Re: [Boost-users] Question regarding template

From: YYW (yyw1_25_at_[hidden])
Date: 2006-10-17 02:44:09

template <class T>
class CMatrix{
    matrix<T>mat_;
 public:
    CMatrix(const identity_matrix<T>& m) { mat_ = m; }
    static CMatrix<T> eye(int size)   { return identity_matrix<T>(size); }
};

template<class T>
void operator << (ostream& os, CMatrix<T>& m)//////////// line A
{}

int main(){
    CMatrix<double> I = CMatrix<double>::eye(5);
    cout<<I;
    cout<<CMatrix<double>::eye(5);//////////////// line B
}
-------------------------------------------
In the codes above, "line B" will causes an error in g++ 3.4.2: no match for 
'operator<<' in 'std::cout << CMatrix<T>::eye(int) [with T = double]()'

Arkadiy Vertleyb advised me to insert "const" in "line A". So, the new 
version of "line A" is:
void operator << (ostream& os, const CMatrix<T>& m)

OK,  there's no error.
But, why?

"Arkadiy Vertleyb" <vertleyb_at_[hidden]> 
??????:eh0dec$ih5$1_at_sea.gmane.org...
> "YYW" <yyw1_25_at_[hidden]> wrote
>
>> why the codes below is wrong in g++ 3.4.2?
>> an error occurs in the line: cout<<CMatrix<double>::eye(5);
>> --------------------------------------
>> #include <ostream>
>> #include <boost/numeric/ublas/matrix.hpp>
>> #include <boost/numeric/ublas/io.hpp>
>>
>> using namespace boost::numeric::ublas;
>> using namespace std;
>>
>> template <class T>
>> class CMatrix
>> {
>>    matrix<T>mat_;
>>
>> public:
>>    CMatrix(const identity_matrix<T>& m) { mat_ = m; }
>>    static CMatrix<T> eye(int size) { return identity_matrix<T>(size); }
>>
>>    template <class T_>
>>        friend ostream& operator << (ostream& os, CMatrix<T_>& m);
>> };
>>
>> template<class T_>
>> ostream& operator << (ostream& os, CMatrix<T_>& m)
>
> Try this:
>
> ostream& operator << (ostream& os, const CMatrix<T_>& m)
>
> Regards,
> Arkadiy