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| <h4 class="subsection">8.3.3 Method signatures</h4> |
| |
| <p>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. |
| |
| <p>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. |
| |
| <p>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. |
| |
| <p>The “signature” is a null-terminated string, composed of the following: |
| |
| <ul> |
| <li>The return type, including type qualifiers. For example, a method |
| returning <code>int</code> would have <code>i</code> here. |
| |
| <li>The total size (in bytes) required to pass all the parameters. This |
| includes the two hidden parameters (the object <code>self</code> and the |
| method selector <code>_cmd</code>). |
| |
| <li>Each argument, with the type encoding, followed by the offset (in |
| bytes) of the argument in the list of parameters. |
| |
| </ul> |
| |
| <p>For example, a method with no arguments and returning <code>int</code> would |
| have the signature <code>i8@0:4</code> if the size of a pointer is 4. The |
| signature is interpreted as follows: the <code>i</code> is the return type |
| (an <code>int</code>), the <code>8</code> is the total size of the parameters in |
| bytes (two pointers each of size 4), the <code>@0</code> is the first |
| parameter (an object at byte offset <code>0</code>) and <code>:4</code> is the |
| second parameter (a <code>SEL</code> at byte offset <code>4</code>). |
| |
| <p>You can easily find more examples by running the “strings” program |
| on an Objective-C object file compiled by GCC. You'll see a lot of |
| strings that look very much like <code>i8@0:4</code>. They are signatures |
| of Objective-C methods. |
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