Reference as class member initialization

Question

I want to initialize a property of a class that holds a reference to another class by passing such a reference as a parameter to the constructor. However I receive an error:

“'TaxSquare::bank' must be initialized in constructor base/member initializer list”. What is wrong in the following code of the classes?

#ifndef TAXSQUARE_H #define TAXSQUARE_H #include "Square.h"  class Bank;  class TaxSquare : public Square {     public:       TaxSquare(int, int, Bank&);       virtual void process();      private:       int taxAmount;       Bank& bank;  }; #endif 

#include <iostream> #include "TaxSquare.h" #include "Player.h" #include "Bank.h" using namespace std;  TaxSquare::TaxSquare(int anID, int amount, Bank& theBank) : Square(anID) {   taxAmount = amount;   bank = theBank; } 

#ifndef BANK_H #define BANK_H  class Bank { public:   Bank(int, int, int);   void getMoney(int);   void giveMoney(int);   void grantHouse();   void grantHotel();  private:   int sumMoney;   int numOfHouses;   int numOfHotels;  };  #endif 

Answer 1

You are attempting to assign to bank, not initialize it:

TaxSquare::TaxSquare(int anID, int amount, Bank& theBank) : Square(anID) {     // These are assignments     taxAmount = amount;     bank = theBank; } 

bank is a reference, and therefore it must be initialized. You do so by putting it in the initializer list:

TaxSquare::TaxSquare(int anID, int amount, Bank& theBank) : Square(anID), taxAmount(amount), bank(theBank) {} 

Answer 2

“'TaxSquare::bank' must be initialized in constructor base/member initializer list”. What is wrong in the following code of the classes?

What is wrong is that TaxSquare::bank is not being initialized in the constructor base/member initialization list, exactly as it says.

"The constructor base/member initialization list" is the initialization list for the constructor in question, TaxSquare::TaxSquare(int, int, Bank&). You're already using it to initialize the base (Square). You must use it to initialize the bank member, because it is of a reference type. Things not specified in the initialization list get default-initialized, and there is no default-initialization for references, because they must always reference something, and there is no default something for them to reference.

Honestly, I find that using references for data members in C++ is more trouble than it's worth, 99% of the time. You're probably better off with a smart pointer, or even a raw one. But you should still initialize that with the initialization list, even if you could get away without. Same goes for the taxAmount, really.

// TaxSquare::TaxSquare(int anID, int amount, Bank& theBank) : Square(anID) // That thing after the colon is the initialization list:      ^^^^^^^^^^^^ // So add the other members to it, and then notice that there is nothing left // for the constructor body to do: TaxSquare::TaxSquare(int anID, int amount, Bank& theBank) :  Square(anID), taxAmount(amount), bank(theBank) {}