How to evaluate a nested preprocessor macro

Question

let's say I want to select the behaviour of a certain preprocessor directive evaluating at compile time the concatenation of a constant string and the result of another macro.

#define CASE1 text1
#define CASE2 text2
#define CASE3 text3
#define SCENARIO 3
/** the following won't work - for examplification purposes only**/
#define FUNCTION CASE##SCENARIO

/** whenever I write FUNCTION, I expect to see text3 **/

I am having an hard time thinking of a viable solution, as the preprocessor is a one-pass beast. Is that even feasible ?

Answer 1

It's possible, you just need to add some extra layers of macros. The key is that when you use the token-pasting operator ##, the preprocessor will not expand its operands. But, if you add another layer of macros, the preprocessor will expand those arguments. For example:

#define CASE1 text1
#define CASE2 text2
#define CASE3 text3
#define SCENARIO 3

#define TOKENPASTE_HELPER(x, y) x ## y
#define TOKENPASTE(x, y) TOKENPASTE_HELPER(x, y)
#define FUNCTION TOKENPASTE(CASE, SCENARIO)

When the preprocessor expands FUNCTION, it expands TOKENPASTE. When it expands TOKENPASTE, it expands its arugments (so SCENARIO gets replaced by 3), since neither of its arguments are operands of the token-pasting operator. Next, it expands TOKENPASTE_HELPER, which does the actual token pasting to make CASE3. Finally, it expands the CASE3 macro to get text3.