friend of a class

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
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asarConnect With a Mentor Commented:
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.
Tommy HuiEngineerCommented:
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?
thorstenAuthor Commented:
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
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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.
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.

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...

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.


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
thorstenAuthor Commented:
Sorry I made a mistake. I have declared my constructor without the statement public. After Declaring it public everything works well.
Thank you
Does this meen I get the points?
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