When Should You Use 'Friend' in C++

When should you use 'friend' in C++?

Firstly (IMO) don't listen to people who say friend is not useful. It IS useful. In many situations you will have objects with data or functionality that are not intended to be publicly available. This is particularly true of large codebases with many authors who may only be superficially familiar with different areas.

There ARE alternatives to the friend specifier, but often they are cumbersome (cpp-level concrete classes/masked typedefs) or not foolproof (comments or function name conventions).

Onto the answer;

The friend specifier allows the designated class access to protected data or functionality within the class making the friend statement. For example in the below code anyone may ask a child for their name, but only the mother and the child may change the name.

You can take this simple example further by considering a more complex class such as a Window. Quite likely a Window will have many function/data elements that should not be publicly accessible, but ARE needed by a related class such as a WindowManager.

class Child
{
//Mother class members can access the private parts of class Child.
friend class Mother;

public:

  string name( void );

protected:

  void setName( string newName );
};

When should you use friend classes?

Friend is used for granting selective access, just like the protected access specifier. It's also hard to come up with proper use case where use of protected is really useful.

In general, friend classes are useful in designs where there is intentional strong coupling: you need to have a special relationship between two classes. More specifically, one class needs access to another classes's internals and you don't want to grant access to everyone by using the public access specifier.

The rule of thumb: If public is too weak and private is too strong, you need some form of selected access: either protected or friend (the package access specifier in Java serves the same kind of role).

Example design

For instance, I once wrote a simple stopwatch class where I wanted to have the native stopwatch resolution to be hidden, yet to let the user query the elapsed time with a single method and the units to be specified as some sort of variable (to be selected by user preferences, say). Rather than, have say elapsedTimeInSeconds(), elapsedTimeInMinutes(), etc. methods, I wanted to have something like elapsedTime(Unit::seconds). To achive both of these goals, I can't make the native resolution public nor private, so I came up with the following design.

Implementation overview

class StopWatch;

// Enumeration-style class.  Copy constructor and assignment operator lets
// client grab copies of the prototype instances returned by static methods.
class Unit
{
friend class StopWatch;
    double myFactor;
    Unit ( double factor ) : myFactor(factor) {}
    static const Unit native () { return Unit(1.0); }
public:
        // native resolution happens to be 1 millisecond for this implementation.
    static const Unit millisecond () { return native(); }

        // compute everything else mostly independently of the native resolution.
    static const Unit second () { return Unit(1000.0 / millisecond().myFactor); }
    static const Unit minute () { return Unit(60.0 / second().myFactor); }
};

class StopWatch
{
    NativeTimeType myStart;
    // compute delta using `NativeNow()` and cast to
    // double representing multiple of native units.
    double elapsed () const;
public:
    StopWatch () : myStart(NativeNow()) {}
    void reset () { myStart = NativeNow(); }
    double elapsed ( const Unit& unit ) const { return elapsed()*unit.myFactor; }
};

As you can see, this design achieves both goals:

1. native resolution is never exposed
2. desired time unit can be stored, etc.

Discussion

I really like this design because the original implementation stored the multiple of native time units and performed a division to compute the elapsed time. After someone complained the division was too slow, I changed the Unit class to cache the dividend, making the elapsed() method (a little) faster.

In general, you should strive for strong cohesion and weak coupling. This is why friend is so little used, it is recommended to reduce coupling between classes. However, there are situations where strong coupling gives better encapsulation. In those cases, you probably need a friend.

When to use friend class in C++

I agree with the comments that say the friend keyword can improve encapsulation if used wisely. I'd just add that the most common (legitimate!) use for friend classes may be testing. You may want a tester class to have a greater degree of access than other client classes would have. A tester class could have a good reason to look at internal details that are deliberately hidden from other classes.

where should friend functions be written?

where should friend functions be written?

Same as member functions. Either inline in the header, or non-inline in a single translation unit. If inline, it can be outside or inside the class definition. If outside, then it must be declared inline explicitly.

I get this error

undefined reference to...

See this: What is an undefined reference/unresolved external symbol error and how do I fix it?

and when I write the body in the .h file I get this error

multiple definition of..

This means that you failed to declare the function as inline, and consequently failed to follow the One Definition Rule. Solution is to declare the function inline.

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