C interpreters

[Heiko Wundram (Beenic)]

Am Donnerstag, 31. Januar 2008 14:48:15 schrieb Jim Stapleton:
> as a secondary (probably stupid) question: how hard is it to write a
> library in C++ and allow C programs to use it?

To write a library in C++ to which C programs have access, you'll have to 
write a set of wrapper functions for every method of a class you want to 
expose to C which basically get an object pointer as the first parameter and 
the actual method arguments as the rest. For example:

test.cc
-------

#include "test.hh"
#include "test.h"

Test::Test()
{
}

int Test::something(int data)
{
	return 0;
}

extern "C" {

	TestObject NewTest() {
		return new Test();
	}

	int TestSomething(TestObject ob, int data) {
		return reinterpret_cast<Test*>(ob)->something(data);
	}

}

test.hh
-------

#ifndef TEST_HH
#define TEST_HH

class Test
{
	Test();
	int something(int data);
};

#endif // TEST_HH

test.h
------

#ifndef TEST_H
#define TEST_H

typedef void* TestObject;

#ifdef __cplusplus
extern "C" {
#endif /* __cplusplus */

	TestObject NewTest();
	int TestSomething(TestObject ob, int data);

#ifdef __cplusplus
}
#endif /* __cplusplus */

#endif /* TEST_H */

test.c
------

#include "test.h"

int main(int argc, char** argv)
{
	TestObject testob;

	testob = NewTest();
	TestSomething(testob,1);
}

This lets you use the compiled test.cc (for example, as a library, to get 
around the problem of having to link your C-program against libstdc++) 
together with a C program.

Be aware of the fact that C doesn't know function overloading, so you'll 
basically have to implement that by defining different methods for every type 
of overloaded function you want to accept.

Depending on how large the C++ framework is which you're trying to wrap (and 
in how much it uses "advanced" C++ features), this is an easy (i.e., 
repetitive) or a hard/close to impossible task, especially when it comes to 
templates.

YMMV.

-- 
Heiko Wundram
Product & Application Development