Why static function should be explicitly declaied outside the class?

Answer 1

This depends a little bit on your language, but in a sense all functions defined outside a class are static, if your language allows them at all (C++ does, Java does not) A static member is a member that is not tied to a particular instance of a class.

Typically a static method (it's called a method, not a function when it's part of a class) is used when you want to allow access to a method without requiring an instance of the class to be created. For example, all the methods of the Java Math object are static. You don't ever create an instance of the Math class. Instead, you refer to the entire class: Math.round(), for example. The static method is still defined inside the class. It can be called without an instance of the class being available.

The most common static method (at least in Java) is main() It needs to be static because it is called before any classes are instantiated. Usually it is used to 'bootstrap' the program by instantiating its own class.

In C++, the static members will have a scope outside the class instance, so they should be prototyped like any other top-level function. Prototyping isn't absolutely necessary, but it makes it much easier to write your code as you don't have to worry about the order in which things are written.