Friend Function in Multiple Classes (OOP with C++)

Question

Hi! I just want to learn friend function syntax in multiple classes, properly. According to what I've learned from the online courses, first of all we should define friend function to each classes in which we want to use it. When we do that, we write (for example) 'friend void functionname (classname1, classname2);'. But when I write this comment line I get an error in different compilers. So, because of that, compiler cannot run the program but even the error, codelite (one of the IDE) can run the program. In addition, when I change comment line as 'friend void functionname (classname_whichclass_I_am_in); problem solves, program runs without any error in every compiler. But I google the syntax of friend function and the true defining of ff is the first one which gives me errors. May you help me to figure out?

The error I get:
Unknown type name 'classname2'

My complete program:
https://code.sololearn.com/c4EXMSVMQMa0/?ref=app

Answer 1

Ipang

Thank you so much for your help😊

Answer 2

You're welcome Sultan Nur Korkusuz 😁

Answer 3

Sultan Nur Korkusuz
Thanks for sharing the code 👍
Please don't mark my answer, I haven't helped you with anything just yet.

~ swim ~
Is forward declaration necessity be the problem here? I see the `bus` class is defined before `microbus` class. What do you think bro?

Answer 4

Seems to me probably not having a declaration or definition of `classname2` be the cause, rather than the function signature (from the error message). But to be clear, I suggest you share your code link on the Description of your original question, so more people can help clear the doubt.

(Edit)
Also add C++ in the Relevant Tags

Answer 5

Thank you so much. I've added the link as you suggested me to be more clear.
Edit again for relevant tags

Answer 6

~swim~
Thanks to you I figure out the problem😊 I'll always declare the class before friend function comment line.👍