"Candidate function not viable" from g++/gcc compiler. What is wrong here?

I'm trying to compile my main.cpp, but I keep getting this error for two hours now. The issue here is to pass a function as a parameter but I think I'm doing something wrong. The compiler says it cannot find the function, but I included "newt_rhap(params)" in the "functions.h" already. I did the returnType (*functionName)(paramType), but I may have skipped something here. The code of my friends doesn't need the recently mentioned syntax. What is wrong here? I tried using both -std=c++11 and -std=c++98. The gcc/g++ compiler came from my Xcode command line tools. ``` g++ (or gcc) -std=c++98(or 11) main.cpp -o main.out ``` There was no difference in error. ``` **error: no matching function for call to 'newt_rhap'** ./functions.h:5:8: note: candidate function not viable: no known conversion from 'double' to 'double (*)(double)' for 1st argument double newt_rhap(double deriv(double), double eq(double), double guess); ``` Here is the code. ``` // main.cpp #include <cmath> #include <cstdlib> #include <iostream> #include "functions.h" using namespace std; // function declarations // ============= // void test(double d); // ============= int main(int argc, char const *argv[]) { // // stuff here excluded for brevity // // ============= do { // line with error guess = newt_rhap(eq1(guess),d1(guess),guess); // more brevity } while(nSig <= min_nSig); // ============= cout << "Root found: " << guess << endl; return 0; } ``` Then functions.h and functions.cpp, respectively ``` // functions.h #ifndef FUNCTIONS_H_INCLUDED #define FUNCTIONS_H_INCLUDED // =========== double newt_rhap(double deriv(double), double eq(double), double guess); // =========== // =========== double eq1(double x); double d1(double x); // =========== #endif // functions.cpp #include <cmath> #include "functions.h" using namespace std; // =========== double newt_rhap(double (*eq)(double ) , double (*deriv)(double ), double guess) { return guess - (eq(guess)/deriv(guess)); } // =========== // =========== double eq1(double x) { return exp(-x) - x; } double d1(double x) { return -exp(-x) - 1; } ```