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paidojoao-groups@yahoo.co
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 Posted: Tue Jun 29, 2004 12:13 pm Post subject: Template Friend |
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Given this code:
#include <iostream>
#include <string>
class timestamp
{
public:
timestamp() {
std::cout << "--> default constructor" << std::endl;
}
template
friend
std::basic_ostream<charT, traits>&
operator<< (std::basic_ostream
private:
// implementation not relevant
};
template<class charT, class traits>
std::basic_ostream<charT, traits>&
operator<<(std::basic_ostream
{
using namespace std;
strm << "--> operator<<()";
return strm;
}
int main()
{
timestamp p;
std::cout << p << std::endl; // line 36
return 0;
}
-------------------------------------------
VC 7.1 (13.10.3077) complains with
bug.cpp(36) : error C2563: mismatch in formal parameter list
bug.cpp(36) : error C2568: '<<' : unable to resolve function overload
Borland C++ 5.6.4 and G++ 3.2 compile without errors.
Who is wrong and who is right?
Changing to
friend
std::ostream&
operator<< (std::ostream& strm, const timestamp& ts);
and the compiler stops complaining.
Josue Gomes
josuegomes at yahoo dot com
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Rob Williscroft
Guest
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| Posted: Tue Jun 29, 2004 12:58 pm Post subject: Re: Template Friend |
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[email]paidojoao-groups (AT) yahoo (DOT) com[/email] wrote in news:49a09e7e.0406290413.3a5c018
@posting.google.com in comp.lang.c++:
| Quote: | template<class charT, class traits
std::basic_ostream
operator<<(std::basic_ostream
const timestamp& ts)
{
using namespace std;
strm << "--> operator<<()";
return strm;
}
int main()
{
timestamp p;
std::cout << p << std::endl; // line 36
return 0;
}
-------------------------------------------
VC 7.1 (13.10.3077) complains with
bug.cpp(36) : error C2563: mismatch in formal parameter list
bug.cpp(36) : error C2568: '<<' : unable to resolve function overload
Borland C++ 5.6.4 and G++ 3.2 compile without errors.
Who is wrong and who is right?
|
VC7.1 is wrong you've found a bug.
| Quote: |
Changing to
friend
std::ostream&
operator<< (std::ostream& strm, const timestamp& ts);
and the compiler stops complaining.
|
Yes, but this isn't the same function as your template function
above, simply removing the friend declaration also works-around
the bug.
Workaround: define the friend inline in the class, if its implementation
is unsutible for inlining have it forward to a non-inline template member
instead.
#include
#include <ostream>
class timestamp
{
template < typename charT, typename traits>
friend std::basic_ostream<charT, traits>& operator<< (
std::basic_ostream
)
{
return ts.m_print( strm );
}
private:
template < typename charT, typename traits>
std::basic_ostream<charT, traits>&
m_print( std::basic_ostream<charT, traits>& ) const;
};
template < typename charT, typename traits>
std::basic_ostream<charT, traits>&
timestamp::m_print( std::basic_ostream<charT, traits>& strm) const
{
strm << "--> operator<<()";
return strm;
}
int main()
{
timestamp p;
std::cout << p << std::endl;
}
Unfortunatly this doesn't work for Borland C++ 5.6.4, so your
left with making m_print() public and just delaring/defining
operator << () at namespace scope and forwarding to m_print.
Rob.
--
http://www.victim-prime.dsl.pipex.com/
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