You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
167 lines
7.9 KiB
HTML
167 lines
7.9 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html>
|
|
<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
|
|
|
|
Permission is granted to copy, distribute and/or modify this document
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
|
any later version published by the Free Software Foundation; with the
|
|
Invariant Sections being "Funding Free Software", the Front-Cover
|
|
Texts being (a) (see below), and with the Back-Cover Texts being (b)
|
|
(see below). A copy of the license is included in the section entitled
|
|
"GNU Free Documentation License".
|
|
|
|
(a) The FSF's Front-Cover Text is:
|
|
|
|
A GNU Manual
|
|
|
|
(b) The FSF's Back-Cover Text is:
|
|
|
|
You have freedom to copy and modify this GNU Manual, like GNU
|
|
software. Copies published by the Free Software Foundation raise
|
|
funds for GNU development. -->
|
|
<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<title>Vague Linkage (Using the GNU Compiler Collection (GCC))</title>
|
|
|
|
<meta name="description" content="Vague Linkage (Using the GNU Compiler Collection (GCC))">
|
|
<meta name="keywords" content="Vague Linkage (Using the GNU Compiler Collection (GCC))">
|
|
<meta name="resource-type" content="document">
|
|
<meta name="distribution" content="global">
|
|
<meta name="Generator" content="makeinfo">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<link href="index.html#Top" rel="start" title="Top">
|
|
<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|
<link href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" rel="up" title="C++ Extensions">
|
|
<link href="C_002b_002b-Interface.html#C_002b_002b-Interface" rel="next" title="C++ Interface">
|
|
<link href="Restricted-Pointers.html#Restricted-Pointers" rel="prev" title="Restricted Pointers">
|
|
<style type="text/css">
|
|
<!--
|
|
a.summary-letter {text-decoration: none}
|
|
blockquote.indentedblock {margin-right: 0em}
|
|
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
|
|
blockquote.smallquotation {font-size: smaller}
|
|
div.display {margin-left: 3.2em}
|
|
div.example {margin-left: 3.2em}
|
|
div.lisp {margin-left: 3.2em}
|
|
div.smalldisplay {margin-left: 3.2em}
|
|
div.smallexample {margin-left: 3.2em}
|
|
div.smalllisp {margin-left: 3.2em}
|
|
kbd {font-style: oblique}
|
|
pre.display {font-family: inherit}
|
|
pre.format {font-family: inherit}
|
|
pre.menu-comment {font-family: serif}
|
|
pre.menu-preformatted {font-family: serif}
|
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
|
pre.smallexample {font-size: smaller}
|
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
|
pre.smalllisp {font-size: smaller}
|
|
span.nolinebreak {white-space: nowrap}
|
|
span.roman {font-family: initial; font-weight: normal}
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
|
ul.no-bullet {list-style: none}
|
|
-->
|
|
</style>
|
|
|
|
|
|
</head>
|
|
|
|
<body lang="en">
|
|
<a name="Vague-Linkage"></a>
|
|
<div class="header">
|
|
<p>
|
|
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>
|
|
</div>
|
|
<hr>
|
|
<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>
|
|
<hr>
|
|
<div class="header">
|
|
<p>
|
|
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>
|
|
</div>
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|