c-preprocessor

问题 Probably a naïve question - I used to program 20 years ago and haven't coded much since. My memory of how the C preprocessor works has atrophied significantly since then... I am writing a very simple C program and I am trying to declare a few static global arrays, but the size of the arrays would be dependent (on a non-trivial way) on a MODE variable. Something like the simplified example below. Two quick points: I know I could just size the arrays according to the largest size needed by any

问题 I have a function that I need to macro'ize. The function contains temp variables and I can't remember if there are any rules about use of temporary variables in macro substitutions. long fooAlloc(struct foo *f, long size) { long i1, i2; double *data[7]; /* do something */ return 42; } MACRO Form: #define ALLOC_FOO(f, size) \ {\ long i1, i2;\ double *data[7];\ \ /* do something */ \ } Is this ok? (i.e. no nasty side effect - other than the usual ones : not "type safe" etc). BTW, I know "macros

问题 -Og is a relatively new optimization option that is intended to improve the debugging experience while apply optimizations. If a user selects -Og , then I'd like my source files to activate alternate code paths to enhance the debugging experience. GCC offers the __OPTIMIZE__ preprocessor macro, but its only set to 1 when optimizations are in effect. Is there a way to learn the optimization level, like -O1 , -O3 or -Og , for use with the preprocessor? 回答1: I believe this is not possible to

问题 I have some C++ code, and want to perform an action if the __APPLE__ or __linux macros are defined. If I did it as a normal if conditional, it would be easy using || : if (something || something) { .. code .. } But as of what I know there is no || operator for #ifdef statements. How would I check if __APPLE__ or __linux is defined using a single #ifdef statement? 回答1: You can't in a single #ifdef would a single #if do instead? #if defined(__APPLE__) || defined(__linux) this also works if you

问题 A while ago I have discovered an (rather ancient) C Compiler, which scanned macros this way (Pseudo code): if line.startswith("#include") or line.startswith("#define"): ... .. Which kind of raised the question for me where macros should really be placed, at the beginning of a line, like so: void stuff() { #if defined(WIN32) || defined(_WIN32) ... #else #if defined(__GNUC__) ... #else ... #endif #endif } Or rather like so (as that's the way I do it, for improved readability): void stuff() {

问题 I want to know the reason behind the output of this code. I couldn't come up with an answer. #define f(a,b) a##b #define g(a) #a #define h(a) g(a) void main() { printf("%s %s",h(f(1,2)),g(f(1,2))); } PS: output is 12 f(1,2) . I thought it was 12 12 or f(1,2) f(1,2) . 回答1: h(f(1,2)) f(1,2) is substituted for a . a is not the subject of a # or ## operator so it's expanded to 12 . Now you have g(12) which expands to "12" . g(f(1,2)) f(1,2) is substituted for a . The # operator applied to a

问题 Is the #include <file> meant to be used for headers only or is it simply a mechanical "inject this code here" that can be used anywhere in the code? What if I use it in the middle of a cpp function to just "inject" code from a single source? will this work or will compilers scream about this? 回答1: It is a mechanical inject the code here device. You can include a text file containing Goethe's Faust if you wish to. You can put it anywhere, even in the middle of a function (of course, #include

问题 In order to make my code shorter and easier to change I want to replace something like enum{ E_AAA, E_BBB, E_CCC }; static const char *strings{"AAA", "BBB", "CCC" }; With a macro, like INIT(AAA, BBB, CCC); but when I try doing a macro with variable arguments, and stringification I get an error as the arguments are not declared. Any idea on how to do this? 回答1: Here a solution I learned a few days ago. The simplified version that attends your question is: #define ENUM_MACRO(name, v1, v2, v3,

问题 Why does M(0) and N(0) have different results? #define CAT_I(a, b) a ## b #define CAT(a, b) CAT_I(a, b) #define M_0 CAT(x, y) #define M_1 whatever_else #define M(a) CAT(M_, a) M(0); // expands to CAT(x, y) #define N_0() CAT(x, y) #define N_1() whatever_else #define N(a) CAT(N_, a)() N(0); // expands to xy 回答1: In fact, it depends on your interpretation of the language standard. For example, under mcpp, a preprocessor implementation that strictly conforms to the text of the language standard,

问题 The question title should say it all, but here's an example of what sort of thing I'm looking for: #ifndef THE_IDENTIFIER_THAT_WOULD_INDICATE_BEING_COMPILED_AS_CPLUSPLUS /* * Example of something that would matter. */ typedef enum _bool bool; enum _bool { false, true }; #endif What is the identifier? It's bugging me severely, as I know I've seen code that does this before. I'm using GCC, by the way. (I'm surprised I couldn't find a duplicate somewhere on SO. If someone else can find one, feel