It's true that calling virtual function in constructor and destructor is not a good practice, and should be avoided. It's because virtual functions are affected by subclasses, but in constructing or destructing phase subclasses are not yet constructed(in constructing) or already destructed(in destructing).
However what happens if a virtual final function is invoked in constructor or destructor? I assume that there should be no problem, since it's not logically wrong.
Calling virtual function in constructor and destructor is forbidden because accessing to subclass' variable, not initialized yet, can occur in overridden version of virtual function, which is declared in the subclass.
While virtual final function is not, it's final and there's no way to access to subclass' variables.
But this is my assumption, and there could be any more reasons that calling virtual function in constructor or destructor is not reasonable.
So, in conclusion,
Is calling virtual final function in constructing/destructing phase is allowed in C++ standard?
Calling a virtual function during construction/destruction is well defined and completely legal except in the case of pure virtual functions.
Calling virtual function in constructor and destructor is forbidden
I don't know (nor cares) who says it's "bad" or "forbidden" from a stylistic point of view, code maintenance point of view... The ability to maintain code depends first on knowing the relevant language and tools well; not knowing what virtual calls do during these phases (*) will lead to misunderstand on the part of the maintainers which is fixed by selecting more experienced maintainers and not dumbing down the programming style.
(*) which aren't technically part of the "lifetime" of the object, which isn't even a very useful concept as objects are usable and used in their constructor (before their lifetime has started) in any non trivial program (I think the standard should simply suppress this unneeded concept).
accessing to subclass' variable, not initialized yet, can occur in overridden version of virtual function, which is declared in the subclass.
It can't. During construction of a base class subobject B
(say by constructor B::B()
), the type of the object is being constructed is by definition B
.
overridden version of virtual function, which is declared in the subclass.
No, there is no existing subclass object at that point, so there is no overriding.
While virtual final function is not, it's final and there's no way to access to subclass' variables.
It makes no difference.
The dynamic type of a polymorphic object is established by a constructor, after the constructors for base classes and before constructing members.
If so, is it widely implemented to most C++ compilers?
In practice all compilers implement setting the dynamic type of an object by changing the one or many vtable pointers to point to appropriate vtables for the type; that is done as part of construction.
It means that during construction, the vptr value changes as derived objects are constructed.