Question

I have got a problem with calling a global function, which takes a pointer to a function as a parameter.
Here is the declaration of the global function:

int lmdif ( minpack_func_mn fcn, void *p, int m, int n, double *x, 
            double *fvec, double ftol)

The “minpack_func_mn” symbol is a typedef for a pointer to a function, defined as:

typedef int (*minpack_func_mn)(void *p, int m, int n, const double *x, 
              double *fvec, int iflag );

I want to call the “lmdif” function with a pointer to a function which is a member of a class I created, and here is the declaration of this class function:

int LT_Calibrator::fcn(void *p, int m, int n, const double *x, 
                       double *fvec,int iflag)

I am calling a global function like this:

info=lmdif(&LT_Calibrator::fcn, 0, m, n, x, fvec, ftol)

Unfortunately, I get a compiler error, which says:
“error C2664: ‘lmdif’ : cannot convert parameter 1 from ‘int (__thiscall LT_Calibrator::* )(void *,int,int,const double *,double *,int)’ to ‘minpack_func_mn’
1> There is no context in which this conversion is possible”

Is there any way to solve that problem?

Answer 1

You need a non-member or static member function; a member function pointer can’t be used in place of your function type because it requires an instance to call it on.

If your function doesn’t need access to a LT_Calibrator instance, then you can simply declare it static, or make it free function. Otherwise, it looks like you can use the first argument (void *p) to pass an instance pointer into a “trampoline” function, which can then call a member function. Something along the lines of this:

// member function
int LT_Calibrator::fcn(int m, ...);

// static (or non-member) trampoline
static int fcn_trampoline(void *p, int m, ...)
{
    return static_cast<LT_Calibrator*>(p)->fcn(m,...);
}

info = lmdif(&fcn_trampoline, this, m, ...);