Calling C++ member function from C

Question

Following is my C++ code:

#include<iostream>
#include<string.h>
extern "C" void wrapper(struct NB*, int );

struct AssetDbFilter {
    std::string attribute;
    std::string value;
  };

class NB {

public:
    void myTest(int a);
};

Following is C++ code:

#include<iostream>
#include<string.h>
 #include "test.h"
using namespace std;

void NB :: myTest (int a) {

cout << a;
}
void wrapper(NB* nb, int a) {
nb->myTest(a);
}

Following is my c code:

#include<stdio.h>
#include "test.h"
void wrapper(struct NB* ,int );
int main()
{
    AssetDbFilter* assetdb;
    struct NB* nb;
    wrapper(nb, 5);
}

I am not able to execute this code. I am calling C++ member function from a C code. When I execute this C file as g++ test.c I am getting following error:

/tmp/ccr1IaLa.o: In function `main':
test1.c:(.text+0x15): undefined reference to `wrapper'
collect2: error: ld returned 1 exit status

Can anyone tell me how can I resolve this?

Comments are not for extended discussion; this conversation has been moved to chat. — Samuel Liew
– Samuel Liew, Commented Jan 15, 2019 at 22:09

Lightness Races in Orbit · Accepted Answer · 2019-01-15 13:38:23Z

4

You built test.c, great! Now you need to build the C++ file as well so that definitions are present.

Currently your toolchain thinks you want to compile and link test.c into an application, but test.c isn't your whole source code.

You can compile test.c without linking:

gcc test.c -c

Then do the same with your C++ file:

g++ whatever.cpp -c

Then link them together to get an executable:

g++ test.o whatever.o -o myProgram

Unfortunately your program will still not work because you are invoking a member function on a dangling/uninitialised pointer. Since you cannot instantiate an NB from your C code, this approach can't work.

I recommend NB contain only C-compatible members. Then you can put its definition into a shared header file and instantiate it whereever you please.

answered Jan 15, 2019 at 13:38

Lightness Races in Orbit

387k77 gold badges670 silver badges1.1k bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

rustyx Over a year ago

... or just delegate object creation to C++: extern "C" struct NB* createNB() { return new NB(); }

luk32 Over a year ago

IMHO it's worth of noting, for the sake of completness, that in pure c++ producing object files for external-linking is technically counterproductive, because there is name mangling and (I think) argument passing mode which are not standardized, so finding and using compiled functions is fragile. Keywords C++ ABI, external and internal linkage. The counter measure is of course to specify C-linkage, which c++-compliant compiler should provide and has its own implications.

eerorika Over a year ago

@rustyx in which case be careful to also provide freeNB or similarly named function and document it in immediate vicinity of createNB, lest a C user of the API becomes confused about how to deal with the memory.

Collectives™ on Stack Overflow

Calling C++ member function from C

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related