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179 lines
8.4 KiB
HTML
179 lines
8.4 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<!-- Copyright (C) 1988-2018 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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Invariant Sections being "Funding Free Software", the Front-Cover
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(see below). A copy of the license is included in the section entitled
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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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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>C++ Attributes (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="C++ Attributes (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="C++ Attributes (Using the GNU Compiler Collection (GCC))">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" rel="up" title="C++ Extensions">
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<link href="Function-Multiversioning.html#Function-Multiversioning" rel="next" title="Function Multiversioning">
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<link href="Bound-member-functions.html#Bound-member-functions" rel="prev" title="Bound member functions">
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</head>
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<body lang="en">
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<a name="C_002b_002b-Attributes"></a>
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<div class="header">
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<p>
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Next: <a href="Function-Multiversioning.html#Function-Multiversioning" accesskey="n" rel="next">Function Multiversioning</a>, Previous: <a href="Bound-member-functions.html#Bound-member-functions" accesskey="p" rel="prev">Bound member functions</a>, Up: <a href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" accesskey="u" rel="up">C++ Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="C_002b_002b_002dSpecific-Variable_002c-Function_002c-and-Type-Attributes"></a>
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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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</p>
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<dl compact="compact">
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<dt><code>abi_tag ("<var>tag</var>", ...)</code></dt>
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<dd><a name="index-abi_005ftag-function-attribute"></a>
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<a name="index-abi_005ftag-variable-attribute"></a>
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<a name="index-abi_005ftag-type-attribute"></a>
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<p>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>
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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>-Wabi-tag</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>
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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>
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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>
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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>
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<p>The <samp>-Wabi-tag</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>
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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>-Wabi-tag</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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</p>
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</dd>
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<dt><code>init_priority (<var>priority</var>)</code></dt>
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<dd><a name="index-init_005fpriority-variable-attribute"></a>
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<p>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>
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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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</p>
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<div class="smallexample">
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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></div>
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<p>Note that the particular values of <var>priority</var> do not matter; only their
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relative ordering.
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</p>
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</dd>
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<dt><code>warn_unused</code></dt>
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<dd><a name="index-warn_005funused-type-attribute"></a>
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<p>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>
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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::lock_guard</code>.
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</p>
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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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</p>
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</dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Function-Multiversioning.html#Function-Multiversioning" accesskey="n" rel="next">Function Multiversioning</a>, Previous: <a href="Bound-member-functions.html#Bound-member-functions" accesskey="p" rel="prev">Bound member functions</a>, Up: <a href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" accesskey="u" rel="up">C++ Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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