Pass reference to external function in class constructor - c++

Question

There are other questions regarding how to pass an external function as an argument to a function in C++, but I am struggling to apply those answers to the case where the target function is the constructor function for a novel class.
Here is what I am trying to do:
#include <iostream>
double external_function(double x, double y) {
return x * y;
}
class foo {
public:
// Initialize a function (?) as a method of class foo
double func(double, double);
// Construct an instance of the class by supplying a pointer to an external function
foo(double(*func_in)(double, double)) {
func = *func_in;
}
// When calling this method, evaluate the external function
double eval(double x, double y) {
return func(x, y);
}
};
int main() {
foo foo_instance(&external_function);
std::cout << foo_instance.eval(1.5, 2); // Should print 3
return 0;
}
I would like to have func as a method of the foo class because I later will write methods of foo that do other things with func, e.g. search for a local minimum.
By analogy, here is working code that passes an external constant instead of a function:
#include <iostream>
double external_value = 1.234;
class bar {
public:
// Initialize a value as a method of class bar
double val;
// Construct an instance of the class by supplying a pointer to an external value
bar(double* val_in) {
val = *val_in;
}
// When calling this method, return the external function
double eval() {
return val;
}
};
int main() {
bar bar_instance(&external_value);
std::cout << bar_instance.eval(); // 1.234
return 0;
}

Answer 1

Like this
class foo {
public:
// Initialize a function pointer as a method of class foo
double (*func)(double, double);
// Construct an instance of the class by supplying a pointer to an external function
foo(double(*func_in)(double, double)) {
func = func_in;
}
// When calling this method, evaluate the external function
double eval(double x, double y) {
return func(x, y);
}
};
In your version func was an undefined method of the class not the function pointer you wanted it to be.

Answer 2

Here is the example using std::function:
#include <functional>
class foo {
public:
// Initialize a function as a method of class foo
std::function<double(double, double)> func;
// Construct an instance of the class by supplying a pointer to an external function
foo(std::function<double(double, double)> func_in) {
func = func_in;
}
// When calling this method, evaluate the external function
double eval(double x, double y) {
return func(x, y);
}
};