c++ - 什么时候我应该更喜欢非成员非 friend 函数而不是成员函数？

Question

Meyers 在他的《Effective C++》一书中提到，在某些场景下，非成员非友元函数比成员函数封装得更好。

例子:

// Web browser allows to clear something
class WebBrowser {
public:
  ...
  void clearCache();
  void clearHistory();
  void removeCookies();
  ...
};

许多用户都希望同时执行所有这些操作，因此 WebBrowser 也可能提供一个功能来做到这一点:

class WebBrowser {
public:
  ...
  void clearEverything();  // calls clearCache, clearHistory, removeCookies
  ...
};

另一种方式是定义一个非成员非好友函数。

void clearBrowser(WebBrowser& wb)
{
  wb.clearCache();
  wb.clearHistory();
  wb.removeCookies();
}

非成员函数更好，因为“它不会增加可以访问类私有(private)部分的函数的数量。”，从而导致更好的封装。

clearBrowser 之类的函数是便利函数，因为它们不能提供 WebBrowser 客户端无法通过其他方式获得的任何功能.例如，如果 clearBrowser 不存在，客户端可以自己调用 clearCache、clearHistory 和 removeCookies。

对我来说，便利函数的例子是合理的。但是当非成员(member)版表现出色时，除了便利功能之外还有什么例子吗？

更一般地说，什么时候使用哪个规则？

Answer 1

More generally, what are the rules of when to use which?

以下是 Scott Meyer 的规则 (source):

Scott has an interesting article in print which advocates that non-member non-friend functions improve encapsulation for classes. He uses the following algorithm to determine where a function f gets placed:

if (f needs to be virtual)
    make f a member function of C;
else if (f is operator>> or operator<<)
{
   make f a non-member function;
   if (f needs access to non-public members of C)
      make f a friend of C;
}
else if (f needs type conversions on its left-most argument)
{
   make f a non-member function;
   if (f needs access to non-public members of C)
      make f a friend of C;
}
else if (f can be implemented via C's public interface)
   make f a non-member function;
else
   make f a member function of C;

His definition of encapsulation involves the number of functions which are impacted when private data members are changed.

在我看来，这几乎概括了一切，而且也很合理。