C-preprocessor: extract [0...N] args from __VA_ARGS__

Question

How can I extract [0...N] arguments from VA_ARGS assuming that N will be less or equal to the number of arguments. Example: ``` #define MY_SEQ r0, r1, r2, r3, r4, r5, r6, r7, \ r8, r9, r10, r11, r12, r13, r14, r15 #define EXTRACT_N(n, SEQ) {... magic ...} ... EXTRACT_N(5, (MY_SEQ())); should expand to: {r0, r1, r2, r3, r4}; ``` It's ok to assume that sequence elements are of this form WHATEVER##N where N is Nth element. I'm looking for some nice solution to this problem NOT using BOOST, e.g. I'd like to understand how it can be done. I did it using iterative approach, but I'd like to know if there is some other way to do it. Here's how I implemented it: ``` #define EXTRACT_1(t0) t0 #define EXTRACT_2(t0, t1) EXTRACT_1(t0),t1 #define EXTRACT_3(t0, t1, t2) EXTRACT_2(t0, t1),t2 #define EXTRACT_4(t0, t1, t2, t3) EXTRACT_3(t0, t1, t2),t3 ... ```

Accepted Answer

You cannot do that in the general case. The c preprocessor is not that flexible.

You might have something like

```
#define EXTRACT_N(N,A) EXTRACTTHEM ## N(A)
```

and have

```
#define EXTRACTTHEM1(X, ...) X
#define EXTRACTTHEM2(X,Y, ...) X,Y
```

etc

(it is easy to generate an arbitrary large, but bounded, set of such macros).

There are more powerful processors than `cpp`, e.g. `m4` or gpp

You might consider instead generating your C or C++ code (using your own script, or some generator) and having your building system (e.g. `Makefile`) take care of generating the C code from something different.

C-preprocessor: extract [0...N] args from __VA_ARGS__

Solution

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