Base class constructor automatic call

I have troubles understanding the output of the following code when it comes to the first printed number.

#include <iostream>
#include <string>

using namespace std;

class A{
public:
    int x;
    A(){
        x = 5;
    }
};
class B: public A{
public:
    static int x;
    B(){
        x++;
    }
    B(int i){
        x = x+i;
    }
    B(string s){
        x--;
    }
};

int B::x=10;

int main()
{
B b1;
B b2(2008);
B b3("Random string");
cout << b1.x << " : " << b2.x << " : " << b3.x << endl;
return 0;
}

output (the first "2018" is the one I’m having troubles with)

2018 : 2018 : 2018

>Solution :

The first thing to note is that A::x is different from B::x. B does inherit A::x, but it introduces a new B::x which is static. Hence x in the scope of Brefers to B::x (not to A::x). Ergo, you can remove the base class without changing the output:

#include <iostream>
#include <string>

using namespace std;

class B {
public:
    static int x;
    B(){
        x++;
    }
    B(int i){
        x = x+i;
    }
    B(string s){
        x--;
    }
};

int B::x=10;

int main()
{
    B b1;
    B b2(2008);
    B b3("Random string");
    cout << b1.x << " : " << b2.x << " : " << b3.x << endl;
}

same output::

2018 : 2018 : 2018

Now, B::x starts out as 10 because thats what you initialize it to. Then B b1; increments it (in the default constructor), then B b2(2008); adds 2008 to B::x, it is 2019 now. Eventually B b3("Random string"); decrements it to arrive at the value 2018. Because B::x is static you see 3 times the same output.

The confusion seems to be caused by B::x being a static member and A::x being hidden by B::x. To reiterate the above: static int x; declares x to be a static member. There is only 1 for all instances. When one instance increments it then any other instance will see the incremented value as well. B::x hides A::x because they have the same name. I suppose this is not intentional. If your intention was to let B inherit x from B and use that, then you should remvoe the delcaration and definition of B::x.

It’s not that I don’t understand a static variable, it’s more like me not getting why b1 that (seemingly) call b(),outputs 2018 instead of incrementing the static x to 11

The line B b1; does increment A::x from 10 to 11. Though the constructor does not produce any output. The values are only printed after all constructors finished.