Unresolved external symbol "enum days __cdecl operator++(enum days)" (??E@YA?AW4days@@W40@@Z) referenced in function main

Question

I write a small program to encounter the next day by giving day.

I write a program in day_enum.cpp file:-

#include <ostream>
#include "AllHeader.h"
using namespace std;


  inline days operator++(days d)
 {
   return static_cast<days>((static_cast<int>(d) + 1) % 7);
  }

 ostream& operator<< (ostream& out,const days& d)
  {
  switch (d)
  {
  case SUN:  out <<"SUN";
    break;

case MON:  out <<"MON";
    break;

case TUE:  out<<"TUES";
    break;

case WED:  out <<"WED";
    break;

case THUS:  out <<"THUS";
    break;

case FRI:  out <<"FRI";
    break;

case SAT: out << "SAT";
    break;
default:
    break;
    return out;
   }
  }

Now allheader.h file look like this:-

#pragma once
#ifndef AllHeader
#define AllHeader

typedef enum days { SUN, MON, TUE, WED, THUS, FRI, SAT } days;
inline days operator++(days d);
ostream& operator<< (ostream& out, const days& d) ; 


#endif

In main function:-

days d = MON, e;
e = ++d;
cout << d << '\t' << e << endl;

I am getting error:- LNK2019 unresolved external symbol "enum days __cdecl operator++(enum days)" (??E@YA?AW4days@@W40@@Z) referenced in function main.

As per my understanding I already declare it in allheader.h file.

Answer 1

According to the C++ Standard

An inline function or variable shall be defined in every translation unit in which it is odr-used outside of a discarded statement.

It seems in the translation unit with main there is no definition of your inline function (operator).

Place the definition of the function in the header.