C++: "Field 'object_var' must be initialized" in constructor of Base class

Question

I am programming a polymorphic program in C++ that requires the use of Derived/Inherited classes. My base class is called ClassA and my Derived class is called ClassB. I have a separate class called varClass that is the datatype for a parameter in the constructor of my Base class, ClassA. I get this error message in the following constructor in ClassA.cpp:

Field 'object_var' must be initialized

#include "ClassA.h"
#include "varClass.h"

ClassA::ClassA(const classVar &O, const int &p) {}

Why must objects from a separate concrete class be initialized before allowing them to be used a parameters of functions in separate classes? I have tried putting a forward declaration of the classVar class in ClassA with no luck. How can I fix this to allow an argument of a datatype from a separate class? If I give it an initialization, how can I ensure each value passed to this constructor is unique and not just overridden by the initialization value?

Here is the rest of the relevant code. I am not going to show the Derived classes as I do not believe they are part of the problem, but decided to mention them since I am a novice programmer and that could possibly be the issue.

// ClassA.h

#ifndef CLASSA_H
#define CLASSA_H

#include "varClass.h"

class ClassA {

protected:
    const classVar &object_var; // The problem lies here
    unsigned var1;

 // ... more code follows

public:
    ClassA(const object &O, const int &p);

// ... more code follows

};

#endif

Header and implementation of objectVar class are irrelevant to issue. That class is a standard concrete class.

score 2 · Accepted Answer · edited May 23 '17 at 11:52

2

You should initialize reference in the constructor's initializer list:

ClassA(const classVar &O, const int &p)
 : object_var(O) {    
}

P.S. Make sure constructor's signatures in header and source files match.

edited May 23 '17 at 11:52

Community

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answered Dec 15 '15 at 01:17

Ivan Aksamentov - Drop

12,860
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Thanks! Very helpful. – jshapy8 Dec 15 '15 at 01:20

score 0 · Answer 2 · answered Dec 15 '15 at 01:08

0

object_var needs to be initialized in the constructor because it is a reference. There is no other way to have it refer to something.

answered Dec 15 '15 at 01:08

1201ProgramAlarm

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Can you give an example of the proper way to do that? How can I initialize it to ensure that values passed to it are unique and not just the same as the initialization value? – jshapy8 Dec 15 '15 at 01:15
@revolution9540 Something like `ClassA::ClassA(const classVar &O, const int &p): object_var(O) {} – 1201ProgramAlarm Dec 15 '15 at 01:17
Thanks! I didn't think of doing it that way. – jshapy8 Dec 15 '15 at 01:19

C++: "Field 'object_var' must be initialized" in constructor of Base class

2 Answers2