How Can Recursion Be Emulated with C/C Macros?

Recursive Macros in C/C

Macros in C/C do not support direct recursion. However, it is possible to emulate recursion through a combination of preprocessing techniques and indirection.

Example of a Recursive-like Macro

The following macro simulates recursion using deferred expressions and indirection:

#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))

Execution Process

When pr(5) is encountered, the following steps occur:

1. pr(5) expands to ((5 == 1) ? 1 : DEFER(pr_id)()(5 - 1)).
2. The preprocessor paints pr(5) blue (disabling context), preventing it from expanding further.
3. DEFER(pr_id)()(5 - 1) becomes OBSTRUCT(pr_id)()(5 - 1) and pr_id () is painted blue.
4. EXPAND(pr(5)) expands to ((5 == 1) ? 1 : ((5 - 1 == 1) ? 1 : DEFER(pr_id)()(5 - 1 - 1)))
5. Additional expansions are required to complete the recursion.

Recursive Repeat Macro

A more complex example of a recursive-like macro is a repeat macro that executes a specific code block a given number of times:

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

#define M(i, _) i

This macro can be used as follows:

EVAL(REPEAT(8, M, ~)) // 0 1 2 3 4 5 6 7

Limitations

While emulating recursion through macros is possible, it is generally considered a bad practice due to potential performance implications and code readability issues. Modern C offers alternative mechanisms for recursion, such as lambda expressions and template metaprogramming.

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