Reason to include "-ObjC" in Xcode

Question

We include "-ObjC"in other linker flags of project settings in Xcode if we include any external libraries or frameworks can any one tell me what is the reason for it and what it does if we include it.

Thanks in Advance..

Answer 1

It's explained pretty well in Apple's QA1490.

Key excerpts:

Objective-C does not define linker symbols for each function (or method, in Objective-C) - instead, linker symbols are only generated for each class.

...

To resolve this issue, the target linking against the static library must pass the -ObjC option to the linker. This flag causes the linker to load every object file in the library that defines an Objective-C class or category

Answer 2

Categories are most known use case for this flag (as Apple's QA1490 explains), but they are not the only reason why this flag is needed.

Since Objective-C is a dynamic language which makes things like NSClassFromString() possible, the standard behaviour of C linker of including only actually used symbols (and throwing away all others) does not work, because there is no way to find out whether some symbols are actually used in some indirect ways like concatenating strings then calling NSSelectorFromString or NSClassFromString.

Consider xib/nib files for example: they don't participate in linking stage, yet they may instantiate library classes which are not referred by anything but this nib file. If linker threw away those classes as unused, the nib couldn't load and work properly.

The only way to ensure nothing is lost is to include every Obj-C class from a library at link time, and this flag does that.

Answer 3

-ObjC linker flag causes the linker to load ever object file in the library that defines an Objective-C class or category.

Why to use -ObjC linker flag - Apple Documentation?