operator<<

[mickey]

Hello,
could anyone at first sight tell me about why this below doens't work with keyword "const" (that I guess the right way) and it works without it? thanks

friend std::ostream& operator<<(std::ostream& os, const Apple& a) { // <- "const"
		return os << "kind = " << a.getKind().c_str() << "\t" 
                  << "weight = " << a.getWeight() << "\t" << "status = " << a.getStatus().c_str() << endl;
	}

friend std::ostream& operator<<(std::ostream& os, const Apple& a) { // <- "const"
		return os << "kind = " << a.getKind().c_str() << "\t" 
                  << "weight = " << a.getWeight() << "\t" << "status = " << a.getStatus().c_str() << endl;
	}

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[jacek]

why this below doens't work with keyword "const" (that I guess the right way) and it works without it?

Most likely because at least one of the Apple::getXxx() methods is not const.

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

[mickey]

It was that! Furthermore always at first sight: I have to do this:

class Fruit {
   virtual friend ostream& operator<<(ostream& os, const Fruit& f) { }
};
class Apple : public Fruit {
     friend ostream& operator<<(ostream& os, const Apple& a) { }   
};
//main.cpp
Fruit* f = new Apple;
cout << *f;

class Fruit {
   virtual friend ostream& operator<<(ostream& os, const Fruit& f) { }
};
class Apple : public Fruit {
     friend ostream& operator<<(ostream& os, const Apple& a) { }   
};
//main.cpp
Fruit* f = new Apple;
cout << *f;

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This above doens't work.......what I like obtain is make virtual the operato<< (I guess you can guess it from the piece of code).....

thanks....

[jacek]

You can't define ostream & ostream::operator<< ( const Fruit & a ) in Fruit class, because it belongs to ostream class, and you can't change the definition of ostream class, so the only place you can put that operator<< is outside any class:

class Fruit 
{
   ...
   virtual void storeOrWhatever( ostream & os ) const = 0;
};
 
class Apple : public Fruit 
{
   ...
   void storeOrWhatever( ostream & os ) const { ... }
};
 
ostream& operator<<( ostream& os, const Fruit & a )
{
   a.storeOrWhatever( os );
   return os;
}

class Fruit 
{
   ...
   virtual void storeOrWhatever( ostream & os ) const = 0;
};

class Apple : public Fruit 
{
   ...
   void storeOrWhatever( ostream & os ) const { ... }
};

ostream& operator<<( ostream& os, const Fruit & a )
{
   a.storeOrWhatever( os );
   return os;
}

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This way you can easily call the base class implementation and you don't need any friends.