X-Macros Redefinition

Question

when I include the "xmacro.h" in header file which is used by multiple header files i get linking error: Error LNK2005: "char const * * iD_Strings" (?iD_Strings@@3PAPBDA) already defined in header_file.obj

1.//"xmacro.h"
2.
3.// (1) Define code generating macro
4.
5.   #ifndef XMACRO_H
6.   #define XMACRO_H
7.
8.    #define GENERATE_IDS \
9.   X(_Name, "/Name") \
10.   X(_ID, "/ID")
11.
12.  // (2) Define X-Macro for generating enum members
13.
14.  #define X(id, idString) id, 
15.   enum IDs
16.  {
17.     ID_LOWERBOUND = -1,
18.    GENERATE_IDS
19.   NUM_IDS
20.  };
21.  #undef X
22.
23.  // (3) Define X-Macro for generating string names
24.
25.  #define X(id, idString) idString,
26.  const char* iD_Strings[] =
27.  {   
28.     GENERATE_IDS
29.     NULL
30.  };
31.  #undef X
32.
33.  #endif

it generates an error when i define X-Macro for generating string names at line 23. how would i use a single macro without redefining it?

Answer 1

Your header file contains the definition of iD_Strings. When you include it from different source files, it gets linked in multiple times. That results in a conflict even if the definitions are identical.

You could declare iD_Strings as static (the C way) or wrap it in an anonymous namespace (the C++ way) to get around this problem.

Answer 2

A better way to do X-Macros (which I've always called list macros) is to pass the operator as a parameter to the list, then you can keep multiple "operators" around at the same time:

#define GENERATE_IDS(_) \
_(Name) \
_(Id)

enum IDs
{
    ID_LOWERBOUND = -1,
    #define MK_ID_ENUM(n) _##n,
    GENERATE_IDS(MK_ID_ENUM)
    NUM_IDS
}

const char* iD_Strings[] =
{
    #define MK_ID_STRING(n) "/" #n,
    GENERATE_IDS(MK_ID_STRING)
    NULL
};

Note you still have to declare the identifiers uniquely, but the invocation of the GENERATE_IDS() macro to declare objects really belongs in a .c/.c++ body and not a header anyway.