14.4. Accessor functions¶

By convention, accessor functions have names that begin with get and end with the name of the instance variable they fetch. The return type, naturally, is the type of the corresponding instance variable.

In this case, the accessor functions give us an opportunity to make sure that the value of the variable is valid before we return it. Here’s what getReal looks like:

double Complex::getReal ()
{
  if (cartesian == false) calculateCartesian ();
  return real;
}

If the cartesian flag is true then real contains valid data, and we can just return it. Otherwise, we have to call calculateCartesian to convert from polar coordinates to Cartesian coordinates:

void Complex::calculateCartesian ()
{
  real = mag * cos (theta);
  imag = mag * sin (theta);
  cartesian = true;
}

Assuming that the polar coordinates are valid, we can calculate the Cartesian coordinates using the formulas from the previous section. Then we set the cartesian flag, indicating that real and imag now contain valid data.

As an exercise, write a corresponding function called calculatePolar and then write getMag and getTheta. One unusual thing about these accessor functions is that they are not const, because invoking them might modify the instance variables.

Take a look at the active code below, which uses the getReal accessor function.

Write your implementation of calculatePolar in the commented area of the active code below. Once you’re done with that, write the getMag and getTheta accessor functions. Read the comments in main to see how we’ll test if your functions works. If you get stuck, you can reveal the extra problem at the end for help.

#include <iostream>
#include <cmath>
using namespace std;

class Complex
{
  double real, imag;
  double mag, theta;
  bool cartesian, polar;

public:
  Complex ();
  Complex (double r, double i);
  void calculateCartesian ();
  double getReal ();
  double getImag ();
  void calculatePolar ();
  double getMag ();
  double getTheta ();
};

void Complex::calculatePolar () {
  // ``calculatePolar`` should convert the real and imaginary parts
  // into magnitude and theta. Use the formula in the previous section.
  // Write your implementation here.
}

double Complex::getMag () {
  // ``getMag`` should return the magnitude.
  // Delete the return 0 and write your implementation here.
  return 0;
}

double Complex::getTheta () {
  // ``getMag`` should return the theta.
  // Delete the return 0 and write your implementation here.
  return 0;
}

int main() {
  Complex c1 (0.0, 1.0);
  // Magnitude should be 1, theta should be pi/2, or about 1.5708
  cout << c1.getMag() << ", " << c1.getTheta() << endl;
}
====
Complex::Complex () { cartesian = false;  polar = false; }

Complex::Complex (double r, double i) {
  real = r;  imag = i;
  cartesian = true;  polar = false;
}

void Complex::calculateCartesian () {
  real = mag * cos (theta);
  imag = mag * sin (theta);
  cartesian = true;
}

double Complex::getReal () {
  if (cartesian == false) calculateCartesian ();
  return real;
}

double Complex::getImag () {
  if (cartesian == false) calculateCartesian ();
  return imag;
}

Let’s write the code for the calculatePolar function. Follow the format of the function calculateCartesian.

        void Complex::calculatePolar () {
---
void Complex::calculateCartesian () {                         #paired
---
   mag = sqrt(pow(real, 2) + pow(imag, 2));
---
   mag = pow(real, 2) + pow(imag, 2);                         #paired
---
   theta = atan(imag / real);
---
   polar = true;
}
---
   cartesian = true;                                          #paired
}

Let’s write the code for the getMag function, which should return the magnitude of a Complex object.

        double Complex::getMag () {
---
void Complex::getMag () {                         #paired
---
   if (polar == false) {
---
      calculatePolar ();
   }
---
   return mag;
}

Let’s write the code for the getTheta function, which should return the magnitude of a Complex object.

        double Complex::getTheta () {
---
double Complex::getMag () {                         #paired
---
   if (polar == false) {
---
      calculatePolar ();
   }
---
      calculateCartesian ();                         #paired
   }
---
   return theta;
}

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