Can someone explain to me why is this the output of this code in C++?

Question

Can someone explain to me why is this the output of this code in C++? The question that is asked is what is the output that is written out by the destructors of these objects (a,b,c,d,e,f)? This is part of a course test, and the good answer is supposed to be: for a: 3:0. for b: 3:1. for c: 2:2. for d: 2:1. for e: 3:1. for f: 3:2.

Can someone explain how this question can be solved without compiling the code? Thank you

class A {

public :
static int o;
int oo;

A ( ) :oo(o++) {};
A (const A& po ) : oo(po .oo + 1) {};
~A () {cout << o << ' : ' << oo << '\n' ;}
};
int A ::o = 0;

int main ( ) {
A a
A b(a);
{
A c (b );
A d;
}
A e(a);
A f;
return 0;
}

Answer 1

ugh, this is some ugly code. Gonna have to do it step by step, so bear with me.
Obviously start at the tildes~ for your destructors:
class A's output in printf would look like ("%d:%d\n", o, oo)

so, o is initialized to 0, when A is defined, and is static (ie shared)

Constructing an A object with no arguments sets oo = o++, which is to say oo=o followed by o+=1.
Constructing an A object with another A, referred to as po , as its argument sets oo = po .oo +1

so A a: o = 1, a.oo = 0
A b(a): o = 1, b.oo = 1
scope begins:
A c(b): o = 1, c.oo = 2
A d: o = 2, d.oo = 1
scope ends, c and d are deconstructed. output o:c.oo and o:d.oo, which are 2:2 and 2:1 respectively
A e(a): o = 2, e.oo = 1
A f: o = 3, f.oo = 2
main ends; output o:a.oo, o:b.oo, o:e.oo, and o:f.oo, which are 3:0, 3:1, 3:1, and 3:2 respectively.

This matches the correct answer so I probably didn't screw anything up. phew 😅

Answer 2

those ooooooo's😅.
also, the code would not compile because you have some errors.
' : ' should be ':' or " : ", and missing semicolon on A a
But yes, it's a very good explanation by Orin Cook.
I can't help but run it to see if that is indeed the case.
Let the code explain itself.

https://code.sololearn.com/ckdmaaWcm5OR/?ref=app

Answer 3

the complexity of the code makes it hard to visualise everything in mind. try putting it in some debugger and step over each line to better understand it.

Answer 4

Thank you so much!