title:Wrapping C++ exceptions, templated and classes in C
keywords:c++,c,exeptions,templates,classes
# Wrapping C++ exceptions, templated and classes in C
Sometime some libraries is distributed only as C++ libs or in template
code. Or plug-ins for some programs is only in C or other language.
Here can join C and C++ compiled code and there will no big difference
with language you use. Only need to choose right compiler for compilation.
C++ stuff can be easily hidden and you can use some C++ "only" features
in C without notice it.
Use templates from C.
Templates is most C++'sh part of all. Templates are defined in C++ headers
and there not generate code. Only when you use they generate code.
It reason why cannot include them inside *.so or *.a libraries.
But you can always generated templates function examples that will
generate code. And it means it now can be inside *.so and *.a.
To use templates from C and compile it with C we need to hide C++ headers
from C and create templates generated code
C compatible header code.h
```c
extern float cmpx_add_f(float, float);
extern int cmpx_add_i( int, int );
```
C++ template code.hpp
```c
template <typename T>
inline T cmpx_add( T a, T b )
{
return a+b;
}
```
Tell C++ to generate code for specific templates code.hpp
```c
template int cmpx_add<int>(int, int);
template float cmpx_add<float>(float, float);
```
Write C compatible wrapper in C++
```c
float cmpx_add_f( float a, float b )
{
return cmpx_add<float>(a,b);
}
int cmpx_add_i( int a, int b )
{
return cmpx_add<int>(a,b);
}
```
lets see now object file info
```
readelf -a
```
```c
18: 0000000000000000 63 FUNC GLOBAL DEFAULT 5 cmpx_add_f
20: 0000000000000000 42 FUNC WEAK DEFAULT 13 _Z8cmpx_addIfET_S0_S0_
21: 000000000000003f 31 FUNC GLOBAL DEFAULT 5 cmpx_add_i
22: 0000000000000000 20 FUNC WEAK DEFAULT 12 _Z8cmpx_addIiET_S0_S0_
```
We see that functions inside code.h is globally visible and we can use them
without problem from C.
Use C++ exceptions
Exceptions is usually very C++'sh everyone know that exceptions cannot be
used in C. There is some C libraries that add some kind exceptions for
C. That libraries usually uses setjmp and longjmp for simulating that kind of stuff.
C header
```c
extern void raise_exception( int );
```
C++ code for raising exceptions
```c
extern void raise_exception( int a )
{
try
{
int b;
b = b/a;
printf("Everything ok ;]\n");
}
catch ( int e )
{
printf("Catching up exception number %d \n",e);
}
}
```
C use exceptions
```c
raise_exception( 1 );
raise_exception( 0 );
```
Here is seen that C++ exception raises inside C code. There can be made
some code additions and C++ exceptions can be used in C with some tricks.
Not yet proof of concept. But with pointers to functions
(wasn't they called callbacks?) it cone be done.
Use C++ classes
C completable headers
```c
extern void* clA_init();
extern void clA_add( void*,int, int );
extern int clA_empty( void* );
extern void clA_destroy( void* );
```
C++ class definitions
```c
class A
{
public:
char *p;
A();
~A();
void add( int, int );
bool empty();
};
```
C++ class implementation
```c
A::A()
{
this->p = (char *)malloc(1024);
}
A::~A()
{
printf("Free Class Baby!!!\n");
if (this->p != NULL)
{
free( this->p);
}
}
void A::add( int a, int b )
{
int i = cmpx_add_i( a , b );
printf("%d\n", i);
}
bool A::empty()
{
if (this->p == NULL)
{
return true;
}
else
{
return false;
}
}
```
C++ wrapping functions
```c
extern void* clA_init()
{
A *a = new A;
return (void *)a;
}
extern void clA_add( void *c, int a, int b )
{
A *cl=(A*)c;
cl->add( a, b );
}
extern int clA_empty( void *c )
{
A *cl=(A*)c;
bool b = cl->empty();
if ( b == true )
{
return 1;
} else
{
return 0;
}
}
extern void clA_destroy( void *c )
{
A *cl = (A*)c;
cl->~A();
}
```
It easy to give C++ class to C as void* and then use
C++ class functions (wasnt they called methods?) from C
simply as pointers. It segfault oriented but it the way how it works.
These examples shows that you can stay as much as possible with C
and use C++ compiled code.
## Links
http://linux.die.net/man/1/readelf
http://gcc.gnu.org/codingconventions.html#ExternC
http://www.cplusplus.com/doc/tutorial/classes/
http://linux.die.net/man/3/setjmp
http://linux.die.net/man/3/longjmp
## Downloads
c_cpp_tricks.tar.gz -
2KiB - http://archive.main.lv/files/writeup/wrapping_c___exceptions,_templated_and_classes_in_c/c_cpp_tricks.tar.gz
