CODEWITHSUNDEEP
cmacrosc-preprocessor
charliehorse55
charliehorse55

Reputation: 1990

C macro get typeof argument

I am trying to write a macro to assist with object oriented programming in C. As I store the class information in a constant struct, I need to create a macro that does the following:

An example:

queue_t* p = NULL;
delete(p);

delete should expand to:

destroyObject(&(queue_t_info), p);

I tried using this macro, but I can't get to to work:

#define delete(X) (destroyObject(&(typeof(*X)##_info), X))

I'm having trouble with the typeof part to work correctly.

Upvotes: 4

Views: 12518

Answers (2)

charliehorse55
charliehorse55

Reputation: 1990

I realized that what I was attempting to do was impossible - the C preprocessor doesn't parse and symbolize the code so it doesn't know which type a variable is.

To solve this, I require the type to be passed into the delete function as well. This isn't ideal as it introduces a frequent source of bugs due to mismatched types. If a programmer passes a pointer to object A but specifies object B in the delete function, the wrong destructor would be called. To solve this, I added a typecheck to the macro so that a compiler warning would be generated for any mismatched types. 

#define typecheck(type,x) \
({  type __dummy; \
typeof(x) __dummy2; \
(void)(&__dummy == &__dummy2); \
})

#define delete(P, X) (destroyObject(&(X##_info), P), typecheck(X, *P))
#define new(X, ...) (createObject(&(X##_info), ##__VA_ARGS__))

Normal usage of the macro:

queue_t* p = new(queue_t);
delete(p, queue_t);

However using the wrong type:

queue_t* p = new(queue_t);
delete(p, int);

causes a compiler warning:

Comparison of distinct pointer types ('int *' and 'typeof (*p) *' (aka 'queue_t *'))

Upvotes: 2

nmikhailov
nmikhailov

Reputation: 1423

typeof isn't macro, it is language construction and it is expanded by compiler, not preprocessor. Since preprocessing goes before compilation, macro can't access typeof result.

Your delete(p) is expanded to: (destroyObject(&(typeof(*p)_info), p)). (You can see it by -E gcc flag)

Upvotes: 9

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