Why my output operator<<() function definition results in unresolved external symbol error?

Question

I don't know what's wrong with it.. I can't find where the error is..

The error is:

error LNK2019: unresolved external symbol "class std::basic_ostream > & __cdecl operator<<(class std::basic_ostream \> &,class Table const &)" (??6@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@ABV?$Table@H@@@Z) referenced in function _main

The code is as followed:

Table.h:

#include <cassert>
#include <vector>
#include <iostream>
#include <cstddef> 

template <class T>
class Table {
protected:
    std::vector<int> 
dimensions;
    // vector whose length is the number of table dimensions
    // and whose elements give the size of each dimension
    std::vector<int> 
offsetArray;

    std::vector<T> 
data;
    // vector of table values, ordered according
    // to the offsets given in offsetArray
    int
numCells;
    // total number of cells in the interior of the table
public:
    class iterator {
     public:
    iterator() : myTable(0), index(0) {}
    iterator(Table * t, int i = 0) 
        { myTable = t; index = i; };
        // construct myself to point to index i
        // of my containing Table object t
    iterator & operator++();
        // advance one element and return a reference to me
    iterator operator++(int);
        // advance one element and return my previous value
    iterator & operator=(iterator i);
        // set my table and position to be the same as j's
    bool operator==(const iterator & i);
        // return true if I'm positioned at the same element as i
    bool operator!=(const iterator & i);
        // return true if I'm positioned at a different 
        // element than i
    T & operator*();
        // return a reference to the element at my current position
    int getIndex() { return index;}
        // return my index value

private:
    Table * myTable;
    int index;
};
Table(){};
    // Construct myself to be empty
Table(const std::vector<int> & v);
    // Construct myself to be of the
    // dimensionality given by v, with elements created
    // by the default constructor
Table(const std::vector<int> & v, const T & initVal);
    // Construct myself to be of the
    // dimensionality given by v, with all cells
    // initialized to initVal
T & operator[](const std::vector<int>);
    // return a reference to the cell
    // value indexed by v
const T & operator[](const std::vector<int>) const;
    // return a const reference to the cell
    // value indexed by v
bool operator==(const Table<T> & t);
    // Return true if I'm the same dimensionality
    // as t and hold the same values
std::vector<int> size() const { return dimensions; }
    // Return my dimension vector
std::vector<int> locationFromIndex(int index) const;
    // return a location vector corresponding to 
    // an index into the (linear) data vector
int getNumCells() {return numCells};
    // return the number of cells I contain
iterator begin() {return iterator(this); };
iterator end() {return iterator(this, numCells); };

friend std::ostream & operator<<(std::ostream & out, const Table<T> & a);
};

template<class T>
std::ostream & operator<<(std::ostream & out, const Table<T> & a){
 for(size_t i=0;i<a.data.size();i++){
 out<<a.locationFromIndex(i)<<a.data[i];
 }
 return out;
 }

The main.cpp file is:

  #include <iostream>
  #include <cstdlib>
  #include "Table.h"
  using namespace std;

  void main() {
   // make this a 4x3x2 matrix, initialized with 1's
    vector<int> v;
    v.push_back(4);
    v.push_back(3);
    v.push_back(2);

   Table<int> t(v, 1);

   // print it
    cout << t << endl;

 }

I guess there is something wrong with the ostream function in the Table.h file. But I do not know how to fix it.. Can someone help me? Thanks!

Answer 1

If you compile with with warnings turned on, you'll get a hint. This is from gcc:

warning: friend declaration ‘std::ostream& operator<<(std::ostream&, const Table&)’ declares a non-template function [-Wnon-template-friend]

note: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here)

So the problem is with your friend declaration:

friend std::ostream & operator<<(std::ostream & out, const Table<T> & a);

It should be:

template <typename U>
friend std::ostream & operator<<(std::ostream & out, const Table<U> & a);

Once you fix that, there's other problems in your stream operator. You're trying to stream a vector<int> for which there's no function defined. But this should put you on the right track.