C++ 中的宏扩展 [英] Macro expansion in C++

问题描述

如何定义参数位于行首的宏(或解决方法)?

How can I define a macro (or a workaround for this) where the parameter is at the beginning of the line?

#define SINGLETON_IMPLEMENTATION(className) 
    ##className* ##className::instance_ = NULL;

这会给出编译器警告 (GCC 3.2.3):'##' 不能出现在宏扩展的任一端"

This give a compiler warning (GCC 3.2.3): " '##' cannot appear at either end of a macro expansion"

推荐答案

## 是连接运算符；编译器只是在抱怨这一点.
你不能在没有之前的东西的情况下连接一个标记，即在宏扩展的开头；只需尝试删除第二行开头的 ##.
第二个 ## 似乎也是错误的.如果您只想初始化一个单例指针，请从宏中删除两个 ##.

## is the concatenation operator; the compiler is just complaining about that.
You cannot concatenate a token without something before it, i.e. at the beginning of the macro expansion; just try to remove the ## at the beginning of the second line.
Also the second ## seems wrong. If you just want to initialize a singleton pointer, remove both ##s from your macro.

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