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Document the redeclaration and overriding restrictions on the 

availability attribute.

llvm-svn: 172587
This commit is contained in:
Douglas Gregor 2013-01-16 01:12:31 +00:00
parent 43dc0c7d60
commit 250ee63515
1 changed files with 28 additions and 1 deletions
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29
clang/docs/LanguageExtensions.rst
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@ -439,7 +439,7 @@ succeeds but Clang emits a warning specifying that the function is deprecated.
Finally, if Clang is instructed to compile code for Mac OS X 10.7, the call
fails because ``f()`` is no longer available.
The availablility attribute is a comma-separated list starting with the
The availability attribute is a comma-separated list starting with the
platform name and then including clauses specifying important milestones in the
declaration's lifetime (in any order) along with additional information. Those
clauses can be:
@ -488,6 +488,33 @@ weakly-linked declaration may or may not be present a run-time, and a program
can determine whether the declaration is present by checking whether the
address of that declaration is non-NULL.
If there a multiple declarations of the same entity, the availability
attributes must either match on a per-platform basis or later
declarations must not have availability attributes for that
platform. For example:
.. code-block:: c
void g(void) __attribute__((availability(macosx,introduced=10.4)));
void g(void) __attribute__((availability(macosx,introduced=10.4))); // okay, matches
void g(void) __attribute__((availability(ios,introduced=4.0))); // okay, adds a new platform
void g(void); // okay, inherits both macosx and ios availability from above.
void g(void) __attribute__((availability(macosx,introduced=10.5))); // error: mismatch
When one method overrides another, the overriding method can be more widely available than the overridden method, e.g.,:
.. code-block:: objc
@interface A
- (id)method __attribute__((availability(macosx,introduced=10.4)));
- (id)method2 __attribute__((availability(macosx,introduced=10.4)));
@end
@interface B : A
- (id)method __attribute__((availability(macosx,introduced=10.3))); // okay: method moved into base class later
- (id)method __attribute__((availability(macosx,introduced=10.5))); // error: this method was available via the base class in 10.4
@end
Checks for Standard Language Features
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