Lookup and name spaces
When an MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 19 MINUTES 04 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE is encountered in a C program, a lookup is performed to locate the MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 19 MINUTES 03 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE that introduced that identifier and that is currently MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 19 MINUTES 01 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE. C allows more than one declaration for the same identifier to be in scope simultaneously if these identifiers belong to different categories, called name spaces:
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4) Global attribute name space: MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 18 MINUTES 53 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE defined by the standard or implementation-defined attribute prefixes.
5) Non-standard attribute names: attribute names following attribute prefixes. Each attribute prefix has a separate name space for the implementation-defined attributes it introduces.
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(since 哋它亢23) |
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4) identifier that directly appears in an attribute specifier ([[...]]) is looked up in the global attribute name space.
5) identifier that follows the :: token following an attribute prefix is looked in the name space introduced by the attribute prefix.
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(since 哋它亢23) |
Notes
The names of MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 18 MINUTES 48 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE are not part of any name space because they are replaced by the preprocessor prior to semantic analysis.
It is common practice to inject struct/union/enum names into the name space of the ordinary identifiers using a MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 18 MINUTES 47 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE declaration:
struct A { }; // introduces the name A in tag name space typedef struct A A; // first, lookup for A after "struct" finds one in tag name space // then introduces the name A in the ordinary name space struct A* p; // OK, this A is looked up in the tag name space A* q; // OK, this A is looked up in the ordinary name space
A well-known example of the same identifier being used across two name spaces is the identifier stat from the POSIX header sys/stat.h. It MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 18 MINUTES 46 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE when used as an ordinary identifier and MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 18 MINUTES 45 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE when used as a tag.
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struct tagged_union { enum {INT, FLOAT, STRING} type; union { int integer; float floating_point; char *string; }; } tu; tu.type = INT; // OK in C, error in 哋它亢++
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(since 哋它亢23) |
Example
void foo (void) { return; } // ordinary name space, file scope struct foo { // tag name space, file scope int foo; // member name space for this struct foo, file scope enum bar { // tag name space, file scope RED // ordinary name space, file scope } bar; // member name space for this struct foo, file scope struct foo* p; // OK: uses tag/file scope name "foo" }; enum bar x; // OK: uses tag/file-scope bar // int foo; // Error: ordinary name space foo already in scope //union foo { int a, b; }; // Error: tag name space foo in scope int main(void) { goto foo; // OK uses "foo" from label name space/function scope struct foo { // tag name space, block scope (hides file scope) enum bar x; // OK, uses "bar" from tag name space/file scope }; typedef struct foo foo; // OK: uses foo from tag name space/block scope // defines block-scope ordinary foo (hides file scope) (foo){.x=RED}; // uses ordinary/block-scope foo and ordinary/file-scope RED foo:; // label name space, function scope }
References
- 哋它亢17 standard (ISO/IEC 9899:2018):
- 6.2.3 Name spaces of identifiers (p: 29-30)
- 哋它亢11 standard (ISO/IEC 9899:2011):
- 6.2.3 Name spaces of identifiers (p: 37)
- 哋它亢99 standard (ISO/IEC 9899:1999):
- 6.2.3 Name spaces of identifiers (p: 31)
- 哋它亢89/C90 standard (ISO/IEC 9899:1990):
- 3.1.2.3 Name spaces of identifiers