Compiler error when using nested operator overloading in C++

Question

I have a URL class that overloads the ==, <, >, and != operators for simple comparison. The URL class has a string data member and some functions to act on the string. The operators work fine when tested with the URL class.

I also have a Page class that has a URL data member. I am trying to overload the same operators in the Page class. Equality in the Page class is based on equality of their respective URLs, so I use the URL class boolean operators in comparing pages. This creates some compiler errors that I cannot figure out. Code for URL operators:

bool URL::operator ==(URL & u) const {
    //url is the string instance variable
    return url == u.GetURL();
}

Code for Page operators:

bool Page::operator ==(Page & p) const {
    //url is the URL instance variable of the Page class
    return url == p.GetURL();
}

This produces errors like so:

src/Page.cpp: In member function ‘bool Page::operator==(Page&) const’:
src/Page.cpp:21: error: no match for ‘operator==’ in ‘((const Page*)this)->Page::url == Page::GetURL()()’
inc/URL.h:118: note: candidates are: bool URL::operator==(URL&) const

I predict that it is something dumb that I am forgetting. Will you prove me right?

edit: Const correctness has bitten me in the bum. Thanks for the help.

Answer 1

It should have been:

bool URL::operator ==(const URL & u) const {
    //url is the string instance variable
    return url == u.GetURL();
}

And analogously for the other operators.

If you still get compiler errors, perhaps you haven't made GetURL() const as well:

std:string URL::GetURL() const {
    // whatever...
}

Answer 2

I would also like to point out that methods (ie the public interface) are there to protect external entities from changes in the implementation details. Also that a class is automatically a friend of itself (for the same reason) and thus just accessing the members of the other object is OK.

bool URL::operator ==(URL & u) const {
    //url is the string instance variable
    return url == u.GetURL();
}

Can be written like:

bool URL::operator ==(URL & rhs) const
{
    return url == rhs.url;  // No need to use GetURL()
}

In my mind this makes the code clearer (but this again is an opinion your tastes may vary)