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copied!<p>The last sentence of the normative text that you cite reads as follows:</p> <blockquote> <p>If the virtual function call uses an explicit class member access and the object expression refers to the complete object of <code>x</code> or one of that object’s base class subobjects but not <code>x</code> or one of its base class subobjects, the behavior is undefined.</p> </blockquote> <p>This is, admittedly, rather convoluted. This sentence exists to restrict what functions may be called during construction in the presence of multiple inheritance.</p> <p>The example contains multiple inheritance: <code>D</code> derives from <code>A</code> and <code>B</code> (we'll ignore <code>V</code>, because it is not required to demonstrate why the behavior is undefined). During construction of a <code>D</code> object, both the <code>A</code> and <code>B</code> constructors will be called to construct the base class subobjects of the <code>D</code> object.</p> <p>When the <code>B</code> constructor is called, the type <em>of the complete object of <code>x</code></em> is <code>D</code>. In that constructor, <code>a</code> is a pointer to the <code>A</code> base class subobject of <code>x</code>. So, we can say the following about <code>a->f()</code>:</p> <ul> <li><p>The <em>object under construction</em> is the <code>B</code> base class subobject of a <code>D</code> object (because this base class subobject is the object currently under construction, it is what the text refers to as <code>x</code>).</p></li> <li><p>It uses <em>explicit class member access</em> (via the <code>-></code> operator, in this case)</p></li> <li><p>The type of <em>the complete object of <code>x</code></em> is <code>D</code>, because that is the most-derived type that is being constructed</p></li> <li><p>The <em>object expression</em> (<code>a</code>) refers to a <em>base class subobject</em> of the complete object of <code>x</code> (it refers to the <code>A</code> base class subobject of the <code>D</code> object being constructed)</p></li> <li><p>The base class subobject to which the <em>object expression</em> refers is not <code>x</code> and is not a base class subobject of <code>x</code>: <code>A</code> is not <code>B</code> and <code>A</code> is not a base class of <code>B</code>.</p></li> </ul> <p>Therefore, the behavior of the call is undefined, per the rule we started from at the beginning.</p> <blockquote> <p>Why is the last method call in <code>B::B</code> undefined? Shouldn't it just call <code>a.A::f</code>?</p> </blockquote> <p>The rule you cite states that when a constructor is called during construction, "the function called is the final overrider in the constructor’s class and not one overriding it in a more-derived class."</p> <p>In this case, the constructor's class is <code>B</code>. Because <code>B</code> does not derive from <code>A</code>, there is no final overrider for the virtual function. Therefore the attempt to make the virtual call exhibits undefined behavior.</p>

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