I have been trying to find a way around the following:
I am using a library functor to solve the root of a non linear equation. It passes two doubles and the name of a function that contains the equation to be solved.

Its xtor and operator are

RootFind(double tL,double tR,double (*fn)(double t));

void operator()(double tL,doubleR,double (*fn)(double t));

I want to be able to send another variable to function fn; I don't know what the value of the variable will be ahead of time.
How do I get the library functor to take an extra variable in the function passed?