Const Member Functions The code given below does not compile due to const issues.

Question

Modify the existing class code and add const in the appropriate locations to make it compile with the existing main and printNumber

#include <iostream>

using namespace std;

//Do not modify anything on or above the line below this6

//YOUR_CODE_BELOW

class EvenNumber {

private:

   int value;

public:

   EvenNumber(int startValue) {

       //use our function to do the work...

       setValue(startValue);

  }

​

   int getValue() {

       return value;

  }

   void setValue(int newValue) {

       //if given odd number, increase by 1

       if(newValue % 2 == 0)

           value = newValue;

       else

           value = newValue + 1;

  }

​

   void increment() { value += 2; }

​

   void decrement() { value -= 2;

​

};

//YOUR_CODE_ABOVE

//Do not modify anything on or below the line above this

void printNumber(const EvenNumber& n) {

   cout << n.getValue() << endl;

}

​

int main

{

   EvenNumber e1(4);

   e1.increment();

   printNumber(e1);

​

   return 0;

}

​

Answer 1

If you try to compile that code, you will see the error:

constFunc.cpp:32:13: error: 'this' argument to member function 'getValue' has type 'const EvenNumber', but function is not marked const

cout << n.getValue() << endl;

This is telling you that the call to the "getValue" function is failing because that function is not declared as const, but the instance of the EvenNumber class is a const. The failure happens because Non-const functions can only be called by non-const objects. However, when a function is declared as const, it can be called on any type of object.

In order to fix this error, all we need to do is add the "const" keyword to the getValue() function definition.

#include <iostream>

using namespace std;

//Do not modify anything on or above the line below this6

//YOUR_CODE_BELOW

class EvenNumber {

private:

  int value;

public:

  EvenNumber(int startValue) {

    //use our function to do the work...

    setValue(startValue);

 }

​

  int getValue() const {

    return value;

 }

  void setValue(int newValue) {

    //if given odd number, increase by 1

    if(newValue % 2 == 0)

      value = newValue;

    else

      value = newValue + 1;

 }

​

  void increment() { value += 2; }

​

  void decrement() { value -= 2;

​

};

//YOUR_CODE_ABOVE

//Do not modify anything on or below the line above this

void printNumber(const EvenNumber& n) {

  cout << n.getValue() << endl;

}

​

int main()

{

  EvenNumber e1(4);

  e1.increment();

  printNumber(e1);

​

  return 0;

}