I know that:

#define foo 4  
#define str(s) #s

with str(foo) writes out: "foo", because stringify is executed first of text expansion, but this:

 #define xstr(s) str(s)
 #define str(s) #s
 #define foo 4

with xstr(foo) writes out: "4".

Why? What are the steps involved in the process?

c++ c c-preprocessor stringification
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