Purpose of a static member function in C++?
If each member function is only contained once per class (to be shared by all instances) what exactly is the purpose of declaring a member function static? Is it like a function being declared const, in that it modifies a particular type of data (in this case, static data members)?
Normal member functions require a class instance to run. Static methods can be called directly without first creating an instance of the class.
Normal method:
MyClass myClass;
myClass.NormalMethod();
Static method:
MyClass::StaticMethod();
So normal methods are perfect for functions that work with the class data. If a method doesn't need to work with the class data, then it would be a candidate for possibly being made static.
Class methods, static or otherwise, can access private members of any of that class's objects, not just its own instance. Same goes for static methods, which don't have an instance unless you pass one to them.
You could also use a free function and declare it a friend, but a free function implies a higher level of abstraction that may operate on objects of different classes. A static class method says "I only make sense in light of my class"
One application of static methods is to create instances and return pointers. For example, there may be derived classes that the caller isn't supposed to know about - the "factory" function knows which derived class to use.
Of course when you need to create an object, you probably don't already have an object to use for that, and even if you do that other object isn't relevant.
Basically, sometimes some action is an aspect of the abstraction that a class provides, but that action isn't associated with a specific object - or at least not one that already exists. In that case, you should implement the action as a static function.
Similarly, some data is related to the abstraction provided by a class but not to a particular instance of that class. That data is probably best implemented as static member variables.
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