ is a lot easier than calling C++ from C.
>
> > The case is, I need to integrate
> > some well defined c++ wavelet classes into my c application, but I don't
> > have enough time to rewrite the wavelet classes in plain c. So how can I
do
> > it? Any help or further pointer will be greatly appreciated. Many
thanks.
>
> You can't call C++ methods from C, only plain functions. If you want
> to call methods, you must write wrapper functions; such functions must
> use C linkage.
>
> A simple example is attached.
>
> --
> Glynn Clements <glynn.clements@virgin.net>
>
>


----------------------------------------------------------------------------
----


> /* private.h */
>
> class foo
> {
> public:
> void bar(void);
> };
>
>


----------------------------------------------------------------------------
----


> /* public.h */
>
> #ifdef __cplusplus
> extern "C" {
> #endif
>
> extern void *new_foo(void);
> extern void foo_bar(void *);
>
> #ifdef __cplusplus
> }
> #endif
>
>


----------------------------------------------------------------------------
----


> /* foo.cpp */
>
> #include <iostream.h>
> #include "private.h"
> #include "public.h"
>
> void foo::bar(void)
> {
> cout << "hello, world" << endl;
> }
>
> void foo_bar(void *f)
> {
> ((foo *) f)->bar();
> }
>
> void *new_foo(void)
> {
> return (void *) new foo;
> }
>
>


----------------------------------------------------------------------------
----


> /* main.c */
>
> #include "public.h"
>
> int main(void)
> {
> void *f = new_foo();
> foo_bar(f);
> return 0;
> }
>
>

Re: integrating c code and c++ code

[Glynn Clements]

Jamie Risk wrote:

> It's also useful to surround your c code with "extern c { ... code ... }"
> when compiling c files with a cpp compiler. Doing so will avoid confusion
> when calling C routines compiled by a cpp compiler from C routines compiled
> by a C compiler (name mangling issues).

Using 'extern "C" ...' only forces C linkage; the code is still
compiled according to the rules of C++, which isn't necessarily
compatible with C. The only robust solution is not to compile C code
with a C++ compiler (or ensure that your code is 100% valid as both C
and C++, which is harder than it seems; just getting 100% valid ANSI C
is difficult enough).

> #ifdef __c_plus_plus /* Check this macro defnition, it's a close variant of
> this */

It's actually "__cplusplus".

-- 
Glynn Clements <glynn.clements@virgin.net>