Hi all,
I'm having a problem and for illustration purposes I developed code
that shows what the problem is about. However, any comment on the code
which is not directly about this issue is surely welcome. I have 3
classes, A is abstract, B and C inherit from A and then I create a
vector with B's and C's and print them. Of course, I overload << for
each one of them.
So it is as follows:
#include <iostream>
#include <string>
#include <vector>
using namespace std;
class A {
public:
A(string n) : name(n) {}
virtual ~A() {}
virtual string getName() = 0;
protected:
string name;
private:
friend
ostream& operator<<(ostream& s, A& val) {
return s << "<UNPRINTABLE_A>";
}
};
class B : public A {
public:
B(string n) : A(n) {}
virtual ~B() {}
virtual string getName();
private:
friend
ostream& operator<<(ostream& s, B& val) {
return s << "Class B: " << val.getName();
}
};
string B::getName() {
string s = "My name is " + name + ".";
return s;
}
class C : public A {
public:
C(string n) : A(n) {}
virtual ~C() {}
virtual string getName();
private:
friend
ostream& operator<<(ostream& s, C& val) {
return s << "Class C : " << val.getName();
}
};
string C::getName() {
string s = name + "is my name.";
return s;
}
int main() {
vector<A*> v;
v.push_back(new B("b1"));
v.push_back(new B("b2"));
v.push_back(new C("c1"));
v.push_back(new B("b3"));
for(vector<A*>::iterator i = v.begin(); i != v.end(); i++)
cout << **i << endl;
return 0;
}
When I run the program I get:
$ ./testop
<UNPRINTABLE_A>
<UNPRINTABLE_A>
<UNPRINTABLE_A>
<UNPRINTABLE_A>
This is surely not what I expected. How can I have the << for B and C
called?
It's idiot to even have <UNPRINTABLE_A> printed. It should never be
printed. Initially I didn't even though that I needed to overload <<
for A but I do because **i is an A and I do not know if it is a B or a
C. It's logic but at the same time confusing!
Cheers,
Paulo Matos
