calling C++ functions in C

[kalyani1031]

Hi, is anybody could help me to point out the problem:
My problem is I want to call C++ function in C.
My code is as follows. Here it gives In function `main':
: undefined reference to `cpp_function'
I am using GNU Compiler in LINUX.

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

 //cpp_file.cpp
 
 #include "cpp_file.h"
 #include <iostream>
 void cpp_function(){
   foo f;
 
   f.see();
   cout<<"Hi, I am in Cpp file and called by C file\n"<<flush;
 }

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

//cpp_file.h
 
 #include <iostream.h>
  class foo {
 public:
   int i;
 
   foo() {i=0; }
   ~foo() {}
 
   void see() {
     cout << "--------------I am inside class ------------" << endl;
   }
 
 };
 
 void cpp_function();

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

//c_file.h
 
#ifdef __cplusplus
extern "C" {
#endif
 
void cpp_function( void );
 
#ifdef __cplusplus
}
#endif

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

//c_file.c
 #include "c_file.h"
 int main(){
   cpp_function();
   return(0);
 }

------------------------------------------------------------------
//Makefile

exe: c_file.o cpp_file.o
               g++ -o exe c_file.o cpp_file.o c_file.h cpp_file.h
       c_file.o: c_file.c c_file.h
               g++ -c c_file.c
       cpp_file.o: cpp_file.cpp cpp_file.h
               g++ -c cpp_file.cpp
 
       clean:
               rm *.o

[Banfa]

Your mistake is in declaring void cpp_function(); in cpp_file.h.

c_file.h has the correct declaration of void cpp_function(); what you should do is include c_file.h into ccp_file.cpp.



I function should only ever be declared once (and defined once too). If you have declared it more than once (as in you code example) it is almost certainly and error and very likely to cause you problems now or in the future.

#ifdef __cplusplus
extern "C" {
#endif
 
#ifdef __cplusplus
}
#endif

Bascially states that everything between the { } should use C caling convention. C++ calling convention is different, to allow for function overloading (and is commonly refered to as name mangling). By using the different defintions you told the compiler that the function used C calling convention when you compiled the C file and that it used C++ calling convention when it compiled the C++ file.

These 2 naming conventions produce different names so when the linker runs it can not find the function the code in the C file is requires and you get a link error.

[dex]

You need to wrap both the declaration (prototype) and the definition of your cpp_function in an extern "C" { }

[dex]

Banfa is also quite correct. The declaration of cpp_function in cpp_file.h should be in it's own header file and included in the c_file.c

[Banfa]

You need to wrap both the declaration (prototype) and the definition of your cpp_function in an extern "C" { }

this isn't right, only the declaration needs to be wrapped.