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> Like, are there other =<something> construct? Yes, there's "= 0" to mark a virtual method as not implemented in this class (that is, it must be implemented b

by cesarb 5y ago

> Like, are there other =<something> construct? Yes, there's "= 0" to mark a virtual method as not implemented in this class (that is, it must be implemented by a subclass), which is older than "= delete". Conceptually, it sets the corresponding slot in the virtual method table to null instead of a pointer to the (non-existent) method, except that it actually doesn't do that, it sets that slot to a pointer to a "pure virtual method called" function from the standard library, which AFAIK prints an error message and exits the program (IIRC, this is because there are some situations in which you can actually manage to call such a pure virtual method, like calling it from a function called within the constructor or destructor of the base class.)

jdsully 5y ago
A concrete object must implement all virtual methods including those marked as pure virtual. All =0 means is that someone else lower down the inheretence chain will implement this and the caller using the class type can rely on it existing. If you try to instantiate an object without all methods implemented you'll get a compiler error about the class being "abstract". You should never have a case where there's a null in the vtable. If there is some specific edge case around ctors then forgive me - this is C++ after all! But in practice you don't have to worry about this which is true for most of the C++ minutia.
- cesarb 5y ago
  > If there is some specific edge case around ctors then forgive me - this is C++ after all! Yes, the specific edge case is around ctors and dtors. During the ctor or the dtor, the vtable is the vtable of the base class, not the derived class, so if you call a virtual function within the ctor or dtor of the base class (don't do that), the function which will be called is the one from the (possibly abstract) base class. See for instance this FAQ entry: https://isocpp.org/wiki/faq/strange-inheritance#calling-virtuals-from-ctors https://isocpp.org/wiki/faq/strange-inheritance#calling-virt...

by cesarb 5y ago

jdsully 5y ago

A concrete object must implement all virtual methods including those marked as pure virtual. All =0 means is that someone else lower down the inheretence chain will implement this and the caller using the class type can rely on it existing. If you try to instantiate an object without all methods implemented you'll get a compiler error about the class being "abstract". You should never have a case where there's a null in the vtable. If there is some specific edge case around ctors then forgive me - this is C++ after all! But in practice you don't have to worry about this which is true for most of the C++ minutia.

cesarb 5y ago
> If there is some specific edge case around ctors then forgive me - this is C++ after all! Yes, the specific edge case is around ctors and dtors. During the ctor or the dtor, the vtable is the vtable of the base class, not the derived class, so if you call a virtual function within the ctor or dtor of the base class (don't do that), the function which will be called is the one from the (possibly abstract) base class. See for instance this FAQ entry: https://isocpp.org/wiki/faq/strange-inheritance#calling-virtuals-from-ctors https://isocpp.org/wiki/faq/strange-inheritance#calling-virt...