why is destructor called earlier? - c++

Question

I wrote this code to learn the shared_ptr object creation inside my own class.
Why is A object shared_ptr destroyed even before A's print (obj->print() inside B's print) is called?
#include <iostream>
#include <memory>
using namespace std;
class A {
private: string str;
public:
A(string s){
str = s;
cout<<"class A: "<< str << endl;}
~A() { cout << "destroying A: " << str << endl;}
void print() { cout << "A print called" << endl;}
};
class B {
private: shared_ptr<A> obj;
public:
B() { cout << "default construct called" << endl;}
B(string s) {
cout << "shared_ptr creation" <<endl;
shared_ptr<A> obj = make_shared<A>(s);
cout << "shared_ptr creation done" <<endl;
}
~B() { cout<<"destroying B" << endl;}
void print() { obj->print();cout << "B print called" << endl;}
};
void func( B i) { i.print();}
int main()
{
{
B b("hello world");
func(b);
}
cout << "in main" << endl;
return 0;
}
output:
shared_ptr creation
class A: hello world
shared_ptr creation done
destroying A: hello world
A print called
B print called
destroying B
destroying B
in main

Answer 1

In B::B(string), shared_ptr<A> obj = make_shared<A>(s); is constructing a local object which gets destroyed immediately when the constructor ends; it has nothing to do with the data member obj, which points to nothing as the result.
I think you want:
B(string s) {
cout << "shared_ptr creation" <<endl;
obj = make_shared<A>(s); // assignment to the data member obj
cout << "shared_ptr creation done" <<endl;
}
Or initialize it in member initializer list.
B(string s) : obj(make_shared<A>(s)) { // initialize the data member obj
cout << "shared_ptr creation done" <<endl;
}