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    <p>3.8/1 says:</p> <blockquote> <p><strong>The lifetime of an object of type T begins when</strong>: — storage with the proper alignment and size for type T is obtained, and — if T is a class type with a non-trivial constructor (12.1), <strong>the constructor call has completed</strong>.</p> </blockquote> <p>3.8/5 says:</p> <blockquote> <p><strong>Before the lifetime of an object has started but after the storage which the object will occupy has been allocated</strong> or, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, <strong>any pointer that refers to the storage location where the object will be or was located may be used but only in limited ways</strong>. Such a pointer refers to allocated storage (3.7.3.2), and using the pointer as if the pointer were of type void*, is well-defined. <strong>Such a pointer may be dereferenced but the resulting lvalue may only be used in limited ways, as described below.</strong></p> </blockquote> <p>"Below" is 3.8/6:</p> <blockquote> <p>Such an lvalue refers to allocated storage (3.7.3.2), and using the properties of the lvalue which do not depend on its value is well-defined.</p> </blockquote> <p>...and then a list of things you can't do. Binding to a reference to the same, derived type is not among them.</p> <p>I can't find anything elsewhere that might make your code invalid. Notably, despite the following phrase in 8.3.2/4:</p> <blockquote> <p>A reference shall be initialized to refer to a valid object or function.</p> </blockquote> <p>there doesn't seem to be any definition of "valid object" to speak of.</p> <p>So, after much to-ing and fro-ing, <strong>I must conclude that it is legal</strong>.</p> <hr> <p><em>Of course, that's not to say that it's in any way a good idea!</em> <strong>It still looks like a bad design.</strong></p> <p>For example, if you later change your base constructor and any of the following become relevant (again from 3.8/6):</p> <ul> <li>the lvalue is used to access a non-static data member or call a non-static member function of the object</li> <li>the lvalue is implicitly converted (4.10) to a reference to a base class type</li> <li>the lvalue is used as the operand of a <code>static_cast</code> (5.2.9) (except when the conversion is ultimately to <code>char&amp;</code> or <code>unsigned char&amp;</code></li> <li>the lvalue is used as the operand of a <code>dynamic_cast</code> (5.2.7) or as the operand of <code>typeid</code>.</li> </ul> <p>...then your program will be undefined, and the compiler may emit no diagnostic for this!</p> <hr> <p><strong>Random other observations</strong></p> <p>I notice a couple of other <em>interesting</em> things whilst compiling this answer, and this is as good a place as any to share them.</p> <p>First, 9.3.2 appears to leave the type of <code>this</code> in a <em>ctor-initializer</em> accidentally unspecified. Bizarre!</p> <p>Second, the criteria set on a pointer by 3.8/5 (not the same list that I quoted from 3.8/6) include:</p> <blockquote> <p>If the object will be or was of a non-POD class type, the program has undefined behavior if [..] the pointer is implicitly converted (4.10) to a pointer to a base class type.</p> </blockquote> <p>I believe that this renders the following innocuous-looking code undefined:</p> <pre><code>struct A { A(A* ptr) {} }; struct B : A { B() : A(this) {} }; int main() { B b; } </code></pre>
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