converting from base class to derived class

[Charles Jamieson]

I have two classes, a base class, CBaseClass, and its derived class,
CDerivedClass.

I overload the insertion operator as

ostream& operator << ( ostream&, CBaseClass& );

Then I define an object as

CDerivedClass object;

I then have the line

std::cout << object;

When I compile and link this code (using gcc 3.3.3), I get the following
error message

undefined reference to `operator<<(std::basic_ostream<char,
std::char_traits<char> >&, CDerivedClass&)'

How can I get the compiler to use the overloaded insertion operator as I
defined it?

-charles

 

[David Hilsee]

I have two classes, a base class, CBaseClass, and its derived class,
CDerivedClass.

I overload the insertion operator as

ostream& operator << ( ostream&, CBaseClass& );

Then I define an object as

CDerivedClass object;

I then have the line

std::cout << object;

When I compile and link this code (using gcc 3.3.3), I get the following
error message

undefined reference to `operator<<(std::basic_ostream<char,
std::char_traits<char> >&, CDerivedClass&)'

How can I get the compiler to use the overloaded insertion operator as I
defined it?

The code looks fine, aside from the nitpick that operator<< might take a
const CBaseClass& instead of a CBaseClass&. The error message makes me
think that there's something you didn't post that's causing the problem,
like a declaration of operator<<(std::basic_ostream<char,
std::char_traits<char> >&, CDerivedClass&) that lacks a definition. It
might be helpful if you post a complete example that causes g++ to emit the
same error message.

 

[Aguilar, James]

I have two classes, a base class, CBaseClass, and its derived class,
CDerivedClass.

I overload the insertion operator as

ostream& operator << ( ostream&, CBaseClass& );

Shouldn't this be virtual if you want it to work for subclasses?

 

[Charles Jamieson]

The code looks fine, aside from the nitpick that operator<< might take a
const CBaseClass& instead of a CBaseClass&. The error message makes me
think that there's something you didn't post that's causing the problem,
like a declaration of operator<<(std::basic_ostream<char,
std::char_traits<char> >&, CDerivedClass&) that lacks a definition. It
might be helpful if you post a complete example that causes g++ to emit the
same error message.

David,

You were right, I did have another declaration for the derived class
without a definition. Thanks.

-charles

 

[David Hilsee]

Shouldn't this be virtual if you want it to work for subclasses?

It's not a member function, so it can't be made virtual. However, it could
delegate to a virtual member function (e.g. "virtual ostream&
Print(std:stream&) const;") in CBaseClass if Charles wanted to invoke
functionality provided by subclasses.

 

[Aguilar, James]

It's not a member function, so it can't be made virtual. However, it could
delegate to a virtual member function (e.g. "virtual ostream&
Print(std:stream&) const;") in CBaseClass if Charles wanted to invoke
functionality provided by subclasses.

Oooh, yeah. I'm a noob, please forgive. I forgot that if it's gonna be a
member function, the object of such and so a type has to be on the left
side.