不能为 C++ 中的枚举重载 <<

Question

我创建了一个枚举数据类型来定义可能的飞行长度。我想重载它的

当我编译这个时，我收到以下错误（为了完整性而发布。基本上是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.  

这是声明枚举和重载运算符的数据文件。

/*
 * 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： 我尝试了常量引用、常量值和非常量值。未编译。

Answer 1

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函数。

要么加inline，点赞

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

或者将定义移动到一个实现文件中（你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& cat。或者如果Categoria是一个枚举，则按值传递。

编辑 2：PPS：如果您将定义移动到实现文件中，那么为了减少对客户端代码的不必要依赖，还将 #include <iostream> 移动到实现文件和头文件中然后#include <iosfwd>。

Answer 2

看看你的功能：

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

它只能匹配一个可写的枚举变量...将类型设为Categoria 或const Categoria&。

Answer 3

试试这样：

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

编辑

doh 多重定义。如果你喜欢在头文件中定义一个函数，那么至少把 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;
}