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| <a name="Type-encoding-1"></a> |
| <h3 class="section">8.3 Type encoding</h3> |
| |
| <p>This is an advanced section. Type encodings are used extensively by |
| the compiler and by the runtime, but you generally do not need to know |
| about them to use Objective-C. |
| </p> |
| <p>The Objective-C compiler generates type encodings for all the types. |
| These type encodings are used at runtime to find out information about |
| selectors and methods and about objects and classes. |
| </p> |
| <p>The types are encoded in the following way: |
| </p> |
| |
| <table> |
| <tr><td width="25%"><code>_Bool</code></td><td width="75%"><code>B</code></td></tr> |
| <tr><td width="25%"><code>char</code></td><td width="75%"><code>c</code></td></tr> |
| <tr><td width="25%"><code>unsigned char</code></td><td width="75%"><code>C</code></td></tr> |
| <tr><td width="25%"><code>short</code></td><td width="75%"><code>s</code></td></tr> |
| <tr><td width="25%"><code>unsigned short</code></td><td width="75%"><code>S</code></td></tr> |
| <tr><td width="25%"><code>int</code></td><td width="75%"><code>i</code></td></tr> |
| <tr><td width="25%"><code>unsigned int</code></td><td width="75%"><code>I</code></td></tr> |
| <tr><td width="25%"><code>long</code></td><td width="75%"><code>l</code></td></tr> |
| <tr><td width="25%"><code>unsigned long</code></td><td width="75%"><code>L</code></td></tr> |
| <tr><td width="25%"><code>long long</code></td><td width="75%"><code>q</code></td></tr> |
| <tr><td width="25%"><code>unsigned long long</code></td><td width="75%"><code>Q</code></td></tr> |
| <tr><td width="25%"><code>float</code></td><td width="75%"><code>f</code></td></tr> |
| <tr><td width="25%"><code>double</code></td><td width="75%"><code>d</code></td></tr> |
| <tr><td width="25%"><code>long double</code></td><td width="75%"><code>D</code></td></tr> |
| <tr><td width="25%"><code>void</code></td><td width="75%"><code>v</code></td></tr> |
| <tr><td width="25%"><code>id</code></td><td width="75%"><code>@</code></td></tr> |
| <tr><td width="25%"><code>Class</code></td><td width="75%"><code>#</code></td></tr> |
| <tr><td width="25%"><code>SEL</code></td><td width="75%"><code>:</code></td></tr> |
| <tr><td width="25%"><code>char*</code></td><td width="75%"><code>*</code></td></tr> |
| <tr><td width="25%"><code>enum</code></td><td width="75%">an <code>enum</code> is encoded exactly as the integer type that the compiler uses for it, which depends on the enumeration |
| values. Often the compiler users <code>unsigned int</code>, which is then encoded as <code>I</code>.</td></tr> |
| <tr><td width="25%">unknown type</td><td width="75%"><code>?</code></td></tr> |
| <tr><td width="25%">Complex types</td><td width="75%"><code>j</code> followed by the inner type. For example <code>_Complex double</code> is encoded as "jd".</td></tr> |
| <tr><td width="25%">bit-fields</td><td width="75%"><code>b</code> followed by the starting position of the bit-field, the type of the bit-field and the size of the bit-field (the bit-fields encoding was changed from the NeXT’s compiler encoding, see below)</td></tr> |
| </table> |
| |
| |
| <p>The encoding of bit-fields has changed to allow bit-fields to be |
| properly handled by the runtime functions that compute sizes and |
| alignments of types that contain bit-fields. The previous encoding |
| contained only the size of the bit-field. Using only this information |
| it is not possible to reliably compute the size occupied by the |
| bit-field. This is very important in the presence of the Boehm’s |
| garbage collector because the objects are allocated using the typed |
| memory facility available in this collector. The typed memory |
| allocation requires information about where the pointers are located |
| inside the object. |
| </p> |
| <p>The position in the bit-field is the position, counting in bits, of the |
| bit closest to the beginning of the structure. |
| </p> |
| <p>The non-atomic types are encoded as follows: |
| </p> |
| |
| <table> |
| <tr><td width="20%">pointers</td><td width="80%">‘<samp>^</samp>’ followed by the pointed type.</td></tr> |
| <tr><td width="20%">arrays</td><td width="80%">‘<samp>[</samp>’ followed by the number of elements in the array followed by the type of the elements followed by ‘<samp>]</samp>’</td></tr> |
| <tr><td width="20%">structures</td><td width="80%">‘<samp>{</samp>’ followed by the name of the structure (or ‘<samp>?</samp>’ if the structure is unnamed), the ‘<samp>=</samp>’ sign, the type of the members and by ‘<samp>}</samp>’</td></tr> |
