Rate Tips

Explain the “discards qualifiers” error in C++ using the metaphor of a contractor who has signed a contract not to change instance variables, but then attempts to use subcontractor who hasn’t agreed to the same terms or signed the appropriate forms.

  • The “discards qualifiers” error is difficult to decipher. The error reads as follows:
    • error: passing 'const Example' as 'this' argument of 'int Example::getValue()' discards qualifiers
  • Using this metaphor the contract requires a const but it is not being given what it was promised.
    • To fix this, you have to make the subcontractor agree to the same terms, to make the subcontractor const as well.
  • The necessary fix to the Java example below is to make the accessor method, getValue, a const method.
    • Using this metaphor, isOddValue is the contractor, and getValue is the subcontractor who has not signed the appropriate contract.
    • Example code: class Example { public:     Example();     int getValue() { return myValue;};     bool isOddValue() const; private:     int myValue; }; bool Example::isOddValue() const {     return (getValue() % 10 == 0); }