Is every "complete" object a "most-derived" object?-CodePudding

[..] An object that is not a subobject of any other object is called a complete object [..].

So consider this snippet of code:

struct Base {};
struct Derived : Base {};
struct MostDerived : Derived {};

I can't understand the wording in this quote from the standard:

[object.intro]/6:

If a complete object, a member subobject, or an array element is of class type, its type is considered the most derived class [..] An object of a most derived class type or of a non-class type is called a most derived object.

From the quote what I understand is that a type of a complete object is of "most-derived" class type. I stopped here, I really do not understand the rest of the wording.

Per the question "What does the "most derived object" mean?" I think that (correct me if I am wrong), objects of type "most-derived" class only, like MostDerived, are called "most-derived" objects. Is this true?
If I have created an object of Base like this: Base b_obj = Base(), is the object b_obj a "most-derived" object?
If I have created an object of Derived like this: Derived d_obj = Derived(), is the object d_obj also a "most-derived" object?
Does the word "derived" in "most-derived" mean that the object is an object of a class like MostDerived, or mean that the object has no class subobject in it?

CodePudding user response：

Per the question "What does the "most derived object" mean?" I think that (correct me if I am wrong), objects of type "most-derived" class only, like MostDerived, are called "most-derived" objects. Does this true?.

"Most derived class" is supposed to be dependent on the object under consideration. It is not a property of the class as such.

if I have created an object of Base like this: Base b_obj = Base(), Is the object b_obj is "most-derived" object?

Yes, b_obj is a variable and as such a complete object. A complete object is always a most-derived object.

if I have created an object of Derived like this: Derived d_obj = Derived(), Is the object d_obj is also a "most-derived" object?

Same as above applies.

Is the word "derived" in "most-derived" mean that the object is an object of a class like MostDerived, or mean that the object has no class subobject in it?

"most-derived" means that there is no other object of which the object under consideration is a base class subobject. This is again not a property of classes themselves, but depends on the concrete instance of the class. If for example a Derived object is created with new Derived, then it contains a Base base class subobject and this subobject is not a most-derived object. However the Derived object is the most-derived object, since there isn't e.g. any MostDerived object of which it is a base class subobject.

Is every "complete" object is "most-derived" object

Yes, every complete object is a most-derived object. But the reverse is not true. For example

struct A {
    Base b;
};

A a;

a is an object of type A and itself a most-derived object and a complete object, but a.b is also a most-derived object, although not a complete object.

CodePudding user response：

An object is not a class.
An object is an instantiation of a class, an array, or built-in-type.
Subobjects are class member objects, array elements, or base classes of an object.
Derived objects (and most-derived objects) only make sense in the context of class inheritance.

void foo() {
    int i = 0; // complete object, but not most-derived (not class type)
}

class A {
   int i = 0; // non complete object, not most-derived
}

void bar() {
    A a; // complete object, but not derived, so can't be "most derived"
}

class B : A { }

void biz() {
   B b; // complete object, derived object, and most-derived object
}

Is every "complete" object is "most-derived" object

No. A most-derived object is an object of a most-derived class, and a most-derived class must be of a class type. Objects may be of class type, but non-class type objects also exist.

Every complete object of class-type is a most-derived object only if that class inherits.
A most-derived object may be a subobject, so you cannot infer object completeness from most-derivedness (however, you can infer that the most-derived object is of class type).

So if I have created an object of Base like this: Base b_obj = Base(), Is the object b_obj is "most-derived" object?

Yes. The most-derived object of b_obj is an object of type Base. This is not necessarily a complete object, however, since this could be a class member definition. Again, complete is not synonymous with most-derived.

Also if I have created an object of Derived like this: Derived d_obj = Derived(), Is the object d_obj is also a "most-derived" object?

Yes. The most-derived object of d_obj is an object of type Derived.

If you have an object created as type MostDerived:

MostDerived md;

It is an object of type MostDerived
It is an object of type Derived
It is an object of type Base
If it is not a member subobject, then it is a complete object
Its most-derived object is of type MostDerived
It has a subobject of type Derived, which is neither a complete object nor a most-derived object
Its subobject of type Derived has a subobject of type Base, which is neither a complete object nor a most-derived object.