Macro expansion in C++

问题

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;

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

回答1:

## 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.

回答2:

You only need ## to append a parameter to another string. Your macro can be recast as

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

来源：https://stackoverflow.com/questions/672618/macro-expansion-in-c