Important note: This answer was written before iOS 8 was announced. While the technical details still apply to system frameworks, it is now possible to build your own, dynamically linked frameworks that ship within your app bundle. There are restrictions, e.g., only an app and its extensions can link to the same instance of an … Read more
Aaand I figured it out. For symbols that are not functions (extern const int foobar, for instance), you have to compare against the address of the symbol, not the symbol itself, so: if (&UIApplicationWillEnterForegroundNotification != NULL) etc; Which in retrospect is kind of obvious, but I still fault the entire universe around me for not … Read more
I am puzzled by the answer given by max.haredoom (and that it was accepted). The answer deals with shared libraries and dynamic linking, whereas the question was clearly about the behavior of static linking using static libraries. I believe this is misleading. When linking static libraries, ld does not care about weak/strong symbols by default: … Read more
Go to your project -> Targets -> Build Phases -> Link Binary with Libraries. Then change the library you want to weak-link from “Required” to “Optional”.
You can do it, here is an example in C: /* * pWeakValue MUST be an extern const variable, which will be aliased to * pDefaultWeakValue if no real user definition is present, thanks to the * alternatename directive. */ extern const char * pWeakValue; extern const char * pDefaultWeakValue = NULL; #pragma comment(linker, “/alternatename:_pWeakValue=_pDefaultWeakValue”)
TLDR Current: Swift: if #available(iOS 9, *) Obj-C, iOS: if (@available(iOS 11.0, *)) Obj-C, OS X: if (NSClassFromString(@”UIAlertController”)) Legacy: Swift (versions prior to 2.0): if objc_getClass(“UIAlertController”) Obj-C, iOS (versions prior to 4.2): if (NSClassFromString(@”UIAlertController”)) Obj-C, iOS (versions prior to 11.0): if ([UIAlertController class]) Swift 2+ Although historically it’s been recommended to check for capabilities (or … Read more