Define private function in reference to class constructor

Question

I want to make a class Function that can take a float method and use it to produce useful values from the function. However, I do not understand how to declare a private method inside the constructor as the method passed by argument.

Hopefully this class can help in understanding my intentions:

#include <iostream>
#include <math.h>


class Function {
    float function(float x);
    public:
        Function(float method(float)) {
            // how do I set private function "function" equal to passed function "method"?
        };
        float eval(float x) {
            return function(x);
        }
        float derive(float x, float error = 0.001) {
            return (function(x) - function(x + error)) / error;
        }
        float integrate(float x0, float x1, float partitions = 100) {
            float integral = 0;
            float step = (x1 - x0) / partitions;
            for(float i = 0; i < partitions; i += step) integral += step * function(i);
            return integral;
        }
};


float exampleFunction(float x) {
    return 2 * pow(x, 2) + 5;
}


int main() {
    Function myFunction (exampleFunction);

    std::cout << myFunction.eval(6);
}

Addressing possible duplicate:

The tagged question was asking about invoking the constructor using a pointer to a method in an already constructed instance of the class. I am trying to pass a pointer into a constructor to define the new class's private method.

you mean [function pointer](https://stackoverflow.com/questions/1485983/calling-c-class-methods-via-a-function-pointer?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa)? — Joseph D., May 10 '18 at 03:49
Possible duplicate of [Calling C++ class methods via a function pointer](https://stackoverflow.com/questions/1485983/calling-c-class-methods-via-a-function-pointer) — Joseph D., May 10 '18 at 03:50
I could pass by reference with `Function( float (*method)(float))`, but unless I can somehow pass that reference to `function`, I haven't fixed my issue. — Benjamin Brownlee, May 10 '18 at 03:52

R Sahu · Accepted Answer · 2018-05-10T04:51:38.177

2

Use

  // The member variable that stores a pointer to a function.
  float (*function)(float);

  // The constructor.
  // Store the passed function in the member variable.
  Function(float method(float)) : function(method) {}

Working demo.

edited May 10 '18 at 04:51

answered May 10 '18 at 03:52

R Sahu

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FWIW, you might consider `std::function` instead. – Justin May 10 '18 at 04:43

score 0 · Answer 2 · answered May 10 '18 at 03:53

0

Easier with typedef:

using func_t = float(float);

class Function {
    func_t* function = nullptr;
public:
    Function(func_t* f) : function(f) {}
    // ...
};

answered May 10 '18 at 03:53

Jarod42

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Define private function in reference to class constructor

2 Answers2