CODEWITHSUNDEEP
c++macros
shi.wangs
shi.wangs

Reputation: 43

Is it right to declare variables using macros in header file in C++?

I encountered a problem when declare some global variables using macros. Here is the case:

the original code: 

/* some_config.h */  
static const std::string KEYWORDS_A[] = {"AXX", "AYY"};  
static const std::vector<std::string>  KEYWORDS_A_VEC(KEYWORDS_A, KEYWORDS_A + sizeof(KEYWORDS_A) / sizeof(KEYWORDS_A[0]));  
static const std::string KEYWORDS_B[] = {"BXX", "BYY"};  
static const std::vector<std::string>  KEYWORDS_B_VEC(KEYWORDS_B, KEYWORDS_B + sizeof(KEYWORDS_B) / sizeof(KEYWORDS_B[0]));

the current code: 

/* some_config.h */  
#define REGISTER_VEC(NAME) \  
static const std::vector<std::string>  KEYWORDS_##NAME_VEC(KEYWORDS_##NAME, KEYWORDS_##NAME + sizeof(KEYWORDS_##NAME) / sizeof(KEYWORDS_##NAME[0]))  
static const std::string KEYWORDS_A[] = {"AXX", "AYY"};  
REGISTER_VEC(A);  
static const std::string KEYWORDS_B[] = {"BXX", "BYY"};  
REGISTER_VEC(B);

the some_config.h was included in some.cpp in which the variables KEYWORDS_A_VEC and KEYWORDS_B_VEC were used. However, for the current code, the g++ compiler will give an error of not finding the definition of KEYWORDS_A_VEC and KEYWORDS_A_VEC.
Is there anything wrong with the code? thanks for your help.

Upvotes: 2

Views: 1261

Answers (1)

Oliver Charlesworth
Oliver Charlesworth

Reputation: 272567

Your compilation problem is caused because you need a tokenization thing in every place that you intend to tokenize, e.g.:

#define REGISTER_NAME(NAME) blah_##NAME##_blah

(Note use of ## on both sides of NAME.)

As to whether this is good practice, I would say no. You have a single macro instantiating multiple variables, which is confusing to the reader of your code. Many IDEs and debuggers will get confused by this sort of thing, which means that a reader will have to do a lot of manual hunting to work out where variables are magically being declared.

Upvotes: 3

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