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