three macro need be only defined before use of nine macro. You can even change three before every use of nine: #define nine three*3 #define three 3 int main() { std::cout << nine; //9 #undef three #define three 4 std::cout << nine; //12 #undef three //no `three` macro defined here int three = 2; std::cout … Read more
The Boost Preprocessor (which works for C as well as C++, even though Boost as a whole is a C++ library) library can help with this kind of task. Instead of using an #ifdef within a macro (which isn’t permitted), it helps you include a file multiple times, with different macros defined each time, so … Read more
The C++03 preprocessor is (at least intended to be) similar to the C preprocessor before C99. Although the wording and paragraph numbers are slightly different, the only technical differences I’m aware of between the two are that the C++ preprocessor handles digraphs (two-letter alternative tokens) and universal character names, which are not present in C. … Read more
The preprocessor is not a text processor, it works on the level of tokens. In your code, after the define, every occurence of the token X would be replaced by the token 10. However, there is not token X in the rest of your code. 1e-X is syntactically invalid and cannot be turned into a … Read more
#define STRINGIFY2(X) #X #define STRINGIFY(X) STRINGIFY2(X) #define A 2 Then STRINGIFY(A) will give you “2”. You can concatenate it with other string literals by putting them side by side. “I have the number ” STRINGIFY(A) “.” gives you “I have the number 2.”.
clang -dM -E – < /dev/null will list all the preprocessor definitions for clang.
You need the double expansion trick: #define S(x) #x #define S_(x) S(x) #define S__LINE__ S_(__LINE__) /* use S__LINE__ instead of __LINE__ */ Addendum, years later: It is a good idea to go a little out of one’s way to avoid operations that may allocate memory in exception-handling paths. Given the above, you should be able … Read more
Running it through the preprocessor shows the problem. Using gcc -E (can also use cpp -P, where the -P option also suppresses generated # lines), inline int safe(int i) { return i >= 0 ? i : -i; } int f(); void userCode(int x) { int ans; // increment 1 increment 2 (one of these) … Read more
#define IMPLIES(x, y) (!(x) || (y)) #define COMPARE(x, y) (((x) > (y)) – ((x) < (y))) #define SIGN(x) COMPARE(x, 0) #define ARRAY_SIZE(a) (sizeof(a) / sizeof(*a)) #define SWAP(x, y, T) do { T tmp = (x); (x) = (y); (y) = tmp; } while(0) #define SORT2(a, b, T) do { if ((a) > (b)) SWAP((a), (b), … Read more
GCC has a feature called statement expressions So if define macro like #define FOO(A) ({int retval; retval = do_something(A); retval;}) then you will be able to use it like foo = FOO(bar);