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145 lines
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145 lines
7.4 KiB
HTML
4 years ago
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<title>C++ Attributes - Using the GNU Compiler Collection (GCC)</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<link rel="up" href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" title="C++ Extensions">
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<link rel="prev" href="Bound-member-functions.html#Bound-member-functions" title="Bound member functions">
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<link rel="next" href="Function-Multiversioning.html#Function-Multiversioning" title="Function Multiversioning">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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</head>
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<body>
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<div class="node">
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<a name="C++-Attributes"></a>
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<a name="C_002b_002b-Attributes"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Function-Multiversioning.html#Function-Multiversioning">Function Multiversioning</a>,
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Previous: <a rel="previous" accesskey="p" href="Bound-member-functions.html#Bound-member-functions">Bound member functions</a>,
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Up: <a rel="up" accesskey="u" href="C_002b_002b-Extensions.html#C_002b_002b-Extensions">C++ Extensions</a>
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<hr>
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</div>
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<h3 class="section">7.7 C++-Specific Variable, Function, and Type Attributes</h3>
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<p>Some attributes only make sense for C++ programs.
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<dl>
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<dt><code>abi_tag ("</code><var>tag</var><code>", ...)</code><dd><a name="index-g_t_0040code_007babi_005ftag_007d-function-attribute-4229"></a><a name="index-g_t_0040code_007babi_005ftag_007d-variable-attribute-4230"></a><a name="index-g_t_0040code_007babi_005ftag_007d-type-attribute-4231"></a>The <code>abi_tag</code> attribute can be applied to a function, variable, or class
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declaration. It modifies the mangled name of the entity to
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incorporate the tag name, in order to distinguish the function or
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class from an earlier version with a different ABI; perhaps the class
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has changed size, or the function has a different return type that is
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not encoded in the mangled name.
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<p>The attribute can also be applied to an inline namespace, but does not
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affect the mangled name of the namespace; in this case it is only used
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for <samp><span class="option">-Wabi-tag</span></samp> warnings and automatic tagging of functions and
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variables. Tagging inline namespaces is generally preferable to
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tagging individual declarations, but the latter is sometimes
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necessary, such as when only certain members of a class need to be
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tagged.
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<p>The argument can be a list of strings of arbitrary length. The
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strings are sorted on output, so the order of the list is
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unimportant.
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<p>A redeclaration of an entity must not add new ABI tags,
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since doing so would change the mangled name.
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<p>The ABI tags apply to a name, so all instantiations and
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specializations of a template have the same tags. The attribute will
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be ignored if applied to an explicit specialization or instantiation.
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<p>The <samp><span class="option">-Wabi-tag</span></samp> flag enables a warning about a class which does
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not have all the ABI tags used by its subobjects and virtual functions; for users with code
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that needs to coexist with an earlier ABI, using this option can help
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to find all affected types that need to be tagged.
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<p>When a type involving an ABI tag is used as the type of a variable or
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return type of a function where that tag is not already present in the
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signature of the function, the tag is automatically applied to the
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variable or function. <samp><span class="option">-Wabi-tag</span></samp> also warns about this
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situation; this warning can be avoided by explicitly tagging the
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variable or function or moving it into a tagged inline namespace.
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<br><dt><code>init_priority (</code><var>priority</var><code>)</code><dd><a name="index-g_t_0040code_007binit_005fpriority_007d-variable-attribute-4232"></a>
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In Standard C++, objects defined at namespace scope are guaranteed to be
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initialized in an order in strict accordance with that of their definitions
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<em>in a given translation unit</em>. No guarantee is made for initializations
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across translation units. However, GNU C++ allows users to control the
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order of initialization of objects defined at namespace scope with the
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<code>init_priority</code> attribute by specifying a relative <var>priority</var>,
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a constant integral expression currently bounded between 101 and 65535
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inclusive. Lower numbers indicate a higher priority.
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<p>In the following example, <code>A</code> would normally be created before
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<code>B</code>, but the <code>init_priority</code> attribute reverses that order:
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<pre class="smallexample"> Some_Class A __attribute__ ((init_priority (2000)));
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Some_Class B __attribute__ ((init_priority (543)));
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</pre>
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<p class="noindent">Note that the particular values of <var>priority</var> do not matter; only their
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relative ordering.
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<br><dt><code>java_interface</code><dd><a name="index-g_t_0040code_007bjava_005finterface_007d-type-attribute-4233"></a>
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This type attribute informs C++ that the class is a Java interface. It may
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only be applied to classes declared within an <code>extern "Java"</code> block.
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Calls to methods declared in this interface are dispatched using GCJ's
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interface table mechanism, instead of regular virtual table dispatch.
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<br><dt><code>warn_unused</code><dd><a name="index-g_t_0040code_007bwarn_005funused_007d-type-attribute-4234"></a>
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For C++ types with non-trivial constructors and/or destructors it is
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impossible for the compiler to determine whether a variable of this
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type is truly unused if it is not referenced. This type attribute
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informs the compiler that variables of this type should be warned
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about if they appear to be unused, just like variables of fundamental
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types.
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<p>This attribute is appropriate for types which just represent a value,
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such as <code>std::string</code>; it is not appropriate for types which
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control a resource, such as <code>std::mutex</code>.
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<p>This attribute is also accepted in C, but it is unnecessary because C
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does not have constructors or destructors.
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</dl>
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<p>See also <a href="Namespace-Association.html#Namespace-Association">Namespace Association</a>.
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</body></html>
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