Conflicting enums

Question

What happens if two different libs define the same enum and I need to use both libs in the same project?

enum Month {January=0, February, ..., December}

Thank you.

P.S. It is C. No namespaces. I won't be able to keep them separated. Need some workaround.

What it the enum linkage? Internal or external? C libs used in C++ project. C++ tag applies.

Answer 1

C libs used in C++ project. C++ tag applies

Since they're used in C++ projects, then you can use namespace when including them in C++ code as:

// C files
//libone.h 
enum Month {January=0, February, ..., December}

//libtwo.h
enum Month {January=0, February, ..., December}

///////////////////////////////////////////////////////////////////
//C++ files

//lib.hpp
namespace LibOne 
{
     #include "libone.h"
}
namespace LibTwo 
{
     #include "libtwo.h"
}

//Usage in C++
LibOne::Month m1 = LibOne::January;
LibTwo::Month m2 = LibTwo::January;

Answer 2

Ultimate magic evil wizard workaround: if namespaces don't help (for whatever reason), and you absolutely can't avoid including both definitions in the same file, use a macro:

#define Month File_A_Month
#include "file_a.h"
#define Month File_B_Month
#include "file_b.h"
#undef Month

In the file, never use Month, only File_A_Month or File_B_Month. I'm not certain of the standards-correctness of this practice.

You might have to define all the enum members similarly to prevent clashes, and you probably want to put this evil hackery in a file called files_a_and_b.h. Of course, in your case the definitions are identical, so you shouldn't need to resort to this, but I'm leaving this answer here for less fortunate posterity.

Answer 3

Neither the enum itself nor the enumeration constants are objects, so they don't have any linkage at all - in this respect, they're like struct tags or typedef names.

This means that you have two workarounds:

• Ensure that the conflicting headers are never, directly or indirectly, #included in the same translation unit (.c file); or
• Launder one or both of the headers to change the conflicting names.

The header laundering can be done either by creating a private copy of the header with the names changed, or using the preprocessor as in Chris Lutz's answer.