Overloading operator<< to output private class-data

Hi,

to output the private data of a class I want to overload the operator<<. The
Output should be written in a string-variable.

To do this i have written the attached code:

But the program-compilation aborts with the following error message:

StreamKoord.cpp: In function `int main()':
StreamKoord.cpp:44: error: no match for 'operator<<' in 'std::operator<<
[with _Traits = std::char_traits<char>]((&oss), "String: Scheitel=") <<
scheitel'
/usr/include/g++/bits/ostream.tcc:63: error: candidates are:
***std::basic_ostream<_CharT,*_Traits>&*std::basic _ostream<_CharT,
***_Traits>::operator<<(std::basic_ostream<_CharT,
***_Traits>&(*)(std::basic_ostream<_CharT,*_Traits >&))*[with*_CharT*=*char,
***_Traits*=*std::char_traits<char>]
*...

A lot of further candidates are listed. But my additional method wasn't
listed.

What is wrong in my code and how can i solve this problem?

Many thanks in advance,
Andreas

PS: gcc: 3.3.4 / linux 2.6.8-24.11-smp

re: Overloading operator<< to output private class-data

Andreas skrev:[color=blue]
> Hi,
>
> to output the private data of a class I want to overload the operator<<. The
> Output should be written in a string-variable.
>[/color]
<snip>[color=blue]
>
> ostringstream & operator<<(ostringstream & os, punkt & v)[/color]

std::ostream& operator<<(std::ostream& os, punkt& v)
[color=blue]
> {
> os << "(";
> os.width(4);
> os << v.p.x << ",";
> os.width(4);
> os << v.p.y << ",";
> os.width(4);
> os << v.p.z << ")";
> return os;
> };[/color]

<snip>


--
TB @ SWEDEN

re: Overloading operator<< to output private class-data

Hi TB,

many thanks for your help!

Whats the rule behind this? I thought that it is necessary to use the same
Output-type for operator<< as defined for oss (like my example).

Sincerely,
Andreas

TB wrote:
[color=blue]
> Andreas skrev:[color=green]
>> Hi,
>>
>> to output the private data of a class I want to overload the operator<<.
>> The Output should be written in a string-variable.
>>[/color]
> <snip>[color=green]
>>
>> ostringstream & operator<<(ostringstream & os, punkt & v)[/color]
>
> std::ostream& operator<<(std::ostream& os, punkt& v)
>[color=green]
>> {
>> os << "(";
>> os.width(4);
>> os << v.p.x << ",";
>> os.width(4);
>> os << v.p.y << ",";
>> os.width(4);
>> os << v.p.z << ")";
>> return os;
>> };[/color]
>
> <snip>
>
>[/color]

re: Overloading operator<< to output private class-data

the best way is .. .

friend std::ostream& operator <<(std::ostream& , const <TYPE>);

don't miss the *friend* keyword

re: Overloading operator<< to output private class-data

Andreas skrev:[color=blue]
> Hi TB,
>
> many thanks for your help!
>
> Whats the rule behind this? I thought that it is necessary to use the same
> Output-type for operator<< as defined for oss (like my example).
>[/color]

std::ostringstream::operator<<() returns std::ostream&.
(std::ostream is a common base class for all output stream classes)

Your original declaration was:
ostringstream & operator<<(ostringstream & os, punkt & v);

And it will work with this code:
std::ostringstream os;
punkt p;
os << p;

But not here:
os << p << p;

If you ask why, reread this reply from the top and look at the error
message your compiler produces.
[color=blue]
> TB wrote:
>[color=green]
>> Andreas skrev:[color=darkred]
>>> Hi,
>>>
>>> to output the private data of a class I want to overload the operator<<.
>>> The Output should be written in a string-variable.
>>>[/color]
>> <snip>[color=darkred]
>>> ostringstream & operator<<(ostringstream & os, punkt & v)[/color]
>> std::ostream& operator<<(std::ostream& os, punkt& v)
>>[color=darkred]
>>> {
>>> os << "(";
>>> os.width(4);
>>> os << v.p.x << ",";
>>> os.width(4);
>>> os << v.p.y << ",";
>>> os.width(4);
>>> os << v.p.z << ")";
>>> return os;
>>> };[/color]
>> <snip>
>>
>>[/color]
>
>[/color]

--
TB @ SWEDEN

re: Overloading operator<< to output private class-data

TB skrev:[color=blue]
> Andreas skrev:[color=green]
>> Hi TB,
>>
>> many thanks for your help!
>>
>> Whats the rule behind this? I thought that it is necessary to use the
>> same
>> Output-type for operator<< as defined for oss (like my example).[/color]
>
> std::ostringstream::operator<<() returns std::ostream&.
> (std::ostream is a common base class for all output stream classes)
>
> Your original declaration was:
> ostringstream & operator<<(ostringstream & os, punkt & v);
>
> And it will work with this code:
> std::ostringstream os;
> punkt p;
> os << p;
>
> But not here:
> os << p << p;[/color]

I meant:

os << "text" << p;

--
TB @ SWEDEN

re: Overloading operator<< to output private class-data

Hi,

thanks!

Another small step for the mankind, but a big step for me.

I believe now its clear why it is necassary to use the baseclass to overload
operators!

Sincererly,
Andreas

TB wrote:
[color=blue]
> TB skrev:[color=green]
>> Andreas skrev:[color=darkred]
>>> Hi TB,
>>>
>>> many thanks for your help!
>>>
>>> Whats the rule behind this? I thought that it is necessary to use the
>>> same
>>> Output-type for operator<< as defined for oss (like my example).[/color]
>>
>> std::ostringstream::operator<<() returns std::ostream&.
>> (std::ostream is a common base class for all output stream classes)
>>
>> Your original declaration was:
>> ostringstream & operator<<(ostringstream & os, punkt & v);
>>
>> And it will work with this code:
>> std::ostringstream os;
>> punkt p;
>> os << p;
>>
>> But not here:
>> os << p << p;[/color]
>
> I meant:
>
> os << "text" << p;
>[/color]

--
Viele Grüße,
Andreas