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8.3.3 Method Signatures

This section documents the encoding of method types, which is rarely needed to use Objective-C. You should skip it at a first reading; the runtime provides functions that will work on methods and can walk through the list of parameters and interpret them for you. These functions are part of the public API and are the preferred way to interact with method signatures from user code.

But if you need to debug a problem with method signatures and need to know how they are implemented (i.e., the ABI), read on.

Methods have their signature encoded and made available to the runtime. The signature encodes all the information required to dynamically build invocations of the method at runtime: return type and arguments.

The signature is a null-terminated string, composed of the following:

For example, a method with no arguments and returning int would have the signature i8@0:4 if the size of a pointer is 4. The signature is interpreted as follows: the i is the return type (an int), the 8 is the total size of the parameters in bytes (two pointers each of size 4), the @0 is the first parameter (an object at byte offset 0) and :4 is the second parameter (a SEL at byte offset 4).

You can easily find more examples by running the strings program on an Objective-C object file compiled by GCC. Youll see a lot of strings that look very much like i8@0:4. They are signatures of Objective-C methods.

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