C++, static vs. namespace vs. singleton

Question

I already read a lot of posts and articles all over the net, but I couldn't find a definite answer about this.

I have some functions with similar purposes that I want to have out of the global scope. Some of them need to be public, others should be private (because they are only helper functions for the "public" ones). Additionally, I don't have only functions, but also variables. They are only needed by the "private" helper functions and should be private, too.

Now there are the three ways:

• making a class with everything being static (contra: potential "Cannot call member function without object" - not everything needs to be static)
• making a singleton class (contra: I WILL need the object)
• making a namespace (no private keyword - why should I put it in a namespace at all, then?)

What would be the way to take for me? Possible way of combining some of these ways?

I thought of something like:

1. making a singleton, the static functions use the helper function of the singleton object (is this possible? I'm still within the class, but accessing an object of it's type)
2. constructor called at programm start, initializes everything (-> making sure the statics can access the functions from the singleton object)
3. access the public functions only through MyClass::PublicStaticFunction()

Thanks.

Answer 1

As noted, using global variables is generally bad engineering practice, unless absolutely needed of course (mapping hardware for example, but that doesn't happen THAT often).

Stashing everything in a class is something you would do in a Java-like language, but in C++ you don't have to, and in fact using namespaces here is a superior alternative, if only:

• because people won't suddenly build instances of your objects: to what end ?
• because no introspection information (RTTI) is generated for namespaces

Here is a typical implementation:

// foo.h #ifndef MYPROJECT_FOO_H_INCLUDED #define MYPROJECT_FOO_H_INCLUDED  namespace myproject {   void foo();   void foomore(); }  #endif // MYPROJECT_FOO_H_INCLUDED  // foo.cpp #include "myproject/foo.h"  namespace myproject {  namespace {   typedef XXXX MyHelperType;    void bar(MyHelperType& helper); } // anonymous  void foo() {   MyHelperType helper = /**/;   bar(helper); }  void foomore() {   MyHelperType helper = /**/;   bar(helper);   bar(helper); } } // myproject 

The anonymous namespace neatly tucked in a source file is an enhanced private section: not only the client cannot use what's inside, but he does not even see it at all (since it's in the source file) and thus do not depend on it (which has definite ABI and compile-time advantages!)

Answer 2

Don't make it a singleton

For public helper functions that don't directly depend on these variables, make them non-member functions. There's nothing gained by putting them in a class.

For the rest, put it in a class as normal non-static members. If you need a single globally accessible instance of the class, then create one (but don't make it a singleton, just a global).

Otherwise, instantiate it when needed.