related post: operator_specifier, not just a name, this post is about a question raised from it.

From [Why string concat macro doesn't work for this “+” case?](https://stackoverflow.com/questions/25072193/why-string-concat- macro-doesnt-work-for-this-case), I learnt that the preprocessing operator `##` can only produce valid preprocessing tokens from other ones. > The following tokens are considered as valid preprocessing tokens(N3797): > header-name > > identifier > pp-number > character-literal > user-defined-character-literal > string-literal > user-defined-string-literal > preprocessing-op-or-punc > each non-white-space character that cannot be one of the above (The quote above is from [Why string concat macro doesn't work for this “+” case?](https://stackoverflow.com/questions/25072193/why-string-concat-macro-doesnt-work-for-this-case))

The reason that `#define DEF(OP) #operator##OP` can't work is because operator and OP(which can be +, - ...) are 2 different tokens, so thye can't be concatenated into one valid preprocessing token.