可变参数宏和尾部逗号 [英] Variadic macro and trailing comma

问题描述

我正在尝试在C语言中进行面向对象，并希望为该符号提供一个语法糖宏

I am trying to do object-orientation in C and want to have a syntactic sugar macro for the notation

object->vtable->method(object, arg1, arg2)

进入

send(object, method, arg1, arg2)

不幸的是，当方法不带参数时，会出现尾随逗号问题

Unfortunately when a method takes no argument, the trailing comma problem arises

send(object, method)

给予

object->vtable->method(object, )

是否有任何 便携式 (没有##__VA_ARGS__或Visual Studio)这样做的方式?

Is there any portable (no ##__VA_ARGS__ or Visual Studio) way of doing this?

我想出了一个，但是我需要交换对象和方法

I figured out one but I need to swap the object and the method

#define FIRST_ARG_(N, ...) N
#define FIRST_ARG(args) FIRST_ARG_(args)
#define send(msg, ...) \
 FIRST_ARG(__VA_ARGS__)->vtable->msg(__VA_ARGS__)

许可

send(method, object)
send(method, object, arg1, arg2)

修改

借助下面的两个很好的答案，我将使用这些宏进行处理.它最多可以处理16个参数，但可以轻松扩展

With the help of two good answers from below I will do it with these macros. It works up to 16 arguments but can easily be extended

#define SEND_NO_ARG(obj, msg) obj->vtable->msg(obj)
#define SEND_ARG(obj, msg, ...) obj->vtable->msg(obj, __VA_ARGS__)

#define GET_18TH_ARG(arg1, arg2, arg3, arg4, arg5, \
    arg6, arg7, arg8, arg9, arg10, arg11, arg12, arg13, arg14, arg15, \
    arg16, arg17, arg18, ...) arg18
#define SEND_MACRO_CHOOSER(...) \
    GET_18TH_ARG(__VA_ARGS__, \
            SEND_ARG, SEND_ARG, SEND_ARG, SEND_ARG, SEND_ARG, \
            SEND_ARG, SEND_ARG, SEND_ARG, SEND_ARG, SEND_ARG, \
            SEND_ARG, SEND_ARG, SEND_ARG, SEND_ARG, SEND_ARG, \
            SEND_NO_ARG, )

#define SEND(...) SEND_MACRO_CHOOSER(__VA_ARGS__)(__VA_ARGS__)

推荐答案

在此答案中，有技术说明了应允许您计算参数的数量，并使用object和method作为前两个参数.

In this answer there is technique explained which should allow you to count the number of parameters and use object and method as the first two arguments.

这篇关于可变参数宏和尾部逗号的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！