c++ - What are static variables?

Question

What are static variables designed for? What's the difference between static int and int?

Answer 1

The static keyword has four separate uses, only two of which are closely related:

• static at global and namespace scope (applied to both variables and functions) means internal linkage
  • this is replaced by unnamed namespaces and is unrelated to the rest
  • in particular, others tend to imply some sort of uniqueness, but internal linkage means the opposite: you can have many objects with the same name, as long as each has internal linkage and you only have one per translation unit
• static data members are "shared" among all instances of the class
  • it's more like they are independent of any class instance
  • this is sometimes grouped with static methods
• static methods do not "operate" on a current instance
  • no this pointer; can call without an instance
• static local variables (in functions) persist across the scope of each function call

Both static data members and static local variables can become hidden global state, and should be used carefully.

Now which two are closely related? It's not the two class members—the warning about global state gives it away. You can consider static data members as static local variables, where the functions to which they belong are all methods of the class, instead of a single function.

I found many related questions, but, surprisingly, no duplicates.