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The ## preprocessing operator is used to merge two tokens into one while expanding macros. This , which is called token pasting or token concatenation. When a macro is expanded, the two tokens on either side of each ## operator are combined into a single token , which that replaces the ## and the two original tokens in the macro expansion [FSF 2005].
Token pasting is most useful when one or both of the tokens come from a macro argument. If either of the tokens next to an a ## is a parameter name, it is replaced by its actual argument before ## executes. The actual argument is not macro expanded first.
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Parameters are not replaced inside string constants, but you can use the # preprocessing operator can be used instead. When a macro parameter is used with a leading #, the preprocessor replaces it with the literal text of the actual argument converted to a string constant [FSF 2005].
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To stringify the result of expansion of a macro argument, you must use two levels of macros must be used:
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#define xstr(s) str(s) #define str(s) #s #define foo 4 |
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