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    <p>I suspect that this might be compiler-specific behavior. Here's my theory:</p> <p>Because (in this particular case) an implicitly-defined T() is a <em>trivial</em> constructor (as defined in 12.1(5) of the standard), the compiler doesn't even attempt to generate a body for T(). Since there's no ctor body, there are no exceptions that could possibly be generated during "construction" (of which there isn't any, really), so there's no need to generate a dtor call, and so no need to generate a dtor body, only to find out that the base class's dtor is private.</p> <p>But as soon as T() becomes non-trivial (even if it remains implicitly-defined), a ctor body must be generated, and you get the error. Something as simple as adding a member to class T that has a user-defined constructor would make the implicitly-defined T() become non-trivial.</p> <p>A separate, but related, issue is that <code>new T()</code> doesn't generate a dtor call (since you don't have a corresponding <code>delete</code> anywhere). In contrast, if I just replace <code>new T()</code> with <code>T dummy</code> in your code, then I get the following from <code>gcc</code>, suggesting that it's now doing the full check for dtor accessibility (as a consequence of having to generate a dtor call):</p> <pre><code>test.cpp: In destructor 'T::~T()': test.cpp:3: error: 'indestructible_base::~indestructible_base()' is private test.cpp:7: error: within this context test.cpp: In function 'int main()': test.cpp:12: note: synthesized method 'T::~T()' first required here test.cpp:12: warning: unused variable 'dummy' </code></pre>
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    1. POWhy must a base class destructor be accessible only when a custom constructor is declared?
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    1. USjjlin
    1. USjjlin
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      1. VTUpMod
    1. CORight. But a `noexcept` user-defined constructor also can't throw, so why does that fail?
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      1. USBen Voigt
    2. CO@BenVoigt. It seems jjlin is correct. Look again at 12.1.5. Seems to not have anything to do with exceptions, but instead if the constructor is 'trivial' or not. For a constructor to be trivial all the members and bases must have trivial constructors (+ other requirements). (indestructible_base could as well be a member.) No destructor is generated in case of a trivial constructor. Also, declaring a constructor makes it non-trivial (except when using = default in c++11). The code is fine because nobody called that (non existing) destructor. Also note the comment jjlin makes about delete;
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      1. USJohan Lundberg
    3. COLooking through the gcc source, I get the impression that whether the ctor throws or not isn't checked. Of course, the source is pretty complex, so I can't say for sure, but it may simply be that the compiler writers just take the conservative approach and generate a dtor if they have to generate a ctor.
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