FW: a nifty feature for c preprocessor

[David Brown]

On 28/12/2011 07:48, R A wrote:
>
> i'm an amateur programmer that just started learning C. i like most
> of the features, specially the c preprocessor that it comes packed
> with. it's an extremely portable way of implementing metaprogramming
> in C.
>
> though i've always thought it lacked a single feature -- an
> "evaluation" feature.
>


I think you have missed the point about the C pre-processor.  It is not 
a "metaprogramming" language - it is a simple text substitution macro 
processor.  It does not have any understanding of the symbols (except 
for "#") in the code, nor does it support recursion - it's pure text 
substitution.  Your suggestion would therefore need a complete re-design 
of the C pre-processor.  And the result is not a feature that people 
would want.

Many uses of the C pre-processor are deprecated with modern use of C and 
C++.  Where possible, it is usually better programming practice to use a 
"static const" instead of a simple numeric "#define", and a "static 
inline" function instead of a function-like macro.  With C++, even more 
pre-processor functionality can be replaced by language features - 
templates give you metaprogramming.  There are plenty of exceptions, of 
course, but in general it is better to use a feature that is part of the 
language itself (C or C++) rather than the preprocessor.

It looks like you are wanting to get the compiler to pre-calculate 
results rather than have them calculated at run-time.  That's a good 
idea - so the gcc developers have worked hard to make the compiler do 
that in many cases.  If your various expressions here boil down to 
constants that the compiler can see, and you have at least some 
optimisation enabled, then it will pre-calculate the results.


If you have particular need of more complicated pre-processing, then 
what you want is generally some sort of code generator.  C has a simple 
enough syntax - write code in any language you want (C itself, or 
anything else) that outputs a C file.  I've done that a few times, such 
as for scripts to generate CRC tables.

And if you really want to use a pre-processing macro style, then there 
are more powerful languages suited to that.  You could use PHP, for 
example - while the output of a PHP script is usually HTML, there is no 
reason why it couldn't be used as a C pre-processor.




> say i have these definitions: #define MACRO_1           (x/y)*y
> #define MACRO_2           sqrt(a) #define MACRO_3           calc13()
> .... #define MACRO_15         (a + b)/c
>
>
> now, all throughout the codebase, whenever and whichever of MACRO_1,
> or MACRO_2 (or so forth) needs to be called, they are conveniently
> "indexed" by another macro expansion:
>
> #define CONCAT(a, b)                  a##b #define CONCAT_VAR(a, b)
> CONCAT(a, b)
>
> #define MASTER_MACRO(N)         CONCAT_VAR(MACRO_, N)
>
> now, if we use MASTER_MACRO with a "direct" value:
>
> MASTER_MACRO(10) or #define N              10 MASTER_MACRO(10) both
> will work.
>
>
> but substitute this with:
>
> #define N                                    ((5*a)/c + (10*b)/c +
> ((5*a) % c + (10*b) % c)/c)
>
> and MASTER_MACRO expands to: MACRO_((5*a)/c + (10*b)/c + ((5*a) % c +
> (10*b) % c)/c)
>
> which, of course is wrong. there are other workarounds or many times
> this scheme can be avoided altogether. but it can be made to work
> (elegantly) by adding an "eval" preprocessor operation:
>
> so we redefine MASTER_MACRO this way: #define MASTER_MACRO(N)
> CONCAT_VAR(MACRO_, eval(N)) which evaluates correctly.
>
> this nifty trick (though a bit extended than what i elaborated above)
> can also be used to *finally* have increments and decrements (among
> others). since "eval" forces the evaluation of an *arithmetic*
> expression (for now), it will force the evaluation of an expression,
> then define it to itself. this will of course trigger a redefinition
> flag from our beloved preprocessor, but the defined effect would be:
>
> #define X             (((14*x)/y)/z)    /* say this evaluates to
> simply 3 */
>
> incrementing X, will simply be: #define X             eval(eval(X) +
> 1)    /* 1) will be evaluated as 4 before any token substitution */
> #define X             eval(eval(X) + 1)    /* 2) will be evaluated as
> 5 before any token substitution */
>
> that easy.
>
> to suppress the redef warnings, we can have another directive like
> force_redef (which can only work in conjunction with eval)
> #force_redef  X             eval(eval(X) + 1)
>
>
> i'm just confused :-S... why hasn't this been suggested? i would love
> to have this incorporated (even just on test builds) to gcc. it would
> make my code so, so much more manageable and virtually extensible to
> more platforms.
>
> i would love to have a go at it and probably modify the gcc
> preprocessor, but i since i know nothing of it's implementation
> details, i don't know where to begin. i was hoping that this being a
> gnu implementation, it's been heavily modularized (the fact that gcc
> was heavily revised back then to use abstract syntax trees, gimple,
> etc, past version 2.95 -- ???). so i can easily "interrupt" the
> parsing operation (i wouldn't dare implement a pre-preprocessing
> operation, being big and redundant), then substitute the eval, then
> make the whole prasing go again.
>
> any advice for a novice? thnx.
>