How do I pass a C++ callback to a C library function? - c++

Question

I'm developing my code using C++ and want to use MPFIT nonlinear curve fitting library, which is developed in C but allows to compile in C++.
For example I have a class named "myClass", and this class has a function myClass::Execute()
I include "mpfit.h" to myClass.h file. And try to call a function called mpfit from Execute().
int status = mpfit(ErrorFunction, num1, num2, xsub_1D, 0, 0, (void *) &variables, &result);
The problem is ErrorFunction is a function of myClass. So compiler gives error when I try to use this. I tried to carry the ErrorFunction out of the class object, but this time I take the error given below:
Error when the ErrorFunction is outside of the class:
Error 4 error C2664: 'mpfit' : cannot convert parameter 1 from 'int
(__cdecl *)(int,int,double *,double,double *,void *)' to
'mp_func'
Error when the ErrorFunction is inside the class:
Error 3 error C3867: 'myClass::ErrorFunction': function call missing argument list; use '&myClass::ErrorFunction' to
Definition of error function:
int ErrorFunction(int dummy1, int dummy2, double* xsub, double *diff, double **dvec, void *vars)
How can I call this function and parse it into mpfit, which is a C function?
mp_func is defined as:
/* Enforce type of fitting function */
typedef int (*mp_func)(int m, /* Number of functions (elts of fvec) */
int n, /* Number of variables (elts of x) */
double *x, /* I - Parameters */
double *fvec, /* O - function values */
double **dvec, /* O - function derivatives (optional)*/
void *private_data); /* I/O - function private data*/

Answer 1

Make sure that your calling conventions match. C libraries use the C calling convention, or cdecl (__cdecl). If you're using the mp_func typedef within C++, it could be defaulting to the compiler's standard calling convention, or stdcall (__stdcall). Either make a new typedef or change it to the following:
typedef int __cdecl (*mp_func)(int m, /* Number of functions (elts of fvec) */
int n, /* Number of variables (elts of x) */
double *x, /* I - Parameters */
double *fvec, /* O - function values */
double **dvec, /* O - function derivatives (optional)*/
void *private_data); /* I/O - function private data*/
And when you declare ErrorFunction, also declare it as __cdecl:
int __cdecl ErrorFunction(int, int, double*, double *, double **, void *);
If the compiler still complains when calling the mpfit function, you can try casting your function pointer to the mp_func typedef with cdecl:
int status = mpfit((mp_func)ErrorFunction, num1, num2, xsub_1D, 0, 0, (void *) &variables, &result);

Answer 2

Given the definitions of mpfit() and mp_func that you have shown, you would need to use the private_data parameter of mp_func to pass your class's this pointer around. You are currently using that parameter to pass your variables item around instead. Make variables be a member of your class (if it is not already) and then pass this to mpfit() instead:
class MyClass
{
private:
TheDataType variables;
static int ErrorFunction(int m, int n, double *x, double *fvec, double **dvec, MyClass *pThis);
public:
void DoIt();
};
void MyClass::DoIt()
{
// ...
int status = mpfit((mp_func)&ErrorFunction, num1, num2, xsub_1D, 0, 0, this, &result);
// ...
}
int MyClass::ErrorFunction(int m, int n, double* x, double *fvec, double **dvec, MyClass *pThis)
{
// use pThis->variables as needed ...
}
Or:
class MyClass
{
private:
static int MPFitErrorFunction(int m, int n, double *x, double *fvec, double **dvec, MyClass *pThis);
int MyErrorFunction(int m, int n, double *x, double *fvec, double **dvec);
public:
void DoIt();
};
void MyClass::DoIt()
{
// ...
int status = mpfit((mp_func)&MPFitErrorFunction, num1, num2, xsub_1D, 0, 0, this, &result);
// ...
}
int MyClass::MPFitErrorFunction(int m, int n, double* x, double *fvec, double **dvec, MyClass *pThis)
{
return pThis->MyErrorFunction(m, n, x, fvec, dvec);
}
int MyClass::MyErrorFunction(int m, int n, double* x, double *fvec, double **dvec)
{
// use this->variables as needed ...
}

Answer 3

Looks like instead of:
int ErrorFunction(int dummy1, int dummy2, double* xsub, double diff, double *dvec, void *vars)
it should be:
int ErrorFunction(int dummy1, int dummy2, double* xsub, double *diff, double **dvec, void *vars)
to match your
typedef int (*mp_func)(int m, /* Number of functions (elts of fvec) */
int n, /* Number of variables (elts of x) */
double *x, /* I - Parameters */
double *fvec, /* O - function values */
double **dvec, /* O - function derivatives (optional)*/
void *private_data); /* I/O - function private data*/

Answer 4

Your callback must be declared extern "C" for this to work.
Edit: I see people are having hard time grasping this fact. The standard says (7.5/1):
Two function types with different language linkages are distinct types
even if they are otherwise identical.

Answer 5

There's a standard idiom for C++ - to - C, using the pimpl idiom:
foo_c.h:
#ifdef __cplusplus
extern "C" {
#endif
//forward declaration. clients of foo_c.h should only hold pointers to Foo_c
typedef struct Foo_c Foo_c;
int someMethod(Foo_c* foo);
#ifdef __cplusplus
}
#endif
foo_c.cpp:
#include <foo.h>
struct Foo_c {
Foo foo;
}
int someMethod(Foo_c* foo) {
try {
foo->foo.someMethod();
return 0; //no error
}
catch(...) {
return 1; //error
}
}
(Edited for extern "C"'s per below answer.)