重载我自己的课程的输出流

Question

I have a base class Animal that class Cat and Dog derive from.

I'm trying to overload the output stream so I can do something like this:

Animal *a = (animal *) 0;
while(read_animal(cin, a) && a)       // a has been assigned a derived class
    cout << *a << '\n';

And here's how I've overloaded it:

 ostream& operator<<(ostream &a, const animal* animal) {
     return a << animal->describe() << endl;
}

But I get an error:

Error 20 error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'animal' (or there is no acceptable conversion)

Ideas?

Answer 1

Your overload expects const animal* but what you are supplying is *a ie animal , not animal* . So you may try:

cout << a << '\n';

Answer 2

Try

cout << a << '\n';

Since your overload expects a const animal* animal

Alternatively change the signature of your function to:

 ostream& operator<<(ostream &a, const Animal& animal) {
     return a << animal.describe() << endl;
}