I am asking this question from an educational/hacking point of view, (I wouldn't really want to code like this).
Is it possible to implement a while loop only using C preprocessor directives. I understand that macros cannot be expanded recursively, so how would this be accomplished?
The C preprocessor is a macro preprocessor (allows you to define macros) that transforms your program before it is compiled. These transformations can be the inclusion of header files, macro expansions, etc. All preprocessing directives begin with a # symbol. For example, #define PI 3.14.
The #define creates a macro, which is the association of an identifier or parameterized identifier with a token string. After the macro is defined, the compiler can substitute the token string for each occurrence of the identifier in the source file.
Description. In the C Programming Language, the #ifdef directive allows for conditional compilation. The preprocessor determines if the provided macro exists before including the subsequent code in the compilation process.
If you want to implement a while loop, you will need to use recursion in the preprocessor. The easiest way to do recursion is to use a deferred expression. A deferred expression is an expression that requires more scans to fully expand:
#define EMPTY() #define DEFER(id) id EMPTY() #define OBSTRUCT(id) id DEFER(EMPTY)() #define EXPAND(...) __VA_ARGS__ #define A() 123 A() // Expands to 123 DEFER(A)() // Expands to A () because it requires one more scan to fully expand EXPAND(DEFER(A)()) // Expands to 123, because the EXPAND macro forces another scan
Why is this important? Well when a macro is scanned and expanding, it creates a disabling context. This disabling context will cause a token, that refers to the currently expanding macro, to be painted blue. Thus, once its painted blue, the macro will no longer expand. This is why macros don't expand recursively. However, a disabling context only exists during one scan, so by deferring an expansion we can prevent our macros from becoming painted blue. We will just need to apply more scans to the expression. We can do that using this EVAL
macro:
#define EVAL(...) EVAL1(EVAL1(EVAL1(__VA_ARGS__))) #define EVAL1(...) EVAL2(EVAL2(EVAL2(__VA_ARGS__))) #define EVAL2(...) EVAL3(EVAL3(EVAL3(__VA_ARGS__))) #define EVAL3(...) EVAL4(EVAL4(EVAL4(__VA_ARGS__))) #define EVAL4(...) EVAL5(EVAL5(EVAL5(__VA_ARGS__))) #define EVAL5(...) __VA_ARGS__
Next, we define some operators for doing some logic(such as if, etc):
#define CAT(a, ...) PRIMITIVE_CAT(a, __VA_ARGS__) #define PRIMITIVE_CAT(a, ...) a ## __VA_ARGS__ #define CHECK_N(x, n, ...) n #define CHECK(...) CHECK_N(__VA_ARGS__, 0,) #define NOT(x) CHECK(PRIMITIVE_CAT(NOT_, x)) #define NOT_0 ~, 1, #define COMPL(b) PRIMITIVE_CAT(COMPL_, b) #define COMPL_0 1 #define COMPL_1 0 #define BOOL(x) COMPL(NOT(x)) #define IIF(c) PRIMITIVE_CAT(IIF_, c) #define IIF_0(t, ...) __VA_ARGS__ #define IIF_1(t, ...) t #define IF(c) IIF(BOOL(c))
Now with all these macros we can write a recursive WHILE
macro. We use a WHILE_INDIRECT
macro to refer back to itself recursively. This prevents the macro from being painted blue, since it will expand on a different scan(and using a different disabling context). The WHILE
macro takes a predicate macro, an operator macro, and a state(which is the variadic arguments). It keeps applying this operator macro to the state until the predicate macro returns false(which is 0).
#define WHILE(pred, op, ...) \ IF(pred(__VA_ARGS__)) \ ( \ OBSTRUCT(WHILE_INDIRECT) () \ ( \ pred, op, op(__VA_ARGS__) \ ), \ __VA_ARGS__ \ ) #define WHILE_INDIRECT() WHILE
For demonstration purposes, we are just going to create a predicate that checks when number of arguments are 1:
#define NARGS_SEQ(_1,_2,_3,_4,_5,_6,_7,_8,N,...) N #define NARGS(...) NARGS_SEQ(__VA_ARGS__, 8, 7, 6, 5, 4, 3, 2, 1) #define IS_1(x) CHECK(PRIMITIVE_CAT(IS_1_, x)) #define IS_1_1 ~, 1, #define PRED(x, ...) COMPL(IS_1(NARGS(__VA_ARGS__)))
Next we create an operator, which we will just concat two tokens. We also create a final operator(called M
) that will process the final output:
#define OP(x, y, ...) CAT(x, y), __VA_ARGS__ #define M(...) CAT(__VA_ARGS__)
Then using the WHILE
macro:
M(EVAL(WHILE(PRED, OP, x, y, z))) //Expands to xyz
Of course, any kind of predicate or operator can be passed to it.
Take a look at the Boost preprocessor library, which allows you to write loops in the preprocessor, and much more.
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With