Namespace + functions versus static methods on a class

Let's say I have, or am going to write, a set of related functions. Let's say they're math-related. Organizationally, should I:

1. Write these functions and put them in my MyMath namespace and refer to them via MyMath::XYZ()
2. Create a class called MyMath and make these methods static and refer to the similarly MyMath::XYZ()

Why would I choose one over the other as a means of organizing my software?

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 that are in the same namespace as a class is, and that have that class as a parameter, belong to that class' interface (because ADL will search those functions when resolving function calls).

For example:

• let's say you have a namespace N
• let's say you have a class C, declared in namespace N (in other words, its full name is N::C)
• let's say you have a function F, declared in namespace N (in other words, its full name is N::F)
• let's say that function F has, among its parameters, a parameter of type C

... Then N::F is part of N::C's public interface.

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 method 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 coolness of a namespace is that in some code, you can avoid mentioning it, if you use the keyword using:

#include <string>
#include <vector>

// Etc.
{
   using namespace std ;
   // Now, everything from std is accessible without qualification
   string s ; // Ok
   vector v ; // Ok
}

string ss ; // COMPILATION ERROR
vector vv ; // COMPILATION ERROR

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

#include <string>
#include <vector>

{
   using std::string ;
   string s ; // Ok
   vector v ; // COMPILATION ERROR
}

string ss ; // COMPILATION ERROR
vector vv ; // COMPILATION ERROR

This "pattern" is mandatory for the 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.

You can even bring the full content of a namespace inside another, with using namespace, as shown with namespace D!

namespace A
{
   void AAA();
   void AAA2();
}

namespace B
{
   void BBB();
}

namespace C
{
   using A::AAA;
   using B::BBB;
}

namespace D
{
   using namespace A;
   using namespace B;
}

void foo()
{
   C::AAA();
   // C::AAA2(); // ERROR, won't compile
   C::BBB();
}

void bar()
{
   D::AAA();
   D::AAA2();
   D::BBB();
}

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 a solution to different problems.

Don't use classes when you need namespaces.

And in your case, you need namespaces.

There are a lot of people who would disagree with me, but this is how I see it:

A class is essentially a definition of a certain kind of object. Static methods should define operations that are intimately tied to that object definition.

If you are just going to have a group of related functions not associated with an underlying object or definition of a kind of object, then I would say go with a namespace only. Just for me, conceptually, this is a lot more sensible.

For instance, in your case, ask yourself, "What is a MyMath?" If MyMath does not define a kind of object, then I would say: don't make it a class.

But like I said, I know there are plenty of folks who would (even vehemently) disagree with me on this (in particular, Java and C# developers).

• If you need static data, use static methods.
• If they're template functions and you'd like to be able to specify a set of template parameters for all functions together then use static methods in a template class.

Otherwise, use namespaced functions.

In response to the comments: yes, static methods and static data tend to be over-used. That's why I offered only two, related scenarios where I think they can be helpful. In the OP's specific example (a set of math routines), if he wanted the ability to specify parameters - say, a core data type and output precision - that would be applied to all routines, he might do something like:

template<typename T, int decimalPlaces>
class MyMath
{
   // routines operate on datatype T, preserving at least decimalPlaces precision
};

// math routines for manufacturing calculations
typedef MyMath<double, 4> CAMMath;
// math routines for on-screen displays
typedef MyMath<float, 2> PreviewMath;

If you don't need that, then by all means use a namespace.