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Concatenating Tokens
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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 \[[FSF 05|AA. C References#FSF 05]\]. |
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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 \[[FSF 05|AA. C References#FSF 05]\]. |
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The following definition for static_assert() from DCL03-A. Use a static assertion to test the value of a constant expression uses the JOIN() macro to concatenate the token assertion_failed_at_line_ with the value of __LINE__.
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#define static_assert(e) \
typedef char JOIN(assertion_failed_at_line_, __LINE__) \
[(e) ? 1 : -1]
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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 \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]. |
If the intention is to expand the __LINE__ macro, which is likely the case here, the following definition for JOIN() is non-compliant:
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#define JOIN(x, y) x ## y |
because the __LINE__ is not expanded, and the character array is subsequently named assertion_failed_at_line___LINE__.
Compliant Solution
To get the macro to expand, a second level of indirection is required, as shown by this compliant solution:
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\[[FSF 05|AA. C References#FSF 05]\] 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]"
\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] 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"
\[[Saks 08|AA. C References#Saks 08]\] Dan Saks, Stephen C. Dewhurst. Presentation. Sooner Rather Than Later: Static Programming Techniques for C++. |
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PRE04-A. Do not reuse a standard header file name 01. Preprocessor (PRE) PRE06-A. Enclose header files in an inclusion guard