Can't overload

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	ui.o src	ripulante.o src	ipoaviao.o src\manga.o src\main.o src\datahora.o src\companhiaaerea.o src\aviao.o src\aeroporto.o src	ripulante.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	ipoaviao.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\Francisc…

Accepted Answer

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 `Categoria`is 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 ` to the implementation file, and in the header file then `#include `.

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

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