cool@archpony ~/D/e/p/c/names [1]> cat main.cpp
#include <iostream>

using namespace std;

class Car
{
public:
    static int number_of_cars = 0;
    Car()
    {
        number_of_cars++;
    }
    ~Car()
    {
        number_of_cars--;
    }
};

int main()
{
    Car first_car;
    cout << first_car.number_of_cars << endl;
    Car second_car;
    cout << first_car.number_of_cars << endl;
    return 0;
}
cool@archpony ~/D/e/p/c/names> clang++ main.cpp -o main -Wall -Wextra -Weverything
main.cpp:8:16: error: non-const static data member must be initialized out of line
    static int number_of_cars = 0;
               ^                ~
1 error generated.