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Subject: Re: [std-proposals] Make abstract classes non-deletable if no virtual destructor available
From: Kilian Henneberger (kilis-mail_at_[hidden])
Date: 2020-03-24 14:24:26


While my initial proposal would require changing the C++ language, a
(not equal but similar) approach would be
to only change the implementation of std::default_delete to:

template<class T>
struct default_delete {
 Â  void operator()(T* ptr) {
 Â Â Â  static_assert(!is_abstract_v<T> || has_virtual_destructor_v<T>);
 Â Â Â  delete ptr;
 Â  }
};

As (hopefully) most people use a abstract base class together with
unique_ptr,
this would also make the language more safe.

On Mon, Mar 23, 2020 at 3:19 PM Kilian Henneberger via Std-Proposals
<std-proposals_at_[hidden] <mailto:std-proposals_at_[hidden]>>
wrote:

    Hello,

    an abstract class (a class that either defines or inherits at least one
    function for which the final overrider is pure virtual)
    cannot be instantiated. However it can be destroyed. But deleting a
    Derived-object through a pointer to Base-object
    leads to undefined behaviour if the destructor of the Base-class is not
    virtual.
    There also exists a CppCoreGuideline addressing this topic:
    http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-dtor-virtual

    I want to propose that we make the attempt to delete a pointer to an
    abstract class ill-formed,
    if the abstract class does not have a (public) virtual destructor.

    This would turn what currently is UB into a compiler error.
    But it also might imply that code which compiles today (even if leading
    to UB), wont compile any longer after this change.

    I am looking forward to any feedback.
    Best regards,
    Kilian Henneberger

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