How Can Recursive Functionality Be Achieved with C/C Macros?

Recursive Macros in C/C

C/C macros do not directly support recursion. However, there are ways to achieve recursive functionality using preprocessor tricks.

Approach Using Deferred Expressions:

This approach utilizes deferred expressions to prevent the macro from being painted blue, thus allowing it to be expanded recursively.

#define EMPTY(...)
#define DEFER(...) __VA_ARGS__ EMPTY()
#define OBSTRUCT(...) __VA_ARGS__ DEFER(EMPTY)()
#define EXPAND(...) __VA_ARGS__

#define pr_id() pr
#define pr(n) ((n==1)? 1 : DEFER(pr_id)()(n-1))

While this approach allows recursion, it requires multiple expansions to complete the recursion.

Example: Recursive Repeat Macro:

Here's an example of a recursive repeat macro:

#define REPEAT(count, macro, ...) \
    WHEN(count) \
    ( \
        OBSTRUCT(REPEAT_INDIRECT) () \
        ( \
            DEC(count), macro, __VA_ARGS__ \
        ) \
        OBSTRUCT(macro) \
        ( \
            DEC(count), __VA_ARGS__ \
        ) \
    )
#define REPEAT_INDIRECT() REPEAT

Example of Usage:

// An example of using the recursive repeat macro
#define M(i, _) i
EVAL(REPEAT(8, M, ~)) // 0 1 2 3 4 5 6 7

Conclusion:

While C/C macros do not directly support recursion, it is possible to achieve it through preprocessor tricks like deferred expressions and custom macros. These methods can be useful in specialized cases where recursive functionality is required in macro definitions.

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