Can Recursive Macros Be Implemented in C/C ?

Exploring Recursive Macros in C/C : The Art of Expansion

Can Recursive Macros Exist?

While macros lack inherent recursive capabilities in C/C , there exists a path to achieving recursive-like behavior. Using a combination of deferred expressions and indirection, one can create elaborate macros that simulate recursion.

How it Works

To craft a recursive macro, we must embrace a multi-step process:

1. Obstruct Self-Expansion: Prevent the macro from halting its expansion by creating a disabling context.
2. Indirection: Use indirection to defer the expansion to a later stage.
3. Expansion: Apply multiple scans to complete the expansion process.

Example: Recursive Repeat Macro

Here's an illustration 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__ \
        ) \
    )

This macro can be invoked like this:

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

Sample Execution Issue

Your provided code:

# define pr(n) ((n==1)? 1 : pr(n-1))
void main ()
{
    int a=5;
    cout<<"result: "<< pr(5) <<endl;
    getch();
}

does not execute due to a recursive macro issue. The pr macro relies solely on recursion, leading to infinite expansion.

Solution

To fix this issue, you could use the following non-recursive version of the macro:

#define fact(n) ((n == 1)? 1 : n * fact(n - 1))

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