study case in C++. Abstract class and instance [closed] - c++

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I have to understand why this program in C++ isn't working. There were many other errors but I corrected them.
#include <iostream>
using namespace std;
class X {
public :
X(int a) : a(a) {}
virtual void f(ostream& flot) const { flot << " X::f" << endl; }
virtual void g() const = 0;
private :
int a;
};
class Z : public X {
void h() const {cout << "Z::h" << endl;}
public :
Z() : X(0) {}
void f(ostream& flot) const override { flot << "Z::f" << endl;}
void g() const override {cout << "Z::g" << endl;}
};
void operator<< (ostream& flot, const Z& z ) { z.f(flot);}
int main() {
Z z;
cout << z << endl;
Z z2();
z2.h();
X* x1;
x1->f(cout);
z.h();
}
You can't write cout << z << endl; because z does not exist as a normal object, is it correct? it is a kind of imaginary object ? at the contrary of z2 which is constructed and does exists. Can you tell me if I am right?
And I don't understand why the two last lines:
x1->f(cout);
z.h();
are not problematic. I thought that x1 does not exists (as z). So why can you call a method for x1?
And for z.h(), the only problem is that h is, here, in this context, private. But if it would have been inside the public part, it wouldn't be a problem. Why ??? Isn't z imaginary so you can't use any method on it?

Answer 1

Z z;
This defines and constructs an object z of type Z. The default constructor is called.
cout << z << endl;
This calls operator<< for (cout, z), and then uses the return value of that to call operator<< for it and endl. The return value is void, so the second call cannot work. Your operator<< needs to return the output stream for this to work.
Z z2();
This defines a function z2 that takes no arguments and returns a Z. Not a variable, but not actually wrong itself.
z2.h();
At this point it's wrong, because z2 is a function and not an object. Try removing the parentheses, or changing the previous line to Z z2 = Z();.
X* x1;
This defines a variable x1 of type X*. It's uninitialized.
x1->f(cout);
This calls the function X::f on your uninitialized pointer, which is undefined behaviour. Read undefined behaviour as "could do anything, avoid".
z.h();
This tries to call the function h on z, but it's a private function. You'll get a compile error because it's private and not accessible from main context.

Answer 2

To get cout to accept a Z object after the operator <<, overload the operator << to recognize an ostream object on the left and a Z on the right. The overloaded << operator function must then be declared as a friend of class Z.
The first error is solved by adding the following declaration to Z class:
friend ostream& operator<<(ostream& os, const Z& z);
On the other hand, if you want to invoke the method h on the instance z2 you have to make h public method, in your code it is private by default.
As a final note, you should receive a segmentation fault at the sentence:
x1->f(cout);
because the pointer x1 point to a non-allocated object.

Answer 3

There is no "imaginary" object in C++. Either an object is created or it is not. There is nothing wring with Z z;. Since the object exists, there is no problem in using the function Z::h() once you make it public.
Z z2(); doesn't create an object as you intend, since it is parsed as a function taking no argument and returng an object of Z. This is, as pointed out in the first comment, the case of most vexing parse.
Regarding the cout, the operator << should be able to print stuff from your class. Hence, you need to make the operator a friend of your class.
Also, since x1 is declared but not initialized, you will have undefined behavior for x1->f();. Also, you can't even do something like X* x1 = new X(0); because X::g() is pure virtual, and the class X is an abstract base class.
The following code would run wothout issues:
#include <iostream>
using namespace std;
class X {
public :
X(int a) : a(a) {}
virtual ~X() {}
virtual void f(ostream& flot) const { flot << " X::f" << endl; }
virtual void g() const {};
private :
int a;
};
class Z : public X {
public :
Z() : X(0) {}
void f(ostream& flot) const override { flot << "Z::f" << endl;}
void g() const override {cout << "Z::g" << endl;}
void h() const {cout << "Z::h" << endl;}
friend ostream &operator<<(ostream &os, const Z &z);
};
ostream& operator<< (ostream& flot, const Z& z ) { z.f(flot); return flot;}
int main() {
Z z;
cout << z << endl;
//Z z2();
//z2.h();
X * x1 = new X(0);
x1->f(cout);
z.h();
delete x1;
return 0;
}