Virtual << operator?

[Rob McDonald]

I would like to force all the classes in my hierarchy to implement the
<< operator for testing purposes. My base class is a pure virtual
class.

I started out by working with operator overloading in the derived
class. I have been trying to use what I learned there to create an
appropriate virtual class to force overloading.


class Base{

// Can't define operator with two arguments inside Base class
virtual std:stream& operator<<(std:stream& s, Base& b) = 0;

// When I did this for the concrete derived class, it failed because
the
// compiler doesn't seem to 'find' the implementation of << for Base
virtual std:stream& operator<<(std:stream& s) = 0;
}

// For the concrete derived class, I got this two-argument approach to
work.
// However, you can't declare virtual functions outside a class.
virtual std:stream& operator<<(std:stream& s, Base& b) = 0;

Any suggestions are appreciated.

Rob

 

[Frank Birbacher]

Hi!

Why not use an oridnary virtual function instead of an operator for
printing?

Any suggestions are appreciated.

struct Printable
{
virtual void print(std:stream&) const =0;
};

static inline std:stream& operator << (
std:stream& stream, Printable const& p
)
{
if(stream)
p.print(stream);
return stream;
}

Regards,
Frank

 

[Rob McDonald]

Hi!

Why not use an oridnary virtual function instead of an operator for
printing?

Probably because that is far too simple and logical.

Worked great.

Thanks,

Rob

 

[AnonMail2005]

Probably because that is far too simple and logical.

Worked great.

Thanks,

Rob

BTW, this exact scenario is covered in The C++ Programming Language.

 

[Rob McDonald]

BTW, this exact scenario is covered in The C++ Programming Language.

I didn't see it there when I was trying to figure this out. Is it in
the operator overloading section, or the streams section, or someplace
else.

Thanks,

Rob

 

[David Côme]

I would like to force all the classes in my hierarchy to implement the
<< operator for testing purposes. My base class is a pure virtual
class.

I started out by working with operator overloading in the derived
class. I have been trying to use what I learned there to create an
appropriate virtual class to force overloading.


class Base{

// Can't define operator with two arguments inside Base class
virtual std:stream& operator<<(std:stream& s, Base& b) = 0;

// When I did this for the concrete derived class, it failed because
the
// compiler doesn't seem to 'find' the implementation of << for Base
virtual std:stream& operator<<(std:stream& s) = 0;
}

// For the concrete derived class, I got this two-argument approach to
work.
// However, you can't declare virtual functions outside a class.
virtual std:stream& operator<<(std:stream& s, Base& b) = 0;

Any suggestions are appreciated.

Rob

To be virtual a function must be membre of a classe. Or any function
member receive
implicitly this* for first argument. To operator<<, the fisrt argument
must be an stream.
So you can't have operator<< like a function member, specially virtual.

 

[AnonMail2005]

I didn't see it there when I was trying to figure this out.  Is it in
the operator overloading section, or the streams section, or someplace
else.

In section 21.2.3.1 Virtual Output Functions of C++PL Special Edition.
It has a non-member operator<< function which takes as the second
argument
a base class, but the function just calls a virtual function to do
it's
work.

HTH