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107 lines
4.3 KiB
HTML
107 lines
4.3 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Copy Assignment - Using the GNU Compiler Collection (GCC)</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="C_002b_002b-Misunderstandings.html#C_002b_002b-Misunderstandings" title="C++ Misunderstandings">
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<link rel="prev" href="Temporaries.html#Temporaries" title="Temporaries">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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Copyright (C) 1988-2015 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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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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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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software. Copies published by the Free Software Foundation raise
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funds for GNU development.-->
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<style type="text/css"><!--
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span.sansserif { font-family:sans-serif; font-weight:normal; }
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Copy-Assignment"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="Temporaries.html#Temporaries">Temporaries</a>,
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Up: <a rel="up" accesskey="u" href="C_002b_002b-Misunderstandings.html#C_002b_002b-Misunderstandings">C++ Misunderstandings</a>
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<hr>
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</div>
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<h4 class="subsection">12.7.4 Implicit Copy-Assignment for Virtual Bases</h4>
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<p>When a base class is virtual, only one subobject of the base class
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belongs to each full object. Also, the constructors and destructors are
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invoked only once, and called from the most-derived class. However, such
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objects behave unspecified when being assigned. For example:
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<pre class="smallexample"> struct Base{
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char *name;
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Base(char *n) : name(strdup(n)){}
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Base& operator= (const Base& other){
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free (name);
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name = strdup (other.name);
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}
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};
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struct A:virtual Base{
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int val;
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A():Base("A"){}
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};
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struct B:virtual Base{
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int bval;
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B():Base("B"){}
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};
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struct Derived:public A, public B{
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Derived():Base("Derived"){}
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};
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void func(Derived &d1, Derived &d2)
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{
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d1 = d2;
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}
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</pre>
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<p>The C++ standard specifies that ‘<samp><span class="samp">Base::Base</span></samp>’ is only called once
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when constructing or copy-constructing a Derived object. It is
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unspecified whether ‘<samp><span class="samp">Base::operator=</span></samp>’ is called more than once when
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the implicit copy-assignment for Derived objects is invoked (as it is
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inside ‘<samp><span class="samp">func</span></samp>’ in the example).
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<p>G++ implements the “intuitive” algorithm for copy-assignment: assign all
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direct bases, then assign all members. In that algorithm, the virtual
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base subobject can be encountered more than once. In the example, copying
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proceeds in the following order: ‘<samp><span class="samp">val</span></samp>’, ‘<samp><span class="samp">name</span></samp>’ (via
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<code>strdup</code>), ‘<samp><span class="samp">bval</span></samp>’, and ‘<samp><span class="samp">name</span></samp>’ again.
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<p>If application code relies on copy-assignment, a user-defined
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copy-assignment operator removes any uncertainties. With such an
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operator, the application can define whether and how the virtual base
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subobject is assigned.
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</body></html>
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