ostream and ofstream overloading

[Chase Bradford]

Hey all
I have a class Foo, and I'm trying to overload the << operator for both
the ostream and ofstream for it. This way I should have two seperate
formats for output, one for files and another for the screen. Right now the
two declarations are:


std:fstream& operator<<( std:fstream &fos, const Foo &theClass);
std:stream& operator<<( std:stream &fos, const Foo &theClass);


The problem is that I keep getting this error with G++

choosing 'std::basic_ostream<_CharT, _Traits>& std:perator<<
(std::basic_ostream<_CharT, _Traits>&, const std::basic_string<_CharT,
_Traits, _Alloc>&) [with _CharT = char, _Traits = std::char_traits<char>,
_Alloc = std::allocator<char>]' over 'std:fstream& operator<<(
std:fstream &fos, const Foo &theClass)'

because worst conversion for the former is better than the worst conversion
for the latter


Googling for this error didn't turn up anything productive, so I was
wondering if it is possible to even do this?


Thanks.

 

[Ivan Vecerina]

I have a class Foo, and I'm trying to overload the << operator for both
the ostream and ofstream for it. This way I should have two seperate
formats for output, one for files and another for the screen. Right now the
two declarations are:


std:fstream& operator<<( std:fstream &fos, const Foo &theClass);
std:stream& operator<<( std:stream &fos, const Foo &theClass);

I wouldn't do this, because it will not work as expected when
chaining output calls with << :
void test( std:fstream& fos , const Foo& theClass )
{
fos << theClass; // will call the 2nd operator
fos << "The class is: " << theClass; // will call the 1st one
// because the first << operator will return an std:stream
}

The problem is that I keep getting this error with G++

choosing 'std::basic_ostream<_CharT, _Traits>& std:perator<<
(std::basic_ostream<_CharT, _Traits>&, const std::basic_string<_CharT,
_Traits, _Alloc>&) [with _CharT = char, _Traits = std::char_traits<char>,
_Alloc = std::allocator<char>]' over 'std:fstream& operator<<(
std:fstream &fos, const Foo &theClass)'

[ NB: you should provide a complete sample if possible ]
Does the Foo class happen to provide an implicit conversion,
eventually to std::string ? [now this can only be a guess]

I'm afraid you are abusing overloading and implicit conversions.
They can be useful techniques, but are also very error prone.
It would be safer to:
- Use dedicated functions instead of already overloaded operators
when wanting to provide non-standard behavior.
- Implicit conversions should be used sparingly -- named member
functions shall be preferred in most cases.


I hope this helps,
Ivan

 

[Chase Bradford]

Thanks Ivan,

I was worried that might be the case so I had already implemented it with
the Push2File dedicated function. I was just hoping there was a way to do
this using the << operator with the particular streams.

I have a class Foo, and I'm trying to overload the << operator for both
the ostream and ofstream for it. This way I should have two seperate
formats for output, one for files and another for the screen. Right now the
two declarations are:


std:fstream& operator<<( std:fstream &fos, const Foo &theClass);
std:stream& operator<<( std:stream &fos, const Foo &theClass);

I wouldn't do this, because it will not work as expected when
chaining output calls with << :
void test( std:fstream& fos , const Foo& theClass )
{
fos << theClass; // will call the 2nd operator
fos << "The class is: " << theClass; // will call the 1st one
// because the first << operator will return an std:stream
}

The problem is that I keep getting this error with G++

choosing 'std::basic_ostream<_CharT, _Traits>& std:perator<<
(std::basic_ostream<_CharT, _Traits>&, const std::basic_string<_CharT,
_Traits, _Alloc>&) [with _CharT = char, _Traits =

_Alloc = std::allocator<char>]' over 'std:fstream& operator<<(
std:fstream &fos, const Foo &theClass)'

[ NB: you should provide a complete sample if possible ]
Does the Foo class happen to provide an implicit conversion,
eventually to std::string ? [now this can only be a guess]

I'm afraid you are abusing overloading and implicit conversions.
They can be useful techniques, but are also very error prone.
It would be safer to:
- Use dedicated functions instead of already overloaded operators
when wanting to provide non-standard behavior.
- Implicit conversions should be used sparingly -- named member
functions shall be preferred in most cases.


I hope this helps,
Ivan

 

[tom_usenet]

Thanks Ivan,

I was worried that might be the case so I had already implemented it with
the Push2File dedicated function. I was just hoping there was a way to do
this using the << operator with the particular streams.

You could add formatting manipulators for your class. Check out this
article:

http://www.cuj.com/documents/s=7998/cujcexp1902austern/

Tom