The magical usage of "#" and "# #" in C + + macro definition

In C + + development often used in the definition of macro use "#" or "# #", the following is a simple description of the use of two symbols:

Define in the # is to use the argument after the # as a symbol, simply to connect it as a string, # #连接前后2个符号, the define definition in the beginning of the "#" macro substitution is not expanded, the macro beginning with the non-# expansion and replace.

As follows:

#define F (A, b) a# #b

#define D (a) #a

#define S (a) d (a)

Puts (d (f (b))); Output result: F (A, B)

Puts (S (f (b))); Output Result: AB

The process is expanded as follows:

Puts (d (f (b))); ----> because the definition of the D macro begins with #, the parameter in the macro is another macro with # #, so the macro as a parameter does not expand, equivalent to puts (#f (A, A, b));----->puts ("f (A, b)");

Puts (S (f (b))); ----> because the definition of S macro does not start with #, the argument in the macro is another macro, but without # #, so the macro as a parameter expands first, equivalent to

Puts (S (AB)),----->puts (d (AB)),---->puts (#ab),---->puts ("AB");

In summary there are the following 2 cases: 1. Do not start with "#", expand Parameter A, and then replace the code:

Puts (S (f (b)))-->puts (S (AB))-->puts (d (AB))-->puts ("AB")

2. Start with "#", replace directly, do not expand:

Puts (d (f (a)))-->puts ("F (A, b)")

The magical usage of "#" and "# #" in C + + macro definition