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The {{\##}} preprocessing operator is used to merge two tokens into one while expanding macros. This 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 replaces the {{\##}} and the two original tokens in the macro expansion \[[FSF 05|AA. References#FSFBibliography#FSF 05]\]. |
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 ## 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 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 \[[FSF 05|AA. References#FSFBibliography#FSF 05]\]. |
Noncompliant Code Example
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{{\_\_LINE\_\_}} is a predefined macro names which expands to an integer constant representing the presumed line number of the current source line within the current source file \[[ISO/IEC 9899:1999|AA. References#ISOBibliography#ISO/IEC 9899-1999]\]. |
If the intention is to expand the __LINE__ macro, which is likely the case here, the following definition for JOIN() is noncompliant:
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References
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\[[FSF 05|AA. References#FSFBibliography#FSF 05]\] Section 3.4, "[Stringification|http://gcc.gnu.org/onlinedocs/cpp/Stringification.html]" and Section 3.5, "[Concatenation|http://gcc.gnu.org/onlinedocs/gcc-4.3.0/cpp/Concatenation.html#Concatenation]" \[[ISO/IEC 9899:1999|AA. References#ISOBibliography#ISO/IEC 9899-1999]\] Section 6.10.3, "Macro replacement," Section 6.10.3.3, "The {{\##}} operator," Section 6.10.3.2, "The {{\#}} operator," Section 6.10.3.4, "Rescanning and further replacement," and Section 6.10.8, "Predefined macro names" \[[Saks 08|AA. References#SaksBibliography#Saks 08]\] |
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01. Preprocessor (PRE) PRE06-C. Enclose header files in an inclusion guard