Counting number of Class Instances using "private static integer variable" in c++

Asked Apr 11 '18 at 18:45

Active Apr 11 '18 at 18:46

Viewed 35 times

I tried to count the number of instances of a class using :

class shelf
{
private:
    static int count_instances = 0 ;
    book b1;
public :
shelf() {
    count_instances++;
    cout << "this is the "<<count_instances<<" instance"<<endl;
}
};
main (){
shelf s1;
}

but get the compilation error : ISO c++ forbids in-class initialization of non-const static member.

While the following arrangement seems to work

class shelf
{
private:
    static int count_instances ;
    book b1;
public :
shelf() {
    count_instances++;
    cout << "this is the "<<count_instances<<" instance"<<endl;
}
};
int shelf::count_instances =0;

main (){
shelf s1;
}

Can someone help me understand? Also what is the right way to count the number of instances of a class

edited Apr 11 '18 at 18:46

Justin

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asked Apr 11 '18 at 18:45

Agent007

1

What is the question? You already have the solution – NathanOliver Apr 11 '18 at 18:47
int shelf::count_instances =0; Shouldn't this access be disallowed? – Agent007 Apr 11 '18 at 18:49
_"Can someone help me understand?"_ That's too broad a question. Can you ask specifically about what you do not understand? – Drew Dormann Apr 11 '18 at 18:49
The first is the declaration that the variable will exist ... the second creates a version of it. That's just what the syntax is. – UKMonkey Apr 11 '18 at 18:50
oh, So the static private members can be accessed from outside the class? – Agent007 Apr 11 '18 at 18:51
You've got it 100% right, there's nothing else to "understand" here :-) Make sure `int shelf::count_instances =0;` definition is in a CPP file, not in the header. – Sergey Kalinichenko Apr 11 '18 at 18:51
1

@Agent007 You are not accessing private member outside the class, you are providing its definition outside class header, as required by the standard. – Sergey Kalinichenko Apr 11 '18 at 18:52
1

As for counting instances of the class, your attempt fails to count copy-initialized instances. – eerorika Apr 11 '18 at 18:52
@dasblinkenlight , That makes sense. Thanks! – Agent007 Apr 11 '18 at 18:55

Counting number of Class Instances using "private static integer variable" in c++

0 Answers0