c++ preprocessor directive for choosing variable in runtime

Question

I'm trying to achieve something like this: I have

struct {
   int var1;
   int var2;
   int var3;
   ......
   int var31;
}

now I have some integer 'r' that I read on runtime, and I want to print value of var##r (that is, if r=6, print var6 from structure), is it possible via preprocessor directives? I understand I can do this by jumping from var1's address sizeof(int) * r times but I want preprocessor to generate whole if-else chain, can I make it somehow do this?

UPDATE: Forgot to mention, cannot modify given struct, thus cannot use array

UPDATE2: I have those limitations (no arrays, nu jumps) because at the end I'm parsing syntax tree generated by clang and I want to have pure if/else statements there

The preprocessor can't do anything at run-time, it's a compile-time only thing. — Some programmer dude, Jul 17 '17 at 05:35
@Christoph I said I don't want to jump, I want preprocessor to generate cases — Zhani Baramidze, Jul 17 '17 at 05:41
@Krom I'm not sure if the standard garantees anything on that but I'm pretty sure it should work on most systems. `int` will be aligned on `sizeof(int)` (e.g. 4 or 8 bytes) so there will usally be no padding between them if it's not a really wierd int. I guess if the code is so flawed that he can't change the struct then that's his best option. — Christoph Diegelmann, Jul 17 '17 at 05:41
You cannot do that with the normal processor, because it does not know anything about code structure. So you would need to write your own. — t.niese, Jul 17 '17 at 05:45
It probably won't get any more beautiful than [this](https://godbolt.org/g/BPhko7). — Christoph Diegelmann, Jul 17 '17 at 05:57
You could try to construct something out of [conditionals in macros](https://stackoverflow.com/questions/19011019) but I think it is not worth the trouble. — typetetris, Jul 17 '17 at 06:01
*"I have those limitations (no arrays, nu jumps)"* - So basically anything that will make the code less verbose but still well defined. — StoryTeller - Unslander Monica, Jul 17 '17 at 06:23

skyking · Answer 1 · 2017-07-17T06:53:02.183

If course you could generate the if-else chain using the preprocessor if you really want to (you probably don't), but if it's only this occurence it's most probably not worth it. I think you should really reconsider the possibility to use an array or offsets into the struct.

You could to some limited extent construct loops in the preprocessor. The limitation is that you can't write proper loops so you will end up in a construct that will end up with a limited amount of iterations. Basically what you do to loop n times is to expand LOOP(n,...) to LOOP#n(...) and then define LOOPn# for all possible values ofn`.

So what we would want is something like:

 if( 0 )                 ; // Note: empty statement
 else if( r == 1 )       return o->var1;
 else if( r == 2 )       return o->var2;
 // etc

The if(0) is to make each variable be handled textually the same. So we can do the first step to define that:

 #define HANDLE_VAR(n) else if( r == n ) return o->var ## n;

this would mean that we write it

 if(0) ;
 HANDLE_VAR(1)
 HANDLE_VAR(2)
 //  etc

Then we define our loop construct:

 #define LOOP(n,M) LOOP ## n(M)
 #define LOOP(1) M(1)
 #define LOOP(2) M(1) M(2)
 // etc

then our construct would be

 if( 0 ) ; LOOP(31,HANDLE_VAR)

And finally we would wrap it all into a macro:

 #define RETURN_VAR if(0) ; LOOP(31,HANDLE_VAR)

There are other options however. The most normal solution would be to use an array to store the variables.

Another would be to write your own preprocessor that supports proper looping, there are ready template processors (genshi for example) that support that.

A third is to calculate the offset into the struct. The suggested method of doing this is not standard compliant though. What you need to do is to have an array of offsets stored:

int StructName::* var_off[] = {
    NULL,
    &StructName::var1,
    &StructName::var2,
    // etc
};

and then use o.*var_off[r] to access var##r (after checking that r is in range of course).

`offsetof` is only good for standard layout types. There should be a static assert there to verify `StructName` is standard layout. — StoryTeller - Unslander Monica, Jul 17 '17 at 06:27
@StoryTeller I was thinking C, in C++ I don't think offsetof is approprate for this situation - I've modified the answer to use pointer to members instead (I think they should work regardless of standard layout). — skyking, Jul 17 '17 at 06:55

c++ preprocessor directive for choosing variable in runtime

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