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    <p>The behavior is undefined because the destructor is invoked twice for the same object:</p> <ul> <li>Once when you invoke it explicitly</li> <li>Once when the scope ends and the automatic variable is destroyed</li> </ul> <p>Invoking the destructor on an object whose lifetime has ended results in undefined behavior per C++03 §12.4/6:</p> <blockquote> <p>the behavior is undefined if the destructor is invoked for an object whose lifetime has ended</p> </blockquote> <p>An object's lifetime ends when its destructor is called per §3.8/1:</p> <blockquote> <p>The lifetime of an object of type <code>T</code> ends when:</p> <p>— if <code>T</code> is a class type with a non-trivial destructor (12.4), the destructor call starts, or</p> <p>— the storage which the object occupies is reused or released.</p> </blockquote> <p>Note that this means if your class has a trivial destructor, the behavior is well-defined because the lifetime of an object of such a type does not end until its storage is released, which for automatic variables does not happen until the end of the function. Of course, I don't know why you would explicitly invoke the destructor if it is trivial. </p> <p>What is a trivial destructor? §12.4/3 says:</p> <blockquote> <p>A destructor is trivial if it is an implicitly-declared destructor and if:</p> <p>— all of the direct base classes of its class have trivial destructors and</p> <p>— for all of the non-static data members of its class that are of class type (or array thereof), each such class has a trivial destructor.</p> </blockquote> <p>As others have mentioned, one possible result of undefined behavior is your program appearing to continue running correctly; another possible result is your program crashing. Anything can happen and there are no guarantees whatsoever.</p>
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    1. PODoes explicitly calling destructor result in Undefined Behavior here?
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    1. COThank you. I don't have the standard document, and realized that I have to pay to get one. Would you mind providing the definition of "non-trivial destructor"? Is the one in the question a non-trivial destructor? Thanks.
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    2. CO@EXP0: I added the definition of a trivial destructor for you. The destructor in the question is not a trivial destructor.
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