Question

I want to write preprocessor functions/arrays that are evaluated at
compile time. For example, if I define

#define MYARR[] {5,4,3,2,1,0}

then, the code

int x = R[0];

should be presented as

int x = 5;

to the compiler. (Of course only literals can be used in the index).
This is important if code size/memory is critical and we don’t want to
store MYARR, but we need it for coding convenience.

Compile time functions would also be good. For example, something like

#define MYMAP(n)
#if n==1
5
#else
2

So, the statement

int x = MYMAP(4);

should be presented to the compiler as

int x = 2;

Obviously, we have to use a literal as the argument. Is this possible?

Answer 1

Sure it’s possible. While you could do that manually, Boost.Preprocessor already gives you the tools you need:

#include <boost/preprocessor.hpp>
#define SEQ (5)(4)(3)(2)(1)(0)
int x = BOOST_PP_SEQ_ELEM(0, SEQ);

… gets transformed to:

int x = 5;

It also includes arithmetic, comparisons and control structures like IF, FOR, FOR_EACH, enumerations, … You just have to keep in mind that the data types you can work with are rather limited.

Utilizing Boost.PP again, your second example can be done like so:

#define MYMAP(x) BOOST_PP_IF(BOOST_PP_EQUAL(x, 1), 5, 2)

You can of course implement manually what Boost.PP does, but considering the time and effort needed for this I personally wouldn’t bother.

As a C user, you won’t be interested in the rest of Boost, so you might want to use bcp to extract the preprocessor components.