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126 lines
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126 lines
6.4 KiB
HTML
4 years ago
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<title>Vague Linkage - Using the GNU Compiler Collection (GCC)</title>
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<link rel="up" href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" title="C++ Extensions">
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<div class="node">
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<a name="Vague-Linkage"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="C_002b_002b-Interface.html#C_002b_002b-Interface">C++ Interface</a>,
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Previous: <a rel="previous" accesskey="p" href="Restricted-Pointers.html#Restricted-Pointers">Restricted Pointers</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">7.3 Vague Linkage</h3>
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<p><a name="index-vague-linkage-4206"></a>
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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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<dl>
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<dt>Inline Functions<dd>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>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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<br><dt>VTables<dd><a name="index-vtable-4207"></a>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><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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<br><dt><code>type_info</code> objects<dd><a name="index-g_t_0040code_007btype_005finfo_007d-4208"></a><a name="index-RTTI-4209"></a>C++ requires information about types to be written out in order to
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implement ‘<samp><span class="samp">dynamic_cast</span></samp>’, ‘<samp><span class="samp">typeid</span></samp>’ and exception handling.
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For polymorphic classes (classes with virtual functions), the ‘<samp><span class="samp">type_info</span></samp>’
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object is written out along with the vtable so that ‘<samp><span class="samp">dynamic_cast</span></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><span class="samp">type_info</span></samp>’ object when it is used: when
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applying ‘<samp><span class="samp">typeid</span></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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<br><dt>Template Instantiations<dd>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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</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>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>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>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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</body></html>
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