How to define inserter or extractor for a class template

[laikon]

this question is about how to define friend functions for a class
template.
the following is an example.

template <typename T>
class Array
{
private:
T* parr;
int sz;

friend ostream& operator << (ostream& os, const Array<T>& rhs);
};

template <class T>
ostream& operator << (ostream& os, const Array<T>& rhs)
{
for (int i = 0; i < rhs.sz; ++i)
os << rhs.parr[i] << "\t";
return os;
}

these codes work well in VC++ 6.0, while linking errors are given in VC
++ 2005.

[Barry]

Re: How to define inserter or extractor for a class template

On Oct 8, 2:57 pm, laikon <lai...@gmail.c omwrote:

this question is about how to define friend functions for a class
template.
the following is an example.
>
template <typename T>
class Array
{
private:
        T* parr;
        int sz;
>
        friend ostream& operator << (ostream& os, const Array<T>&rhs);

friend ostream& operator<< <T(ostream& os, const Array<T>& rhs);

>
};
>
template <class T>
ostream& operator << (ostream& os, const Array<T>& rhs)
{
        for (int i = 0; i < rhs.sz; ++i)
                os << rhs.parr[i] << "\t";
        return os;
>
}
>
these codes work well in VC++ 6.0, while linking errors are given in VC
++ 2005.

--
Best Regards
Barry

[Barry]

Re: How to define inserter or extractor for a class template

On Oct 8, 3:19 pm, Barry <dhb2...@gmail. comwrote:

On Oct 8, 2:57 pm, laikon <lai...@gmail.c omwrote:
>

this question is about how to define friend functions for a class
template.
the following is an example.

>

template <typename T>
class Array
{
private:
        T* parr;
        int sz;

>

        friend ostream& operator << (ostream& os, const Array<T>& rhs);

>
friend ostream& operator<< <T(ostream& os, const Array<T>& rhs);
>

Note that this is only a work around for VC2005.
According to 10.5.3
We should forward declaring "template operator<<", which requires
forward
declaration of "template class Array".
So the code looks like this:

template <typename T>
class Array;

template <typename T>
ostream& operator<< (ostream&, Array<Tconst&);

template <typename T>
class Array {
...

friend
ostream& operator<< <T(ostream&, Array const&); // Array<Tis
also fine
};

template <typename T>
ostream& operator<< (ostream& ostrm, Array<Tconst& a) {
...
}

>
>

};

>

template <class T>
ostream& operator << (ostream& os, const Array<T>& rhs)
{
        for (int i = 0; i < rhs.sz; ++i)
                os << rhs.parr[i] << "\t";
        return os;

>

}

>

these codes work well in VC++ 6.0, while linking errors are given in VC
++ 2005.

>

--
Best Regards
Barry

[James Kanze]

Re: How to define inserter or extractor for a class template

On Oct 8, 8:57 am, laikon <lai...@gmail.c omwrote:

this question is about how to define friend functions for a
class template.

It's difficult:-).

the following is an example.

template <typename T>
class Array
{
private:
T* parr;
int sz;

friend ostream& operator << (ostream& os, const Array<T>& rhs);

Note that this declares a non-template operator<< as a friend.
That's probably not what you want.

};

template <class T>
ostream& operator << (ostream& os, const Array<T>& rhs)
{
for (int i = 0; i < rhs.sz; ++i)
os << rhs.parr[i] << "\t";
return os;
}

these codes work well in VC++ 6.0, while linking errors are
given in VC ++ 2005.

If you have a declaration of the operator<<, as a template,
before you define the class, I think it should work. In
practice, I've found it more convenient to only use inline
friends, so the problem doesn't come up. Something like:

template< typename T >
class Array
{
// ...
public:
void print( std::ostream& dest ) const ;
friend std::ostream&
operator<<( std::ostream& dest,
Array< T const& array )
{
array.print( dest ) ;
return *this ;
}
} ;

The friend may be a non-template function, but it doesn't
matter, since I don't have to declare it anywhere else.

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
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