优先将非成员非朋友功能发送给成员函数

Question

This question title is taken from the title of item #23 in Effective C++ 3rd Edition by Scott Meyers. He uses the following code:

class WebBrowser {
public:
    void clearCache();
    void clearHistory();
    void removeCookies();

    //This is the function in question.
    void clearEverything();
};

//Alternative non-member implementation of clearEverything() member function.
void clearBrowser(WebBrowser& wb) {
    wb.clearCache();
    wb.clearHistory();
    wb.removeCookies();
};

While stating that the alternative non-member non-friend function below is better for encapsulation than the member function clearEverything(). I guess part of the idea is that there are less ways to access the internal member data for the WebBrowser if there are less member functions providing access.

If you were to accept this and make functions of this kind external, non-friend functions, where would you put them? The functions are still fairly tightly coupled to the class, but they will no longer be part of the class. Is it good practice to put them in the class's same CPP file, in another file in the library, or what?

I come from a C# background primarily, and I've never shed that yearning for everything to be part of a class, so this bewilders me a little (silly though that may sound).

Answer 1

Usually, you would put them in the associated namespace. This serves (somewhat) the same function as extension methods in C#.

The thing is that in C#, if you want to make some static functions, they have to be in a class, which is ridiculous because there's no OO going on at all- eg, the Math class. In C++ you can just use the right tool for this job- a namespace.

Answer 2

So clearEverything is a convenience method that isn't strictly necessary. But It's up to you to decide if it's appropriate.

The philosophy here is that class definitions should be kept as minimal as possible and only provide one way to accomplish something. That reduces the complexity of your unit testing, the difficulty involved in swapping out the whole class for an alternate implementation, and the number of functions that could need to be overridden by sub-classes.

In general, you shouldn't have public member functions that only invoke a sequence of other public member functions. If you do, it could mean either: 1) you're public interface is too detailed/fine-grained or otherwise inappropriate and the functions being called should be made private, or 2) that function should really be external to class.

Car analogy: The horn is often used in conjunction w/ slamming on your brakes, but it would be silly to add a new pedal/button for that purpose of doing both at once. Combining Car.brake() and Car.honk() is a function performed by Driver . However, if a Car.leftHeadLampOn() and Car.rightHeadLampOn() were two separate public methods, it could be an example of excessively fine grained control and the designer should rethink giving Driver a single Car.lightsOn() switch.

In the browser example, I tend to agree with Scott Meyers that it should not be a member function. However, it could also be inappropriate to put it in the browser namespace. Perhaps it's better to make it a member of the thing controlling Web browser, eg part of a GUI event handler. MVC experts feel free to take over from here.

Answer 3

I do this a lot. I've always put them into the same .cpp as the other class member functions. I don't think there is any binary size overhead depending where you put them though. (unless you put it in a header :P)

Answer 4

If you want to go down this route the imlementation of clearEverything should be put in both the header (declaration) and implementation of the class - as they are tightly coupled and seems the best place to put them.

However I would be inclined to place them as a part of the class - as in the future you may have other things to clear or there may be a better or faster implementation to implement clearEverything such as droppping a database an just recreate the tables