168 lines
8.0 KiB
HTML
168 lines
8.0 KiB
HTML
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<head>
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<title>Using the GNU Compiler Collection (GCC): Vague Linkage</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC): Vague Linkage">
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<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Vague Linkage">
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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="C_002b_002b-Interface.html#C_002b_002b-Interface" rel="next" title="C++ Interface">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Vague-Linkage"></a>
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<div class="header">
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<p>
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Next: <a href="C_002b_002b-Interface.html#C_002b_002b-Interface" accesskey="n" rel="next">C++ Interface</a>, Previous: <a href="Restricted-Pointers.html#Restricted-Pointers" accesskey="p" rel="prev">Restricted Pointers</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="Vague-Linkage-1"></a>
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<h3 class="section">7.3 Vague Linkage</h3>
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<a name="index-vague-linkage"></a>
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<p>There are several constructs in C++ that require space in the object
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file but are not clearly tied to a single translation unit. We say that
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these constructs have “vague linkage”. Typically such constructs are
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emitted wherever they are needed, though sometimes we can be more
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clever.
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</p>
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<dl compact="compact">
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<dt>Inline Functions</dt>
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<dd><p>Inline functions are typically defined in a header file which can be
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included in many different compilations. Hopefully they can usually be
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inlined, but sometimes an out-of-line copy is necessary, if the address
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of the function is taken or if inlining fails. In general, we emit an
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out-of-line copy in all translation units where one is needed. As an
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exception, we only emit inline virtual functions with the vtable, since
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it always requires a copy.
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</p>
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<p>Local static variables and string constants used in an inline function
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are also considered to have vague linkage, since they must be shared
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between all inlined and out-of-line instances of the function.
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</p>
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</dd>
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<dt>VTables</dt>
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<dd><a name="index-vtable"></a>
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<p>C++ virtual functions are implemented in most compilers using a lookup
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table, known as a vtable. The vtable contains pointers to the virtual
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functions provided by a class, and each object of the class contains a
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pointer to its vtable (or vtables, in some multiple-inheritance
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situations). If the class declares any non-inline, non-pure virtual
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functions, the first one is chosen as the “key method” for the class,
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and the vtable is only emitted in the translation unit where the key
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method is defined.
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</p>
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<p><em>Note:</em> If the chosen key method is later defined as inline, the
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vtable is still emitted in every translation unit that defines it.
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Make sure that any inline virtuals are declared inline in the class
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body, even if they are not defined there.
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</p>
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</dd>
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<dt><code>type_info</code> objects</dt>
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<dd><a name="index-type_005finfo"></a>
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<a name="index-RTTI"></a>
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<p>C++ requires information about types to be written out in order to
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implement ‘<samp>dynamic_cast</samp>’, ‘<samp>typeid</samp>’ and exception handling.
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For polymorphic classes (classes with virtual functions), the ‘<samp>type_info</samp>’
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object is written out along with the vtable so that ‘<samp>dynamic_cast</samp>’
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can determine the dynamic type of a class object at run time. For all
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other types, we write out the ‘<samp>type_info</samp>’ object when it is used: when
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applying ‘<samp>typeid</samp>’ to an expression, throwing an object, or
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referring to a type in a catch clause or exception specification.
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</p>
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</dd>
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<dt>Template Instantiations</dt>
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<dd><p>Most everything in this section also applies to template instantiations,
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but there are other options as well.
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See <a href="Template-Instantiation.html#Template-Instantiation">Where’s the Template?</a>.
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</p>
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</dd>
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</dl>
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<p>When used with GNU ld version 2.8 or later on an ELF system such as
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GNU/Linux or Solaris 2, or on Microsoft Windows, duplicate copies of
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these constructs will be discarded at link time. This is known as
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COMDAT support.
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</p>
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<p>On targets that don’t support COMDAT, but do support weak symbols, GCC
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uses them. This way one copy overrides all the others, but
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the unused copies still take up space in the executable.
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</p>
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<p>For targets that do not support either COMDAT or weak symbols,
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most entities with vague linkage are emitted as local symbols to
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avoid duplicate definition errors from the linker. This does not happen
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for local statics in inlines, however, as having multiple copies
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almost certainly breaks things.
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</p>
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<p>See <a href="C_002b_002b-Interface.html#C_002b_002b-Interface">Declarations and Definitions in One Header</a>, for
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another way to control placement of these constructs.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="C_002b_002b-Interface.html#C_002b_002b-Interface" accesskey="n" rel="next">C++ Interface</a>, Previous: <a href="Restricted-Pointers.html#Restricted-Pointers" accesskey="p" rel="prev">Restricted Pointers</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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