Why does the "C++ Core Guidelines" recommends the prefer independent functions instead of class members?

Question

The C++ Core Guidelines section C.4 recommends "make a function a member only if it needs direct access to the representation of a class", using the following example:

class Date {
    // ... relatively small interface ...
};

// helper functions:
Date next_weekday(Date);
bool operator==(Date, Date);

I don't understand the reasoning here. In my opinion this would be a better option:

class Date {
    // ... relatively small interface ...

    Date next_weekday(Date const&) const;
    bool operator==(Date const&) const;
};

What is the advantage in this reasoning?

*Why* do you think member functions would be a better option? :) — Quentin, May 11 '17 at 13:07
I'm with you on this, subject to making the "helper functions" `static`. — Bathsheba, May 11 '17 at 13:07
Look at [how-non-member-functions-improve-encapsulation](http://stackoverflow.com/questions/1692084/how-non-member-functions-improve-encapsulation) — Jarod42, May 11 '17 at 13:10
Your'e aware of the C++ code philosophy of "don't pay for what you don't use", I hope. It can be said that breaking encapsulation incurs a price. So this guideline just applies the same philosophy to the price of breaking encapsulation. — StoryTeller - Unslander Monica, May 11 '17 at 13:16
Even though this is a dup, I disagree with the downvoting. Going from Core Guidelines to something written by Scott Meyer is non-trivial. Also, the other question knows this is about encapsulation. Finally, the CG specifies independent and the other post is about non-member. In short, finding the dup is non-trivial for most people. — Roy Falk, Sep 27 '19 at 08:28

score 3 · Accepted Answer · answered May 11 '17 at 13:11

Member functions have access to private members of a class. This means that they can use the internal representation of the class in their logic.

Non-member functions only have access to public members of a class. This means that they can only use the publicly exposed interface of the class, thus improving encapsulation.

score 0 · Answer 2 · answered May 11 '17 at 13:10

It increases encapsulation

When you change the private implementation of Date, you only have to check // ... relatively small interface ... for places where previous assumptions may no longer hold, instead of // ... relatively small interface ... + next_weekday + operator== + ...

Why does the "C++ Core Guidelines" recommends the prefer independent functions instead of class members?

2 Answers2

It increases encapsulation