Thread: 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 ?

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.

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?"

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 -

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 */

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?