C++: String parameter in base class deconstructs when the derived class deconstruction - c++

Question

I have a base class named A containing a string type parameter.
Class B is derived from A.
I define class C have parameter A* a, and
assign it to B.
In the main function, I cannot get the string value of the base class as it became blank when b deconstructs.
I want it to output:
"Hello!"
"Hello!"
end
But the output is:
"Hello!"
end
Here is my code:
class A {
public:
string str;
};
class B : public A {
public:
B(string _str) {
str = _str;
}
};
class C {
public:
A *a;
public:
void printOut() {
B b("Hello!");
a = &b;
cout << a->str << endl;
}
};
int main() {
C c;
c.printOut();
cout << c.a->str << endl;
cout << "end" << endl;
return 0;
}
How can I deal with it?

Answer 1

Correct, because B b("Hello!"); goes out of scope, c.a is now a dangling pointer that will cause undefined behaviour upon being dereferenced. If you want it to outlive the scope you could allocate it on the heap instead :
class A {
public:
string str;
};
class B : public A {
public:
B(string _str) {
str = _str;
}
};
class C {
public:
A *a;
public:
void printOut() {
B* b = new B("Hello!");
a = b;
cout << a->str << endl;
}
};
int main() {
C c;
c.printOut();
cout << c.a->str << endl;
cout << "end" << endl;
delete c.a;
return 0;
}
This gets messsy real fast though because you have to track the memory allocated yourself and call delete appropriately, consider redesigning or using smart pointers.

Answer 2

Why are you storing an A* ? You know that doesn't work, so stop doing it.
Make a copy of the A object, or a copy of the string it contains, and stop trying to do something silly.
class A {
public:
string str;
};
class B : public A {
public:
B(string _str) {
str = _str;
}
};
class C {
public:
string str;
public:
void printOut() {
B b("Hello!");
str = b.str;
cout << str << endl;
}
};
int main() {
C c;
c.printOut();
cout << c.str << endl;
cout << "end" << endl;
return 0;
}