不能超载<<对于 C++ 中的枚举

Question

我创建了一个枚举数据类型来定义可能的飞行长度。我想超载它的 <<运算符，因此表示效果更好。

当我编译这个时，我收到以下错误（为了完整性而发布。基本上运算符的多个定义<<（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.  

这是数据文件，其中两个声明了枚举并且重载了运算符。

/*
 * 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_ */

编辑： 我尝试了 const 引用、const by value 和非 const value。没有编译。

Answer 1

看看你的函数：

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

它只能匹配可写的枚举变量...使类型为 Categoria 或 const Categoria&。

Answer 2

尝试这样：

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

编辑

多重定义。如果您喜欢在标头中定义函数，那么至少放置 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;
}

Answer 3

多个定义链接器错误，因为您在包含在两个或多个编译单元中的头文件中定义了该函数。

添加内联，

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

或者将定义移动到实现文件（那么您仍然需要在头文件中声明）。

编辑：PS：另外，正如其他人提到的（我没有看到），通过引用 const 传递第二个参数，例如 Categoria const&猫。或者，如果 Categoria 是一个枚举，则按值传递它。

编辑2：PPS：如果将定义移动到实现文件，那么为了减少客户端代码不必要的依赖关系，请将 #include 移动到实现文件，然后在头文件中添加 #include 。