C C macro string concatenation

Question

define STR1       quot s quot   define STR2       quot 1 quot   define STR3      STR1    STR2  Is it possible to concatenate STR1 and STR2  to  quot s1 quot   You can do this by passing args to another Macro function  But is there a direct way

User · Accepted Answer

If they re both strings you can just do   define STR3 STR1 STR2  This then expands to   define STR3  quot s quot   quot 1 quot   and in the C language  separating two strings with space as in  quot s quot   quot 1 quot  is exactly equivalent to having a single string  quot s1 quot

User · Answer

You don t need that sort of solution for string literals  since they are concatenated at the language level  and it wouldn t work anyway because  s  1  isn t a valid preprocessor token    Edit  In response to the incorrect  Just for the record  comment below that unfortunately received several upvotes  I will reiterate the statement above and observe that the program fragment   define PPCAT NX A  B  A    B PPCAT NX  s    1     produces this error message from the preprocessing phase of gcc  error  pasting   s   and   1   does not give a valid preprocessing token      However  for general token pasting  try this         Concatenate preprocessor tokens A and B without expanding macro definitions     however  if invoked from a macro  macro arguments are expanded        define PPCAT NX A  B  A    B        Concatenate preprocessor tokens A and B after macro-expanding them       define PPCAT A  B  PPCAT NX A  B    Then  e g   both PPCAT NX s  1  and PPCAT s  1  produce the identifier s1  unless s is defined as a macro  in which case PPCAT s  1  produces  lt macro value of s gt 1   Continuing on the theme are these macros         Turn A into a string literal without expanding macro definitions     however  if invoked from a macro  macro arguments are expanded        define STRINGIZE NX A   A        Turn A into a string literal after macro-expanding it       define STRINGIZE A  STRINGIZE NX A    Then    define T1 s  define T2 1 STRINGIZE PPCAT T1  T2      produces  s1    By contrast   STRINGIZE PPCAT NX T1  T2      produces  T1T2  STRINGIZE NX PPCAT NX T1  T2      produces  PPCAT NX T1  T2     define T1T2 visit the zoo STRINGIZE PPCAT NX T1  T2      produces  visit the zoo  STRINGIZE NX PPCAT T1  T2      produces  PPCAT T1  T2

User · Answer

Hint   The STRINGIZE macro above is cool  but if you make a mistake and its argument isn t a macro - you had a typo in the name  or forgot to  include the header file - then the compiler will happily put the purported macro name into the string with no error   If you intend that the argument to STRINGIZE is always a macro with a normal C value  then   define STRINGIZE A    A  STRINGIZE NX A     will expand it once and check it for validity  discard that  and then expand it again into a string   It took me a while to figure out why STRINGIZE ENOENT  was ending up as  ENOENT  instead of  2     I hadn t included errno h

[c++] C/C++ macro string concatenation

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