關於C++宏定義的一個疑問？

在github上看到別人寫的一段代碼：

#define PP_JOIN(X, Y) PP_DO_JOIN(X, Y)

#define PP_DO_JOIN(X, Y) PP_DO_JOIN2(X, Y)

#define PP_DO_JOIN2(X, Y) X##Y


為什麼不直接寫

#define PP_JOIN(X, Y) X##Y


呢?

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因為PP_JOIN(i_, __LINE__)想要得到i_246，而不是i___LINE__。

C++ 16.3.1

1 After the arguments for the invocation of a function-like macro have been identi?ed, argument substitution takes place. A parameter in the replacement list, unless preceded by a # or ## preprocessing token or followed by a ## preprocessing token (see below), is replaced by the corresponding argument after all macros contained therein have been expanded.

16.3.3

2 If, in the replacement list of a function-like macro, a parameter is immediately preceded or followed by a ## preprocessing token, the parameter is replaced by the corresponding argument』s preprocessing token sequence;

如果直接作為##的參數，其中的宏就不會被擴展，就像這裡的__LINE__；通過增加一層間接層，使其中的宏先行擴展，就可以得到正確的結果。

雖然我不知道為什麼要加兩層間接。。

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http://stackoverflow.com/questions/8231966/why-do-i-need-double-layer-of-indirection-for-macros

有更合適找答案的地方就應該先用

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