How can I keep track of the number of instances of a class

Question

Let's say I have a class Person which has a member variable nr_of_persons. I wish the constructor Person() would increment this variable each time I create a new Person object. I guess that nr_of_persons must be a static variable, but I cannot initialize it, and I can't increment it :( I would like to use such a variable like this:
int getSize(){
return nr_of_persons;
}

Answer 1

good question and good discussion so far thank you

Answer 2

a static variable in an instance you create will change the variable in every instance. say you have a class that draws a circle. In that class it has a static variable for color to be red. You create 5 instances of that class.. if you change that variable to yellow, it will change all of the instances of that circle to yellow.

This is why you can't keep a count with a static variable inside the class your creating the instances of. say your Person class is created and initializes your variable when it starts.

static int nr_of_persons = 0;

everytime you create an instance, it sets nr_of_persons to 0 across all of the instances.

Answer 3

Even if person_count is NOT static, it still works. (In fact for a global variable, I don't know if making it static has any use ;))

Answer 4

@Justin Hill -> Thank you a lot. But I still think that there has to be a more "automatic" way of doing that. After all, why are there static variables at all?

Answer 5

Thank you. I'll check.

Answer 6

I think that you are write. My problem was solved this way:
1) declare a global variable
static int person_count = 0;
2) in the constructor of Person class:
Person(... ..., ... ...){
person_count++;
//bla-bla-bla;
}

Answer 7

Glad you got it working!