[英]Macro expansion in C++
How can I define a macro (or a workaround for this) where the parameter is at the beginning of the line?如何定义参数位于行首的宏(或解决方法)?
#define SINGLETON_IMPLEMENTATION(className) \
##className* ##className::instance_ = NULL;
This give a compiler warning (GCC 3.2.3): " '##' cannot appear at either end of a macro expansion"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”
You only need ## to append a parameter to another string.你只需要## 到 append 一个参数到另一个字符串。 Your macro can be recast as
您的宏可以重铸为
#define SINGLETON_IMPLEMENTATION(className) \
className* className::instance_ = NULL;
##
is the concatenation operator; ##
是连接运算符; the compiler is just complaining about that.编译器只是在抱怨这一点。
You cannot concatenate a token without something before it, ie at the beginning of the macro expansion;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; just try to remove the
##
at the beginning of the second line.只需尝试删除第二行开头的
##
。
Also the second ##
seems wrong.另外第二个
##
似乎是错误的。 If you just want to initialize a singleton pointer, remove both ##
s from your macro.如果您只想初始化 singleton 指针,请从宏中删除两个
##
。
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