| <tr><td width="20%">unions</td><td width="80%">‘<samp>(</samp>’ followed by the name of the structure (or ‘<samp>?</samp>’ if the union is unnamed), the ‘<samp>=</samp>’ sign, the type of the members followed by ‘<samp>)</samp>’</td></tr> |
| <tr><td width="20%">vectors</td><td width="80%">‘<samp>![</samp>’ followed by the vector_size (the number of bytes composing the vector) followed by a comma, followed by the alignment (in bytes) of the vector, followed by the type of the elements followed by ‘<samp>]</samp>’</td></tr> |
| </table> |
| |
| <p>Here are some types and their encodings, as they are generated by the |
| compiler on an i386 machine: |
| </p> |
| <br> |
| |
| <table> |
| <tr><td width="25%">Objective-C type</td><td width="75%">Compiler encoding</td></tr> |
| <tr><td width="25%"><div class="smallexample"> |
| <pre class="smallexample">int a[10]; |
| </pre></div></td><td width="75%"><code>[10i]</code></td></tr> |
| <tr><td width="25%"><div class="smallexample"> |
| <pre class="smallexample">struct { |
| int i; |
| float f[3]; |
| int a:3; |
| int b:2; |
| char c; |
| } |
| </pre></div></td><td width="75%"><code>{?=i[3f]b128i3b131i2c}</code></td></tr> |
| <tr><td width="25%"><div class="smallexample"> |
| <pre class="smallexample">int a __attribute__ ((vector_size (16))); |
| </pre></div></td><td width="75%"><code>![16,16i]</code> (alignment would depend on the machine)</td></tr> |
| </table> |
| |
| <br> |
| |
| <p>In addition to the types the compiler also encodes the type |
| specifiers. The table below describes the encoding of the current |
| Objective-C type specifiers: |
| </p> |
| <br> |
| |
| <table> |
| <tr><td width="25%">Specifier</td><td width="75%">Encoding</td></tr> |
| <tr><td width="25%"><code>const</code></td><td width="75%"><code>r</code></td></tr> |
| <tr><td width="25%"><code>in</code></td><td width="75%"><code>n</code></td></tr> |
| <tr><td width="25%"><code>inout</code></td><td width="75%"><code>N</code></td></tr> |
| <tr><td width="25%"><code>out</code></td><td width="75%"><code>o</code></td></tr> |
| <tr><td width="25%"><code>bycopy</code></td><td width="75%"><code>O</code></td></tr> |
| <tr><td width="25%"><code>byref</code></td><td width="75%"><code>R</code></td></tr> |
| <tr><td width="25%"><code>oneway</code></td><td width="75%"><code>V</code></td></tr> |
| </table> |
| |
| <br> |
| |
| <p>The type specifiers are encoded just before the type. Unlike types |
| however, the type specifiers are only encoded when they appear in method |
| argument types. |
| </p> |
| <p>Note how <code>const</code> interacts with pointers: |
| </p> |
| <br> |
| |
| <table> |
| <tr><td width="25%">Objective-C type</td><td width="75%">Compiler encoding</td></tr> |
| <tr><td width="25%"><div class="smallexample"> |
| <pre class="smallexample">const int |
| </pre></div></td><td width="75%"><code>ri</code></td></tr> |
| <tr><td width="25%"><div class="smallexample"> |
| <pre class="smallexample">const int* |
| </pre></div></td><td width="75%"><code>^ri</code></td></tr> |
| <tr><td width="25%"><div class="smallexample"> |
| <pre class="smallexample">int *const |
| </pre></div></td><td width="75%"><code>r^i</code></td></tr> |
| </table> |
| |
| <br> |
| |
| <p><code>const int*</code> is a pointer to a <code>const int</code>, and so is |
| encoded as <code>^ri</code>. <code>int* const</code>, instead, is a <code>const</code> |
| pointer to an <code>int</code>, and so is encoded as <code>r^i</code>. |
| </p> |
| <p>Finally, there is a complication when encoding <code>const char *</code> |
| versus <code>char * const</code>. Because <code>char *</code> is encoded as |
| <code>*</code> and not as <code>^c</code>, there is no way to express the fact |
| that <code>r</code> applies to the pointer or to the pointee. |
| </p> |
| <p>Hence, it is assumed as a convention that <code>r*</code> means <code>const |
| char *</code> (since it is what is most often meant), and there is no way to |
| encode <code>char *const</code>. <code>char *const</code> would simply be encoded |
| as <code>*</code>, and the <code>const</code> is lost. |
| </p> |
| <table class="menu" border="0" cellspacing="0"> |
| <tr><td align="left" valign="top">• <a href="Legacy-type-encoding.html#Legacy-type-encoding" accesskey="1">Legacy type encoding</a>:</td><td> </td><td align="left" valign="top"> |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="_0040encode.html#g_t_0040encode" accesskey="2">@encode</a>:</td><td> </td><td align="left" valign="top"> |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="Method-signatures.html#Method-signatures" accesskey="3">Method signatures</a>:</td><td> </td><td align="left" valign="top"> |
| </td></tr> |
| </table> |
| |
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