Overloading the << operator

I need to overload the << operator as a virtual function. I've only ever done them as friend functions and friend functions can't be virtual functions so I'm running into problems.

In the base class I'm using it as a pure virtual function:

Code:

//header

virtual void operator<<(ostream &, const Holding &) = 0;

and in the derived class I'm using it like this

Code:

//header

virtual void operator<<(ostream &, const Book &);

Code:

//.cpp

void Book::operator<<(ostream &out, const Book &book){
        
        book.print(out);
        
        }

I'm getting these errors from it:

`void Book::operator<<(std::ostream&, const Book&)' must take exactly one argument
and
In member function `virtual void Book::operator<<(std::ostream&, const Book&)':
and
`const Book' as `this' argument of `virtual void Book::print(std::ostream&)' discards qualifiers
and
cannot allocate an object of type `Book'
because the following virtual functions are abstract:
virtual void Holding::operator<<(std::ostream&, const Holding&)

What do you think might be the problem?

I was able to figure it all out

Feel free to post the solution you found. As I'm sure you noticed, operator<< cannot be a member function, and so it cannot be virtual. Here is more reading for those interested:

http://www.parashift.com/c++-faq-lit...html#faq-15.11

Well I got it to compile and then I checked it different ways and it's not working like it's supposed to. It's not even actually calling the function when I try to output it. I'll read that article and see if I can figure it out. Thanks.

I'm still getting this error

Code:

`const Holding' as `this' argument of `virtual void Holding::print(std::ostream&)' discards qualifiers

I have it set up pretty much the same way as that link you showed. My print function in the Holding class is a pure virtual function. I'm not sure if that is screwing this up or not. Also what does it mean to discard qualifiers?

For anyone who is interested I found an answer.

The standard library includes a templated functional called less. less is a
struct that has an operator() that takes two arguments and applies < to
them. The two arguments are declared as const, which means that it is not
permissible to call any operator on them which is not likewise const.
Calling a non-const operator "discards qualifiers", i.e., discards const.

I just had to make my print functions const functions.

This just allowed me to compile it. When I actually call the overloaded << operator it just returns the memory address the object is at. I'm trying to print out some of the data it holds .

What does your print function look like? What does the code that calls it look like?

Code from the Derived class

Code:

void Book::print(ostream & out)const{
        
        out << "BOOK: " << author << " " << '\"' << title << '\"' << " " << callNumber << endl;
        
        }  // This is a virtual function
ostream & operator<<(ostream & out, const Book & book){
     
     book.print(out);
     return out;
     
     } // This is a friend function

The code that calls it looks like this

Code:

    Book * book;
    Recording * recording;
    Holding * Holdings[5];
    .
    .
    getting variable data
    .
    .
    book = new Book(author, title, callNumber);
    Holdings[i] = book;  // Holdings is an ABC

    cout << Holdings[i] << endl;
    // cout << book << endl;   (they both have the same result)

Should I post more of my code or is that sufficient?

Wow once again I feel dumb. I fixed it. I was getting a memory reference because I wasn't passing in a pointer. Now I am and it works. Saweet!!!

Usually you would fix it the other way around. Instead of changing the function to take a pointer, you would leave the function to take a reference and pass a reference instead of a pointer. If you have a pointer and you want to pass a reference, just dereference it:

Code:

cout << *(Holdings[i]) << endl;
cout << *book << endl;

That is what I did. I guess I didn't make it clear. I didn't change the function. The function always received a constant reference but I wasn't passing in a pointer. Now I am. Thanks for all the help!