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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 \[[FSF 052005|AA. Bibliography#FSF 05]\]. |
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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 \[[FSF 052005|AA. Bibliography#FSF 05]\]. |
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The following definition for static_assert() from guideline DCL03-C. 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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Note also that macro parameters cannot be individually parenthesized when concatenating tokens using the ## operator, converting macro parameters to strings using the # operator, or concatenating adjacent string literals. This is an exception, PRE01-EX2, to guideline PRE01-C. Use parentheses within macros around parameter names.
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Search for vulnerabilities resulting from the violation of this rule on the CERT website.
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This rule appears in the C++ Secure Coding Standard as : PRE05-CPP. Understand macro replacement when concatenating tokens or performing stringification.
Bibliography
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\[[FSF 052005|AA. Bibliography#FSF 05]\] 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]" \[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] 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" \[[Saks 082008|AA. Bibliography#Saks 08]\] |
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