Create a function from another one

Question

more than a general case, I have a very specific example in mind : in GSL (GNU Scientific Library), the main function type used (in order to perform integration, root finding,...) is gsl_function , which have an attribute function whose type is double(*)(double, void *)

Say I want to create a gsl_function from double a_squared(double a) {return a*a};. a__squared 's type is double(*)(double) I would like to create a convert function taking in argument (double(*)(double) f) and returning an object of type double(*)(double, void *) which would satisfy convert(f)(double a, NULL) == f(a)

But after some research, it seems like I can't define another function in my convert function. How to proceed ?

Answer 1

The need to pass a raw function pointer to the GSL API limits your options considerably - you can't use anything based on std::function because there's no way to obtain a function pointer from a std::function (and this rules out lambdas using captures, which would have offered a neat solution).

Given these constraints, here's a possible solution making use of a static wrapper class. You could just as well have put the contents of this class in a namespace, but using the class at least gives some semblance of encapsulation.

typedef double gsl_function_type(double, void*);  // typedef to make things a bit more readable...

// static class to wrap single-parameter function in GSL-compatible interface
// this really just serves as a namespace - there are no non-static members,
// but using a class lets us keep the details private
class Convert
{
    Convert() = delete;  // don't allow construction of this class    

    // pointer to the function to be invoked
    static double (*m_target)(double);

    // this is the function we'll actually pass to GSL - it has the required signature
    static double target(double x, void*) {
        return m_target(x);  // invoke the currently wrapped function
    }

public:
    // here's your "convert" function
    static gsl_function_type* convert(double (*fn)(double)) {
        m_target = fn;
        return ⌖
    }
};

There's a live example here: http://coliru.stacked-crooked.com/a/8accb5db47a0c51d