FAQ 16.12: What is the right strategy for implementing a function that needs to maintain state between calls?

Turn the function into a functionoid. Do not create a function with local static data.

In C, it was common to create a function that maintained state between calls by means of local, static data inside the function body. Since this is unsafe in a multithreaded environment, in C++ such a function should be implemented as a functionoid, which is a fancy name for a class that has one major member function. The local static data from the original C-like function should become nonstatic member data of the functionoid class. The benefit is to allow different callers to have different values for the datum that used to be static. For example, every calling function that wants its own copy of the function's state can simply create its own distinct functionoid object.