Counting macro

Hello,

Is it possible to have a macro expanded to a number incremented after every "instance"?

I am trying to use it to generate case labels, so they have to be compile-time constants.

For example -

Code:

---

#define M [...]

switch(...)
{
case M:
...
case M:
...
case M:
...
}

---

And I want it to preprocess to -

Code:

---

#define M [...]

switch(...)
{
case 0:
...
case 1:
...
case 2:
...
}

---

Any ideas :D?

Thanks!

I just realized, I don't really care about the numbers themselves, as long as they are unique in the switch statement. So I can just use __LINE__ (assuming the switch statement won't be across more than 1 file, and no 2 cases are on the same line).

I'm still curious about the original problem, though.

Hmm... why do you want to do this in the first place? It seems unnecessary when you can um, type the numbers out yourself.

Hehe. This is a very long story. I'm trying to do some kind of macro-based cooperative multitasking... using only switch statements. So there could be a lot of labels (need one per "yield()"), and they will be inserted and removed all the time.

Hopefully I'll have it done today, but here's what I'm trying to do - Piece of Mind » Blog Archive » Primitive Multi-Tasking Using Switch Statement - "Have you moo-ed today?"

Common Predefined Macros - The C Preprocessor
The __COUNTER__ macro

Woohoo it's working now!

Warning - probably the ugliest code you've seen in a very long time.

This is an implementation of cooperative "multitasking" using just switch statements and macros. Why? Piece of Mind » Blog Archive » Primitive Multi-Tasking Using Switch Statement - "Have you moo-ed today?"

Code:

---

#define RESTARTABLE_BEGIN static int restartable_stage = 0; switch(restartable_stage) { case 0:
#define RESTARTABLE_YIELD restartable_stage = __LINE__; return false; case __LINE__:
#define RESTARTABLE_END } restartable_stage = 0; return true;

---

Example:

Code:

---

#include <iostream>


bool task1()
{
        RESTARTABLE_BEGIN;

        std::cout << "t1 -> 1" << std::endl;

        RESTARTABLE_YIELD;

        std::cout << "t1 -> 2" << std::endl;

        RESTARTABLE_YIELD;

        std::cout << "t1 -> 3" << std::endl;

        RESTARTABLE_END;
}

bool task2()
{
        RESTARTABLE_BEGIN;

        std::cout << "t2 -> 1" << std::endl;

        RESTARTABLE_YIELD;

        std::cout << "t2 -> 2" << std::endl;

        RESTARTABLE_YIELD;

        std::cout << "t2 -> 3" << std::endl;

        RESTARTABLE_YIELD;

        std::cout << "t2 -> 4" << std::endl;

        RESTARTABLE_YIELD;

        std::cout << "t2 -> 5" << std::endl;

        RESTARTABLE_END;
}

int main(int argc, char* argv[])
{
        bool task1_done = false;
        bool task2_done = false;

        while (!(task1_done && task2_done))
        {
                if (!task1_done)
                {
                        task1_done = task1();
                }

                if (!task2_done)
                {
                        task2_done = task2();
                }
        }

        return 0;
}

---

May be useful in embedded applications if your requirements are simple and can't be bothered to get a real scheduler.

This program actually triggers a bug in VS2010. It doesn't like __LINE__ as a case label (saying it's not constant) if edit and continue debugging is turned on.

Workaround provided by putty -

Quote:

---

337 * In particular, if you are getting `case expression not constant'
338 * errors when building with MS Visual Studio, this is because MS's
339 * Edit and Continue debugging feature causes their compiler to
340 * violate ANSI C. To disable Edit and Continue debugging:
341 *
342 * - right-click ssh.c in the FileView
343 * - click Settings
344 * - select the C/C++ tab and the General category
345 * - under `Debug info:', select anything _other_ than `Program
346 * Database for Edit and Continue'.
347 */

---

Quote:

---

Originally Posted by Salem

Common Predefined Macros - The C Preprocessor
The __COUNTER__ macro

---

Thanks!

One little problem, though.

If I have

Code:

---

#define M ... __COUNTER__ ...

M;
M;
M;

---

All the M's will get the same counter value. Is there a way around that?

Deviously breaking the token and repasting it together .....

Code:

---

#define M_HELPER(token)  __##token##__
#define M M_HELPER(COUNTER)

---

should do the trick.