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136 lines
5.3 KiB
HTML
136 lines
5.3 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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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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(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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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Copy Assignment (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="Copy Assignment (Using the GNU Compiler Collection (GCC))">
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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-Misunderstandings.html#C_002b_002b-Misunderstandings" rel="up" title="C++ Misunderstandings">
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<link href="Non_002dbugs.html#Non_002dbugs" rel="next" title="Non-bugs">
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<link href="Temporaries.html#Temporaries" rel="prev" title="Temporaries">
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</head>
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<body lang="en">
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<a name="Copy-Assignment"></a>
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<div class="header">
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<p>
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Previous: <a href="Temporaries.html#Temporaries" accesskey="p" rel="prev">Temporaries</a>, Up: <a href="C_002b_002b-Misunderstandings.html#C_002b_002b-Misunderstandings" accesskey="u" rel="up">C++ Misunderstandings</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="Implicit-Copy_002dAssignment-for-Virtual-Bases"></a>
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<h4 class="subsection">13.7.4 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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</p>
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<div class="smallexample">
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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></div>
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<p>The C++ standard specifies that ‘<samp>Base::Base</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>Base::operator=</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>func</samp>’ in the example).
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</p>
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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>val</samp>’, ‘<samp>name</samp>’ (via
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<code>strdup</code>), ‘<samp>bval</samp>’, and ‘<samp>name</samp>’ again.
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</p>
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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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</p>
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</body>
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</html>
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