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copied!<p>There are usually two variants of the constructor (<em>not-in-charge</em> / <em>in-charge</em>) and three of the destructor (<em>not-in-charge</em> / <em>in-charge</em> / <em>in-charge deleting</em>).</p> <p>The <em>not-in-charge</em> ctor and dtor are used when handling an object of a class that inherits from another class using the <code>virtual</code> keyword, when the object is not the complete object (so the current object is "not in charge" of constructing or destructing the virtual base object). This ctor receives a pointer to the virtual base object and stores it.</p> <p>The <em>in-charge</em> ctor and dtors are for all the other cases, i.e. if there is no virtual inheritance involved; if the class has a virtual destructor, the <em>in-charge deleting</em> dtor pointer goes into the vtable slot, while a scope that knows the dynamic type of the object (i.e. for objects with automatic or static storage duration) will use the <em>in-charge</em> dtor (because this memory should not be freed).</p> <p>Code example:</p> <pre><code>struct foo { foo(int); virtual ~foo(void); int bar; }; struct baz : virtual foo { baz(void); virtual ~baz(void); }; struct quux : baz { quux(void); virtual ~quux(void); }; foo::foo(int i) { bar = i; } foo::~foo(void) { return; } baz::baz(void) : foo(1) { return; } baz::~baz(void) { return; } quux::quux(void) : foo(2), baz() { return; } quux::~quux(void) { return; } baz b1; std::auto_ptr<foo> b2(new baz); quux q1; std::auto_ptr<foo> q2(new quux); </code></pre> <p>Results:</p> <ul> <li>The dtor entry in each of the vtables for <code>foo</code>, <code>baz</code> and <code>quux</code> point at the respective <em>in-charge deleting</em> dtor.</li> <li><code>b1</code> and <code>b2</code> are constructed by <code>baz()</code> <em>in-charge</em>, which calls <code>foo(1)</code> <em>in-charge</em></li> <li><code>q1</code> and <code>q2</code> are constructed by <code>quux()</code> <em>in-charge</em>, which falls <code>foo(2)</code> <em>in-charge</em> and <code>baz()</code> <em>not-in-charge</em> with a pointer to the <code>foo</code> object it constructed earlier</li> <li><code>q2</code> is destructed by <code>~auto_ptr()</code> <em>in-charge</em>, which calls the virtual dtor <code>~quux()</code> <em>in-charge deleting</em>, which calls <code>~baz()</code> <em>not-in-charge</em>, <code>~foo()</code> <em>in-charge</em> and <code>operator delete</code>.</li> <li><code>q1</code> is destructed by <code>~quux()</code> <em>in-charge</em>, which calls <code>~baz()</code> <em>not-in-charge</em> and <code>~foo()</code> <em>in-charge</em></li> <li><code>b2</code> is destructed by <code>~auto_ptr()</code> <em>in-charge</em>, which calls the virtual dtor <code>~baz()</code> <em>in-charge deleting</em>, which calls <code>~foo()</code> <em>in-charge</em> and <code>operator delete</code></li> <li><code>b1</code> is destructed by <code>~baz()</code> <em>in-charge</em>, which calls <code>~foo()</code> <em>in-charge</em></li> </ul> <p>Anyone deriving from <code>quux</code> would use its <em>not-in-charge</em> ctor and dtor and take on the responsibility of creating the <code>foo</code> object.</p> <p>In principle, the <em>not-in-charge</em> variant is never needed for a class that has no virtual bases; in that case, the <em>in-charge</em> variant is then sometimes called <em>unified</em>, and/or the symbols for both <em>in-charge</em> and <em>not-in-charge</em> are aliased to a single implementation.</p>

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