c++ - i have done the <<operator overloading but it's not working -

i have overloaded '<<' operator in linked class implementation, , declared .cpp file in main function, it's not working, , error occurs: undefined reference operator<& list)

this declaration of ostream function in linkedlist.h file:

  friend std::ostream& operator<< (std::ostream& os, linkedlist<t>& list); 

and implementation of ostream function:

 template <typename t>    std::ostream& operator<< (std::ostream& os, linkedlist<t> list)   {   list.current = list.start;   while(list.current != null)     {         os<< list.current->info<<" -> ";         list.current = list.current->next;     }     os<<"null"<<endl;     return os;  } 

in main function, have created list has object savingaccount class

linkedlist <savingaccount> list; 

and error occurs in main function in line:

 cout << list <<endl; 

well.. linkedlist class implementation:

#include "linkedlist.h" #include "savingaccount.h" #include <cstddef> using namespace std;  template <typename t> linkedlist<t>::linkedlist() {    start = null;    current = null; } template <typename t> linkedlist<t>::~linkedlist()   {     // add code.   }    template <typename t>    std::ostream& operator<< (std::ostream& os,const linkedlist<t>& list) {     list.current = list.start;     while(list.current != null)     {         os<< list.current->info<<" -> ";         list.current = list.current->next;     }     os<<"null"<<endl;     return os;   } 

and header file of linkedlins class:

#ifndef linkedlist_h #define linkedlist_h #include<iostream>  using namespace std;   template <typename t> struct node{     t info;     node<t> *next; };  template <typename t>  class linkedlist {     node<t> *start;     node<t> *current; public:     linkedlist();     ~linkedlist();     friend std::ostream& operator<< (std::ostream& os, const linkedlist<t>& list);    };  #endif // linkedlist_h 

i hope guys can me that, appreciated

ok, after posted code seems problem split linkedlist template declaration , implementation between header , source file. unless doing explicit instantiation in source file, cannot this: cannot separate template declaration , implementation, have in 1 header.

you need put content of linkedlist.cpp in linkedlist.h, rid of source file altogether, , include updated header need it.

p.s. also, modifying object through const reference not idea, speaking. compiler not it. might want reconsider design.

p.p.s. if want , mean modify data members in objects of class access via const reference, need declare data members mutable.

p.p.p.s. elaborate on template function foo friend declarations template bar class , avoid lengthy discussions in comments.

if defining such function declaration inside class, straightforward thing - use friend keyword. example, friend void foo( bar< t > & ) { ... }.

if defining separately outside class, need use different syntax in declaration , avoid shadowing of template parameters: template< typename u > friend void foo( bar< u > & ) { ... } (assuming u not used in template parameters in bar class declaration). , define outside class.

if want specify default template arguments foo gets more trickier. cannot use default template arguments in friend declaration, have forward declare everything:

template< typename t > class bar;  template< typename t = int > void foo( bar< t > & );  template< typename t > class bar {   template< typename u >   friend void foo( bar< u > & ); };  template< typename t > void foo( bar< t > & ) {  ... } 

and 1 last p.p.p.p.s (i hope!). since using operator << type savingaccount in operator << linkedlist, need define operator << savingaccount too.

note self : don't answer questions in comments' section answer ;).