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| <title>Using the GNU Compiler Collection (GCC): C++ Attributes</title> |
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| <a name="C_002b_002b-Attributes"></a> |
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| <a name="C_002b_002b_002dSpecific-Variable_002c-Function_002c-and-Type-Attributes"></a> |
| <h3 class="section">7.7 C++-Specific Variable, Function, and Type Attributes</h3> |
| |
| <p>Some attributes only make sense for C++ programs. |
| </p> |
| <dl compact="compact"> |
| <dt><code>abi_tag ("<var>tag</var>", ...)</code></dt> |
| <dd><a name="index-abi_005ftag-function-attribute"></a> |
| <a name="index-abi_005ftag-variable-attribute"></a> |
| <a name="index-abi_005ftag-type-attribute"></a> |
| <p>The <code>abi_tag</code> attribute can be applied to a function, variable, or class |
| declaration. It modifies the mangled name of the entity to |
| incorporate the tag name, in order to distinguish the function or |
| class from an earlier version with a different ABI; perhaps the class |
| has changed size, or the function has a different return type that is |
| not encoded in the mangled name. |
| </p> |
| <p>The attribute can also be applied to an inline namespace, but does not |
| affect the mangled name of the namespace; in this case it is only used |
| for <samp>-Wabi-tag</samp> warnings and automatic tagging of functions and |
| variables. Tagging inline namespaces is generally preferable to |
| tagging individual declarations, but the latter is sometimes |
| necessary, such as when only certain members of a class need to be |
| tagged. |
| </p> |
| <p>The argument can be a list of strings of arbitrary length. The |
| strings are sorted on output, so the order of the list is |
| unimportant. |
| </p> |
| <p>A redeclaration of an entity must not add new ABI tags, |
| since doing so would change the mangled name. |
| </p> |
| <p>The ABI tags apply to a name, so all instantiations and |
| specializations of a template have the same tags. The attribute will |
| be ignored if applied to an explicit specialization or instantiation. |
| </p> |
| <p>The <samp>-Wabi-tag</samp> flag enables a warning about a class which does |
| not have all the ABI tags used by its subobjects and virtual functions; for users with code |
| that needs to coexist with an earlier ABI, using this option can help |
| to find all affected types that need to be tagged. |
| </p> |
| <p>When a type involving an ABI tag is used as the type of a variable or |
| return type of a function where that tag is not already present in the |
| signature of the function, the tag is automatically applied to the |
| variable or function. <samp>-Wabi-tag</samp> also warns about this |
| situation; this warning can be avoided by explicitly tagging the |
| variable or function or moving it into a tagged inline namespace. |
| </p> |
| </dd> |
| <dt><code>init_priority (<var>priority</var>)</code></dt> |
| <dd><a name="index-init_005fpriority-variable-attribute"></a> |
| |
| <p>In Standard C++, objects defined at namespace scope are guaranteed to be |
| initialized in an order in strict accordance with that of their definitions |
| <em>in a given translation unit</em>. No guarantee is made for initializations |
| across translation units. However, GNU C++ allows users to control the |
| order of initialization of objects defined at namespace scope with the |
| <code>init_priority</code> attribute by specifying a relative <var>priority</var>, |
| a constant integral expression currently bounded between 101 and 65535 |
| inclusive. Lower numbers indicate a higher priority. |
| </p> |
| <p>In the following example, <code>A</code> would normally be created before |
| <code>B</code>, but the <code>init_priority</code> attribute reverses that order: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">Some_Class A __attribute__ ((init_priority (2000))); |
| Some_Class B __attribute__ ((init_priority (543))); |
| </pre></div> |
| |
| <p>Note that the particular values of <var>priority</var> do not matter; only their |
| relative ordering. |
| </p> |
| </dd> |
| <dt><code>java_interface</code></dt> |
| <dd><a name="index-java_005finterface-type-attribute"></a> |
| |
| <p>This type attribute informs C++ that the class is a Java interface. It may |
| only be applied to classes declared within an <code>extern "Java"</code> block. |
| Calls to methods declared in this interface are dispatched using GCJ’s |
| interface table mechanism, instead of regular virtual table dispatch. |
| </p> |
| </dd> |
| <dt><code>warn_unused</code></dt> |
| <dd><a name="index-warn_005funused-type-attribute"></a> |
| |
| <p>For C++ types with non-trivial constructors and/or destructors it is |
| impossible for the compiler to determine whether a variable of this |
| type is truly unused if it is not referenced. This type attribute |
| informs the compiler that variables of this type should be warned |
| about if they appear to be unused, just like variables of fundamental |
| types. |
| </p> |
| <p>This attribute is appropriate for types which just represent a value, |
| such as <code>std::string</code>; it is not appropriate for types which |
| control a resource, such as <code>std::mutex</code>. |
| </p> |
| <p>This attribute is also accepted in C, but it is unnecessary because C |
| does not have constructors or destructors. |
| </p> |
| </dd> |
| </dl> |
| |
| <p>See also <a href="Namespace-Association.html#Namespace-Association">Namespace Association</a>. |
| </p> |
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