Result of the code step by step in C++

Question

Hello, can someone please help me through this code step by step, how does it come to the solution (which is at the end of the code at the bettem of the page)? Thank you

https://ibb.co/ZMDqGDc

class C1 {
static int sa;
int a;
public:
C1(int _a) : a(sa) { sa=_a;}
~C1() { ++sa; cout << a;}
};
class C2 {
static int sb;
int b;
public:
C2() : b(++sb) {}
~C2() {cout << b;}
};

int C1::sa=1;
int C2::sb=0;

{
C1 a1(2);
C2 b1;
C1 a2(1);
} //1
cout << endl;
{
C2 b1;
C1 a1(7);
c2 b2;
} // 2
cout << endl;

{
C2 b1, b2;
C1 a1(4);
}//3
cout << endl;

Results are:
//1: 2,1,1
//2: 3,3,2
//3: 8,5,4

Answer 1

can you copy paste the code here?

Answer 2

Yes, of course.

class C1 {
static int sa;
int a;
public:
C1(int _a) : a(sa) { sa=_a;}
~C1() { ++sa; cout << a;}
};
class C2 {
static int sb;
int b;
public:
C2() : b(++sb) {}
~C2() {cout << b;}
};

int C1::sa=1;
int C2::sb=0;

{
C1 a1(2);
C2 b1;
C1 a2(1);
} //1
cout << endl;
{
C2 b1;
C1 a1(7);
c2 b2;
} // 2
cout << endl;

{
C2 b1, b2;
C1 a1(4);
}//3
cout << endl;

Results are:
//1: 2,1,1
//2: 3,3,2
//3: 8,5,4

Answer 3

Ok, so: sa and sb are static, and start at 1 and 0 respectively
When you create any C1, it sets its own this.a = sa, and then changes the shared sa to the argument passed to it. When it is destroyed (eg by going out of scope), it increments the shared sa and prints its own a.
When you create a C2, it first increments the shared sb, and then sets b = sb. When destroyed, it just prints b. So:
Scope 1:
C1 a1(2): a1.a is 1, sa is 2
C2 b1: sb is 1, b1.b is 1
C1 a2(1): a2.a is 2, sa is 1
Scope ends, prints (in LIFO order) a2.a, b1.b, and a1.a == 211
sa is also incremented twice by C1 destructors, so sa is now 3
Scope 2:
b1: sb is 2, b1.b is 2
a1(7): a1.a is 3, sa is 7
b2: sb is 3, b2.b is 3
Scope ends, print 332, increment sa (now 8)
Scope 3:
b1: sb is 4, b1.b is 4
b2: sb is 5, b2.b is 5
a1(4): a1.a is 8, sa is 4
Scope ends, print 854
(sa is incremented but it doesn't matter)

final output:
211
332
854

Answer 4

Thank you very much.

The part which I have problem with is the following:

C1 a2(1): a2.a is 2, sa is 1

I understand why sa is 1 here, I just don't understand exactly why a2.a is 2.

Answer 5

Because of the way it's written, the C1 constructor sets a=sa before doing anything else, as part of the initialization of the instance, and then sa = _a second, when the constructor actually executes.

Answer 6

Okay, now I get the whole picture. Thank you so much!