How, exactly, does the double-stringize trick work?

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Yes, it’s guaranteed.

It works because arguments to macros are themselves macro-expanded, except where the macro argument name appears in the macro body with the stringifier # or the token-paster ##.

6.10.3.1/1:

… After the arguments for the
invocation of a function-like macro
have been identified, 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…

So, if you do STR1(THE_ANSWER) then you get “THE_ANSWER”, because the argument of STR1 is not macro-expanded. However, the argument of STR2 is macro-expanded when it’s substituted into the definition of STR2, which therefore gives STR1 an argument of 42, with the result of “42”.

More Related Contents:

  1. What are the applications of the ## preprocessor operator and gotchas to consider?
  2. Convert a preprocessor token to a string
  3. What does ## mean for the C(C++) preprocessor?
  4. # and ## in macros
  5. Concatenate int to string using C Preprocessor
  6. Opposite of C preprocessor “stringification”
  7. C Macros to create strings
  8. Macros to create strings in C
  9. The need for parentheses in macros in C [duplicate]
  10. #define macro for debug printing in C?
  11. #pragma pack effect
  12. Real-world use of X-Macros
  13. Easy way to use variables of enum types as string in C?
  14. MIN and MAX in C
  15. How do I check OS with a preprocessor directive?
  16. Stringification of a macro value
  17. Stringification – how does it work?
  18. What is the meaning of lines starting with a hash sign and number like ‘# 1 “a.c”‘ in the gcc preprocessor output?
  19. Macro vs Function in C
  20. How to compare strings in C conditional preprocessor-directives
  21. Array-size macro that rejects pointers
  22. Understanding container_of macro in the Linux kernel
  23. Can gcc output C code after preprocessing?
  24. Why does the C preprocessor interpret the word “linux” as the constant “1”?
  25. Finding out what the GCC include path is [duplicate]
  26. Type of #define variables
  27. C circular dependency
  28. Pseudo-generics in C
  29. What is the possible use for “#define for if (false) {} else for”?
  30. Using Parentheses in Define Preprocessor Statements

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