Function pointers working as closures in C++

Question

Is there a way in C++ to effectively create a closure which will be a function pointer? I am using the Gnu Scientific Library and I have to create a gsl_function. This funct

Answer 1

Though bradgonesurfing has given a nice answer that will work for converting closures into gsl_functions without any further thought, I would like to share with you the idiom for doing a direct translation from C++ into C.

Supposing you have the closure:

double a;
[&a](double x){return a+x;}

You would convert translate this into an equivalent function pointer idiom as follows:

struct paramsAPlusX{
    double* a;
    paramsAPlusX(double & a_):a(&a_){}
}
double funcAPlusX(double x, void* params){
   paramsAPlusX* p= (paramsAPlusX*)params;
   return *(p->a) + x;
}

//calling code:
double a;
paramsAPlusX params(a);
gsl_function f;
f.function=funcAPlusX;
f.params=¶msAPlusX;
//use f here.

Many C libraries use this sort of idiom, and they don't all use a struct for it (they frequently pass it as two separate parameters to the function) so automatic conversion isn't always possible.