friend of a class

Posted on 1999-01-18
Last Modified: 2010-04-16
I have defined two classes A and B (in different header and cpp files). I want to use the constructor of class B in the constructor of class A. I did this by declaring class A as a friend of class B. This works but i want to ask wether there is another possibility to use the constructor of class B in class A without using the friend statement
Thanks Thorsten
Question by:thorsten
LVL 15

Expert Comment

by:Tommy Hui
ID: 1184469
What exactly do you mean by using the constructor of B in class A?

Are the classes related by hierarchy (meaning, is class A derived from B)?

Are you trying to create an instance of B in A?

Author Comment

ID: 1184470
Hi thui,
The classes are independent. The constructor of class A should throw an exception from the type of class B if the arguments it gets during initilization are not valid.
This is the constructor of class A
A::A(int a, int b, int c)
day =a;
if (korrekt()==0) throw Falsches_Datum(day,month,year);
korrekt() checkes wether the date is valid or not.
I did this declaring class A and B as friends. My question is which other possibilities do exist to use the constructor Falsches_Datum(..) in the constructor of class A.
Thank you

Expert Comment

ID: 1184471
As i know that is enough to declare class B ih header-file of class A. Then you can use B-constructor in A-methods. Constructor is public.
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Expert Comment

ID: 1184472
really don't understand your question either,
if the constructor of class B is public, why does class A have to be a friend to call it.

the 'friend' keyword have only one meaning, which is not valid in this case.

it gives access for a class'es private!! members/methods,
for a function, another class/struct/union etc.
and nothing else.


Expert Comment

ID: 1184473
You should always try to avoid using Friend classes. It is the underminer of encapsulation. Like Koskia said, a friend class has access to a class's private members/methods.
To solve your problem, just make the constructor of the B-class public and include the header of the B-class in the A-class. Then the A-class has enough access to the B-class.

PS: if there's no way to avoid friend classes, check your software design, maybe it needs an update...

LVL 12

Expert Comment

ID: 1184474
To answer your specific question, you do not need to use the friend statement as the constructor should be public in both A and B (if not ask yourself how anyone is ever to create an instance of class A or B). Class B simply needs to call the constructor of class A.



Expert Comment

ID: 1184475
Make sure the constructor of class B is really public.
There must be word 'public:' above the definition of the constructor of B in .h file.

Best regards

Author Comment

ID: 1184476
Sorry I made a mistake. I have declared my constructor without the statement public. After Declaring it public everything works well.
Thank you

Expert Comment

ID: 1184477
Does this meen I get the points?

Accepted Solution

asar earned 200 total points
ID: 1184478
Constructor as a class can't be called using a friend specifier. So that you can't be able to use it. The main definition of a constructor is a member function ( not a friend function ). A constructor is called only when an object of that class is instanciated. No way you can use as a friend fuction.

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