Can't overload << for Enum in C++

Question

I have created an enumerated data type to define possible flight lengths. I'd like to overload its << operator so the representation is nicer.

When I compile this, I get the following error (posted for completeness' sake. Basically multiple definitions of operator <<(ostream&, Categoria&)):

g++ -oProjectoAEDA.exe src\voo.o src\tui.o src\tripulante.o src\tipoaviao.o src\manga.o src\main.o src\datahora.o src\companhiaaerea.o src\aviao.o src\aeroporto.o
src\tripulante.o: In function `ZlsRSoR9Categoria':
c:/mingw/bin/../lib/gcc/i686-pc-mingw32/4.5.1/../../../../include/c++/4.5.1/new:103: multiple definition of `operator<<(std::ostream&, Categoria&)'
src\voo.o:C:\Users\Francisco\workspace_aeda\ProjectoAEDA\Debug/../src//headers/categoria.h:20: first defined here
src\tipoaviao.o: In function `ZlsRSoR9Categoria':
c:/mingw/bin/../lib/gcc/i686-pc-mingw32/4.5.1/../../../../include/c++/4.5.1/new:103: multiple definition of `operator<<(std::ostream&, Categoria&)'
src\voo.o:C:\Users\Francisco\workspace_aeda\ProjectoAEDA\Debug/../src//headers/categoria.h:20: first defined here
src\manga.o: In function `ZlsRSoR9Categoria':
c:/mingw/bin/../lib/gcc/i686-pc-mingw32/4.5.1/../../../../include/c++/4.5.1/new:103: multiple definition of `operator<<(std::ostream&, Categoria&)'
src\voo.o:C:\Users\Francisco\workspace_aeda\ProjectoAEDA\Debug/../src//headers/categoria.h:20: first defined here
src\main.o: In function `ZlsRSoR9Categoria':
C:\Users\Francisco\workspace_aeda\ProjectoAEDA\Debug/../src//headers/categoria.h:20: multiple definition of `operator<<(std::ostream&, Categoria&)'
src\voo.o:C:\Users\Francisco\workspace_aeda\ProjectoAEDA\Debug/../src//headers/categoria.h:20: first defined here
src\companhiaaerea.o: In function `ZlsRSoR9Categoria':
c:/mingw/bin/../lib/gcc/i686-pc-mingw32/4.5.1/../../../../include/c++/4.5.1/new:103: multiple definition of `operator<<(std::ostream&, Categoria&)'
src\voo.o:C:\Users\Francisco\workspace_aeda\ProjectoAEDA\Debug/../src//headers/categoria.h:20: first defined here
src\aviao.o: In function `ZlsRSoR9Categoria':
c:/mingw/bin/../lib/gcc/i686-pc-mingw32/4.5.1/../../../../include/c++/4.5.1/new:103: multiple definition of `operator<<(std::ostream&, Categoria&)'
src\voo.o:C:\Users\Francisco\workspace_aeda\ProjectoAEDA\Debug/../src//headers/categoria.h:20: first defined here
src\aeroporto.o: In function `ZlsRSoR9Categoria':
c:/mingw/bin/../lib/gcc/i686-pc-mingw32/4.5.1/../../../../include/c++/4.5.1/new:103: multiple definition of `operator<<(std::ostream&, Categoria&)'
src\voo.o:C:\Users\Francisco\workspace_aeda\ProjectoAEDA\Debug/../src//headers/categoria.h:20: first defined here
collect2: ld returned 1 exit status
Build error occurred, build is stopped
Time consumed: 928  ms.

This is the data file where both the enum is declared and the operator overloaded.

/*
 * categoria.h
 *
 *  Created on: 9 de Out de 2010
 *      Author: Francisco
 */

#ifndef CATEGORIA_H_
#define CATEGORIA_H_

#include <iostream>

enum Categoria {
    LongoCurso,
    MedioCurso,
    Domestico
};

std::ostream& operator<<(std::ostream & os, Categoria & cat)
{
  switch (cat) {
  case LongoCurso:
      os << "Longo Curso";
      break;
  case MedioCurso:
      os << "Medio Curso";
      break;
  case Domestico:
      os << "Domestico";
  }
  return os;
}



#endif /* CATEGORIA_H_ */

EDIT: I tried const reference, const by value and non const value. None compiled.

Cheers and hth. - Alf · Accepted Answer · 2014-03-08T03:50:06.153

20

Multiple definition linker errors because you've defined the function in a header file that's included in two or more compilation units.

Either add inline, like

inline std::ostream& operator<<(std::ostream & os, Categoria & cat)

or move the definition to an implementation file (you still need a declaration in the header file then).

EDIT: PS: also, as others have mentioned (I didn't see that), pass the second argument by reference to const, like Categoria const& cat. Or if Categoriais an enum, pass it by value.

EDIT 2: PPS: if you move the definition to an implementation file, then to reduce needless dependencies for client code, move also the #include <iostream> to the implementation file, and in the header file then #include <iosfwd>.

edited Mar 08 '14 at 03:50

answered Oct 25 '10 at 10:48

Cheers and hth. - Alf

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"darn, I can't see that here while editing": while editing there's a grey bar under the question title which you can drag down to reveal as much of the question text as necessary. – Eamon Nerbonne Oct 25 '10 at 10:59
@Cheersandhth "anum" should be "enum" in this answer. I don't have enough rep to make an edit that small. – Trebor Rude Mar 08 '14 at 00:36

score 3 · Answer 2 · answered Oct 25 '10 at 10:36

3

Look at your function:

std::ostream& operator<<(std::ostream & os, Categoria & cat)

It can only match a writable enumeration variable... make the type Categoria or const Categoria&.

answered Oct 25 '10 at 10:36

Tony Delroy

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Shameless plug: [How to pass objects to functions in C++?](http://stackoverflow.com/questions/2139224/how-to-pass-objects-to-functions-in-c/2139254#2139254) – sbi Oct 25 '10 at 10:45
I should have looked more carefully at your compiler errors... sorry! Kudos to Alf. – Tony Delroy Oct 26 '10 at 02:35

BЈовић · Answer 3 · 2010-10-25T10:51:21.683

3

Try like this:

std::ostream &operator<<( std::ostream &os, const Categoria obj );

EDIT

doh multiple definition. If you prefer to define a function in a header, then at least put inline :

inline
std::ostream &operator<<( std::ostream &os, const Categoria cat )
{
  switch (cat) {
  case LongoCurso:
      os << "Longo Curso";
      break;
  case MedioCurso:
      os << "Medio Curso";
      break;
  case Domestico:
      os << "Domestico";
  }
  return os;
}

edited Oct 25 '10 at 10:51

answered Oct 25 '10 at 10:37

BЈовић

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Thank you, that worked, but I moved the implementation to a source file. – F. P. Oct 25 '10 at 10:56

Can't overload << for Enum in C++

3 Answers3