在C ++中的宏扩展 [英] Macro expansion in C++

问题描述

如何定义参数位于行开头的宏（或变通方法）？

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）：'##'结束宏扩展

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;编译器只是抱怨这一点。

你不能连接一个令牌前面没有东西，即在宏展开的开始;只需尝试删除第二行开头的 ## 。

第二个 ## 似乎错了。如果你只是想初始化单例指针，从你的宏中删除 ## 。

## 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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