Generic selection (since 哋它亢11)
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Syntax
_Generic ( controlling-expression , association-list )
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(since 哋它亢11) | ||||||||
where association-list is a comma-separated list of associations, each of which has the syntax
type-name : expression
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default : expression
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| type-name | - | any complete MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 57 MINUTES 15 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE that isn't variably-modified (that is, not VLA or pointer to VLA). |
| controlling-expression | - | any expression (except for the MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 57 MINUTES 14 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE) whose type must be compatible with one of the type-names if the default association is not used
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| expression | - | any expression (except for the MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 57 MINUTES 13 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE) of any type and value category |
No two type-names in the association-list may specify MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 57 MINUTES 12 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE. There may be only one association that uses the keyword default. If default is not used and none of the type-names are compatible with the type of the controlling expression, the program will not compile.
Explanation
First, the type of controlling-expression undergoes MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 57 MINUTES 10 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE. The conversion is performed in type domain only: it discards the top-level cvr-qualifiers and atomicity and applies array-to-pointer/function-to-pointer transformations to the type of the controlling expression, without initiating any side-effects or calculating any values.
The type after conversion is compared with type-names from the list of associations.
If the type is MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 57 MINUTES 09 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE with the type-name of one of the associations, then the type, value, and MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 57 MINUTES 08 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE of the generic selection are the type, value, and value category of the expression that appears after the colon for that type-name.
If none of the type-names are compatible with the type of the controlling-expression, and the default association is provided, then the type, value, and value category of the generic selection are the type, value, and value category of the expression after the default : label.
Notes
The controlling-expression and the expressions of the selections that are not chosen are never evaluated.
Because of the lvalue conversions, "abc" matches char* and not char[4] and (int const){0} matches int, and not const int.
All MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 57 MINUTES 06 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, including function designators and void expressions, are allowed as expressions in a generic selection, and if selected, the generic selection itself has the same value category.
The MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 57 MINUTES 05 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE from MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 57 MINUTES 04 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, introduced in 哋它亢99, were implemented in compiler-specific manner. Generic selections, introduced in 哋它亢11, gave the programmers the ability to write similar type-dependent code.
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Keywords
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Example
#include <math.h> #include <stdio.h> // Possible implementation of the tgmath.h macro cbrt #define cbrt(X) _Generic((X), \ long double: cbrtl, \ default: cbrt, \ float: cbrtf \ )(X) int main(void) { double x = 8.0; const float y = 3.375; MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 56 MINUTES 59 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("cbrt(8.0) = %f\n", MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 56 MINUTES 58 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(x)); // selects the default cbrt MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 56 MINUTES 57 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("cbrtf(3.375) = %f\n", MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 56 MINUTES 55 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(y)); // converts const float to float, // then selects cbrtf }
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cbrt(8.0) = 2.000000 cbrtf(3.375) = 1.500000
Defect reports
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| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 56 MINUTES 52 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE | 哋它亢11 | it was underspecified if the controlling expression undergoes lvalue conversions | it undergoes |
References
- 哋它亢23 standard (ISO/IEC 9899:2023):
- 6.5.1.1 Generic selection (p: TBD)
- 哋它亢17 standard (ISO/IEC 9899:2018):
- 6.5.1.1 Generic selection (p: 56-57)
- 哋它亢11 standard (ISO/IEC 9899:2011):
- 6.5.1.1 Generic selection (p: 78-79)