FAQ 16.06 How is a static data member similar to a global variable?

A static data member is like a global variable with a funny name that does not need to be public:.

If a class has a static data member, there is only one copy of that datum even if there are many instances of the class. This is like a global variable. The difference is that a static data member has a funny (scoped) name (it doesn't pollute the global namespace) and it needn't be public: (static data members can be private:, protected:, or public:).

These factors allow classes to be the logical packaging device; source files are reduced to mere buckets of bits and code. There is no need to use source files for hiding data (for instance, there is no need to use file-scope static data to hide data in a source file) since the data can now be hidden in a class. This distinction allows the physical packaging of software to be different from the logical packaging. For example, physical packaging may be optimized based on page fault analysis or on compile-time performance or for maintainability, and so forth.

Global data is rarely used any more. Normally objects and instance data work fine, but when true global data is required, the right choice is normally to use static member data (that is, class-scope static data members) or to put the data in an unnamed namespace.