C++ static member functions and variables

Question

I am learning C++ by making a small robot simulation and I\'m having trouble with static member functions inside classes.

I have my Environment class defined like this:<

Answer 1

static members are those that using them require no instantiation, so they don't have this, since this require instantiation:

class foo {
public
    void test() {
        n = 10; // this is actually this->n = 10
    }
    static void static_test() {
        n = 10; // error, since we don't have a this in static function
    }
private:
    int n;
};

As you see you can't call an instance function or use an instance member inside an static function. So a function should be static if its operation do not depend on instance and if you require an action in your function that require this, you must think why I call this function static while it require this.

A member variable is static if it should shared between all instances of a class and it does not belong to any specific class instance, for example I may want to have a counter of created instances of my class:

// with_counter.h
class with_counter {
private:
    static int counter; // This is just declaration of my variable
public:
    with_counter() {++counter;}
    ~with_counter() {--counter;}

    static int alive_instances() {
        // this action require no instance, so it can be static
        return counter;
    }
};

// with_counter.cpp
int with_counter::counter = 0; // instantiate static member and initialize it here