Classes part 11 - friend functions (and why you should probably avoid) | Modern Cpp Series Ep. 47

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►Lesson Description: In this lesson I discuss the 'friend' keyword in C++ and why you should probably avoid using it because it breaks encapsulation in C++ classes and structs. The friend keyword allows (in a one-way relationship) functions to access private variables of a class. The friend keyword can also (in a one way relationship) allow classes to access all of the private member variables of another class.
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[@dexter-xi1gl]

omg, you are my video professor in Northeastern !!!!!

[@k0185123]

I just started using friend recently and felt proud of it, because I thought I've made some progress in my cpp learning, but now it seems that was a wrong decision. I shouldn't have used friend. 😂 Actually I even tried to use the "Client-Attorney" concept to hide part of the member functions. And it really doesn't look good, although it eventually works in my case. I wonder how to hide partial information without using friend. I thought that's a good way to encapsulate things, but the problem is it will disclose everything to the friends. Thank you for the explanation!

[@MikeShah]

Cheers! Yes, friend is one of those features that has to be considered carefully. The pimpl idiom can generally be useful for hiding implementation details (czcams.com/video/3mFpXNEB_AA/video.html)

[@k0185123]

@@MikeShah Thank you, Mike! I watched this video and will try to implement pIMPL next time. :D

[@dhanushs1802]

Very nicely explained as always. Thank You.

[@MikeShah]

Thank you for the kind words!

[@thestarinthesky_]

I didn't understand the second reason why we should avoid declaring friends in our classes . It starts at @10:57. Would you please explain that second reason?
Also I think that'd be great if you could post a video on the combination of declaring friends on derived and base classes with different types of inheritance :)
Thanks again for this informative video.

[@MikeShah]

Cheers! The basic issue with friends classes is that they can break encapsulation when used improperly, and I think they're an easier feature to use improperly versus just using a member function in many cases. isocpp.org/wiki/faq/friends

[@thestarinthesky_]

Hello Mike,
I was wondering why I get error on 'd.k;' !
Does it mean our friendship relationship doesn’t provide us access to the private members of the Base class? I thought friendship applies to the whole subject of a class (data members, member functions and Base)
struct Base
{

public:
int i;
protected:
int j;
private:
int k;
};
struct Derived : protected Base
{
friend int main();
public:
int l;
protected:
int m;
private:
int n;
};
int main()
{
Derived d;
d.i = 1;
d.j = 2;
d.k = 3;
d.l = 4;
d.m = 5;
d.n = 6;
return 0;
}

[@sallaklamhayyen9876]

we can create a friend class which can be used as white box testing ?

[@MikeShah]

Certainly could do that! In a perfect world, you'd catch any errors calling the public member functions, but perhaps there are some testing methodologies where you'd want to inject some errorful state into an object :)