Namespace + functions versus static methods on a class 命名空间函数和类的静态方法对比

source:http://stackoverflow.com/questions/1434937/namespace-functions-versus-static-methods-on-a-class

By default, use namespaced functions.

Classes are to build objects, not to replace namespaces.

In Object Oriented code

Scott Meyers wrote a whole Item for his Effective C++ book on this topic, "Prefer non-member non-friend functions to member functions". I found an online reference to this principle in an article from Herb Sutter:http://www.gotw.ca/gotw/084.htm

The important thing to know is that: In C++ functions in the same namespace than a class belong to that class' interface. (because ADL will search those functions when resolving function calls)

namespaced functions, unless declared "friend" have no access to the class' internals, whereas static methods have.

This means, for example, that when maintaining your class, if you need to change your class' internals, you will need to search for side effects in all its methods, including the static ones.

Extension I

Adding code to a class' interface.

In C#, you can add methods to a class even if you have no access to it. But in C++, this is impossible.

But, still in C++, you can still add a namespaced function, even to a class someone wrote for you.

See from the other side, this is important when designing your code, because by putting your functions in a namespace, you will authorize your users to increase/complete the class' interface.

Extension II

A side-effect of the previous point, it is impossible to declare static methods in multiple headers. Every methods must be declared in the same class.

For namespaces, functions from the same namespace can be declared in multiple headers (the almost-standard swap function is the best example of that).

Extension III

The basic cooless of a namespace is that in some code, you can avoid mentioning it, if you use the keyword "using":

#include<string>#include<vector>// Etc.{usingnamespace std ;// Now, everything from std is accessible without qualification
   string s ;// Okvector v ;// Ok}

string ss ;// COMPILATION ERRORvector vv ;// COMPILATION ERROR

And you can even limit the "pollution" to one class:

#include<string>#include<vector>{using std::string ;
   string s ;// Okvector v ;// COMPILATION ERROR}

string ss ;// COMPILATION ERRORvector vv ;// COMPILATION ERROR

This "pattern" is mandatory for proper use of the almost-standard swap idiom.

And this is impossible to do with static methods in classes.

So, C++ namespaces have their own semantics.

But it goes further, as you can combine namespaces in a way similar to inheritance.

For example, if you have a namespace A with a function AAA, a namespace B with a function BBB, you can declare a namespace C, and bring AAA and BBB in this namespace with the keyword using.

Conclusion

Namespaces are for namespaces. Classes are for classes.

C++ was designed so each concept is different, and is used differently, in different cases, as solution to different problems.

Don't use classes when you need namespaces.

And in your case, you need namespaces.