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diff --git a/share/doc/gcc/Copy-Assignment.html b/share/doc/gcc/Copy-Assignment.html new file mode 100644 index 0000000..2072f17 --- /dev/null +++ b/share/doc/gcc/Copy-Assignment.html @@ -0,0 +1,139 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- This file documents the use of the GNU compilers. + +Copyright (C) 1988-2023 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 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>Using the GNU Compiler Collection (GCC): Copy Assignment</title> + +<meta name="description" content="Using the GNU Compiler Collection (GCC): Copy Assignment"> +<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Copy Assignment"> +<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="Indices.html#Indices" rel="index" title="Indices"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="C_002b_002b-Misunderstandings.html#C_002b_002b-Misunderstandings" rel="up" title="C++ Misunderstandings"> +<link href="Non_002dbugs.html#Non_002dbugs" rel="next" title="Non-bugs"> +<link href="Temporaries.html#Temporaries" rel="previous" title="Temporaries"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +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.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en_US" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Copy-Assignment"></a> +<div class="header"> +<p> +Previous: <a href="Temporaries.html#Temporaries" accesskey="p" rel="previous">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="Indices.html#Indices" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Implicit-Copy_002dAssignment-for-Virtual-Bases"></a> +<h4 class="subsection">14.7.4 Implicit Copy-Assignment for Virtual Bases</h4> + +<p>When a base class is virtual, only one subobject of the base class +belongs to each full object. Also, the constructors and destructors are +invoked only once, and called from the most-derived class. However, such +objects behave unspecified when being assigned. For example: +</p> +<div class="smallexample"> +<pre class="smallexample">struct Base{ + char *name; + Base(const char *n) : name(strdup(n)){} + Base& operator= (const Base& other){ + free (name); + name = strdup (other.name); + return *this; + } +}; + +struct A:virtual Base{ + int val; + A():Base("A"){} +}; + +struct B:virtual Base{ + int bval; + B():Base("B"){} +}; + +struct Derived:public A, public B{ + Derived():Base("Derived"){} +}; + +void func(Derived &d1, Derived &d2) +{ + d1 = d2; +} +</pre></div> + +<p>The C++ standard specifies that ‘<samp>Base::Base</samp>’ is only called once +when constructing or copy-constructing a Derived object. It is +unspecified whether ‘<samp>Base::operator=</samp>’ is called more than once when +the implicit copy-assignment for Derived objects is invoked (as it is +inside ‘<samp>func</samp>’ in the example). +</p> +<p>G++ implements the “intuitive” algorithm for copy-assignment: assign all +direct bases, then assign all members. In that algorithm, the virtual +base subobject can be encountered more than once. In the example, copying +proceeds in the following order: ‘<samp>name</samp>’ (via <code>strdup</code>), +‘<samp>val</samp>’, ‘<samp>name</samp>’ again, and ‘<samp>bval</samp>’. +</p> +<p>If application code relies on copy-assignment, a user-defined +copy-assignment operator removes any uncertainties. With such an +operator, the application can define whether and how the virtual base +subobject is assigned. +</p> + + + +</body> +</html> |