macros

问题 Is it possible to do something like this: #define F(x) \ #ifdef DOUBLE \ 2*x \ #else \ x \ #endif so that when I use F , what it expands to depends on whether the macro DOUBLE is defined? I don't think so, but I'm hopeful. GNU expansions are fine. Edit In response to some of the answers, I'm really using this to do some code generation, where the code is slightly different depending on where it gets defined. Because of the order in which some files are included and where the relevant macros

问题 I am trying to implement a macro to recursively converting an infix list into a prefix one. I encounter a problem as follows: ;;this works (defmacro recursive-infix [form] (list (second form) (first form) (if (not (seq? (nth form 2))) (nth form 2) (recursive-infix (nth form 2))))) ;;this doesn't work (defmacro my-recursive-infix [form] `(~(second form) ~(first form) (if (not (seq? ~(nth form 2))) ~(nth form 2) (my-recursive-infix ~(nth form 2))))) (macroexpand '(recursive-infix (10 + 10))) ;

问题 I am trying to implement a macro to recursively converting an infix list into a prefix one. I encounter a problem as follows: ;;this works (defmacro recursive-infix [form] (list (second form) (first form) (if (not (seq? (nth form 2))) (nth form 2) (recursive-infix (nth form 2))))) ;;this doesn't work (defmacro my-recursive-infix [form] `(~(second form) ~(first form) (if (not (seq? ~(nth form 2))) ~(nth form 2) (my-recursive-infix ~(nth form 2))))) (macroexpand '(recursive-infix (10 + 10))) ;

问题 I would like to simplify the following class A { int a; int b; int c; std::vector<int*> addrs; public: A() : addrs{ &a, &b, &c } {} }; so that I don't have the write the list of fields in two places, i.e. the declarations and the initializer for addrs . Is there some way to use a macro to collect the declarations and use them later. E.g., class A { VAR_DECL(a); VAR_DECL(b); VAR_DECL(c); std::vector<int*> addrs; public: A() : addrs{ VAR_ADDRESSES } {} }; For context this is intended for

问题 I'm writing a macro for function / macro composition (mixed combinations are possible). Inside of the macro I have to treat symbols which represent functions and those that name macros differently. It is because result function must work with any number of arguments (if 'lowest' function in composition can), and I cannot apply apply to macros. My question: how to determine what a given symbol represents: function or macro? 回答1: Macro: CL-USER 8 > (macro-function 'bar) NIL CL-USER 9 > (macro

问题 I want a macro to free multiple (variadic number) pointers of different type . Based on similar questions in SO I made this code which seems to work #include <stdio.h> #include <stdarg.h> #include <stdlib.h> /* your compiler may need to define i outside the loop */ #define FREE(ptr1, ...) do{\ void *elems[] = {ptr1, __VA_ARGS__};\ unsigned num = sizeof(elems) / sizeof(elems[0]);\ for (unsigned i=0; i < num; ++i) free(elems[i]);\ } while(0) int main(void) { double *x = malloc(sizeof(double));

问题 I want a macro to free multiple (variadic number) pointers of different type . Based on similar questions in SO I made this code which seems to work #include <stdio.h> #include <stdarg.h> #include <stdlib.h> /* your compiler may need to define i outside the loop */ #define FREE(ptr1, ...) do{\ void *elems[] = {ptr1, __VA_ARGS__};\ unsigned num = sizeof(elems) / sizeof(elems[0]);\ for (unsigned i=0; i < num; ++i) free(elems[i]);\ } while(0) int main(void) { double *x = malloc(sizeof(double));

问题 we have a frequently recurring lines in the code that check for the return status of a function, and if it's a failure return with the status immediately. I thought of defining a macro for this: #define RETURN_IF_FAILED(x) { int stat = (x); if (FAILED(stat)) return stat; } The local variable is because the parameter x could be a function call. As I know there are some weird artifacts of using macros and I am not very proficient in them, I'd like to ask if you see any problems with this macro.

问题 I try to find a way in C++17 to shorten following function calls: GetCurrentContext()->GetEntityManager()->CreateEntity(); maybe to something like: EM()->CreateEntity(); I tried a function pointer, but this is only valid for static functions: constexpr auto &EM = GetContext()->GetEntityManager; // error: reference to non-static member function must be called; Is there a better solution than using a Macro? #define EM GetContext()->GetEntityManager When using an inline function call, i'm afraid

问题 I've never seen this syntax before. #define SNS(s) (s),(sizeof(s)-1) The way i'm reading this is that SNS(s) = sizeof(s)-1 . What is the comma doing? Is it necessary? int ft_display_fatal(const char *err, unsigned len, int fd, int rcode) { UNUSED(write(fd, err, len)); return (rcode); } Main return (ft_display_fatal(SNS("File name missing.\n"), 2, 1)); 回答1: Macros are just text replacement, so they can expand to just about anything you want. In this case, the macro is being used to expand into