Order of calling constructor and destructor: BDDB vs DBBD

Question

class A {
public:
A() { cout <<"a";}
~A() { cout <<"b";}
};
class B{
public:
B() { cout <<"c";}
~B() { cout <<"d";}
};
int main() {
B b;
A a;
}

// outputs cabd

Since object b of class B is sequentially executed before object a of class A,
class B's constructor is executed first.

Next the confusion kicks in...

Why does class A's constructor get executed before class B's destructor?

Class A and class B do NOT have an inheritance relationship,
so wouldn't it make sense to free the memory for the instance of class B ASAP?

Is it a general rule to finish executing all constructors
(in the order of class instances being allocated memory)
before executing destructors?