vect.hpp:13:33: error: declaration of ‘operator<<’ as non-function

Question

I have this error

vect.hpp:13:33: error: declaration of ‘operator<<’ as non-function

for the code :

#include <iostream>

template<unsigned d>
class Vect{
    protected:
        double * coord;
    public:
        Vect() {for(int i=0, i<d, i++){*(coord+i)=0;}}
        ~Vect(){delete coord; coord=nullptr;}
        Vect(const Vect &);
        double operator=(const Vect &);
        double operator[](unsigned i) const{return *(coord+i);}
        friend std::ostream & operator<< <>(std::ostream &, const Vect<d> &); 
};

for the line :

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

Answer 1

The friend declaration refers to the instantiation of the operator<< but there's no primary template declaration. You need to declare the operator template in advance. e.g.

// forward declaration
template<unsigned d>
class Vect;

// primary template declaration of operator<<
template<unsigned d>
std::ostream & operator<< (std::ostream &, const Vect<d> &); 

template<unsigned d>
class Vect{
    protected:
        double * coord;
    public:
        Vect() {for(int i=0; i<d; i++){*(coord+i)=0;}}
        ~Vect(){delete coord; coord=nullptr;}
        Vect(const Vect &);
        double operator=(const Vect &);
        double operator[](unsigned i) const{return *(coord+i);}
        friend std::ostream & operator<< <>(std::ostream &, const Vect<d> &); 
};

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