Re: Virtual Base Class, "Diamond" Inheritance
On 1 Dez., 20:05, Ryan Clark <RyanCl...@alum.berkeley.edu> wrote:
I'm getting a little frustrated with constructor syntax when I use
virtual base classes.
Take the following code (classic diamond inheritance):
class BaseClass
{
int base_value;
public:
BaseClass(int bv) : base_value(0)
{
base_value = bv;
}
This initialization of member base_value looks a bit silly
to me, why did you not use the member initializer list for
that?
virtual ~BaseClass()
{
}
int value() const
{
return base_value;
}
};
class A : public virtual BaseClass
{
public:
A() : BaseClass(1)
{
}
virtual ~A()
{
}
};
You can remove all the empty virtual destructor definitions
on the sub-class-hierarchies of BaseClass, because virtually
derived classes honor virtual member functions of base classes,
see 10.3 [class.virtual]/2 (All my standard references are
relative to the recent draft N2798). Therefore I removed them
in my following quotes.
class B : public virtual BaseClass
{
public:
B() : BaseClass(2)
{
}
[..]
};
class Diamond: public A, public B
{
public:
Diamond(): BaseClass(3)
{
}
[..]
};
int main(int argc, char* argv[])
{
Diamond d;
printf("Hello World! %d\n", d.value());
return 0;
}
This code won't compile without the calls to the BaseClass constructor
in A and B, even though they are never called. Why is that? I have
already told the compiler, "this class will be instantiated somewhere
else" (right?), why do I have to add completely meaningless code to
make the compiler happy?
No, you did *not* tell the compiler that A and B might never be
instantiated, because they are instantiable. Just add the definition
A a;
immediately after the definition of A and you have a valid object
definition.
What's more... if add this class:
class Diamond2 : public Diamond
{
public:
Diamond2(): BaseClass(4)
{
}
[..]
};
And change variable d to type Diamond2, it won't compile without the
constructor call to BaseClass. It seems to me that the call to
BaseClass should be satisfied by the call in the Diamond constructor.
No, it is not, because a virtual base class is constructed in the
most derived object, see 10.1 [class.mi]/4:
"[..] For each distinct base class that is specified virtual, the
most derived object shall contain a single base class subobject
of that type.[..]"
and 12.6.2 [class.base.init]/7:
"All subobjects representing virtual base classes are initialized
by the constructor of the most derived class (1.8). If the
constructor of the most derived class does not specify a mem-
initializer for a virtual base class V, then V?s default
constructor is called to initialize the virtual base class
subobject. If V does not have an accessible default constructor,
the initialization is ill-formed. A mem-initializer naming a
virtual base class shall be ignored during execution of the
constructor of any class that is not the most derived class."
I have two problems with this, (1) code is getting duplicated
unnecessarily, and (2) it undermines the programmer's ability to
design neatly encapsulated classes (for example, if the whole purpose
of class Diamond was to set the correct value in BaseClass).
Is there something I'm missing? I might be less frustrated if I had a
better understanding about what is going on. I don't see any
explanation in Stroustrup. Is there some kind of syntactic trick I can
use to duplicate less code?
You are not the first one who stumbled across this problem, see
e.g. the recent thread "Initialization of base virtual classes":
http://preview.tinyurl.com/6anord
The point is, that the virtual base class-c'tor is invoked
in the most derived object. Which one this is, depends on the
actual object. In your example all types A, B, Diamond, and
Diamond2 can be instantiated, so it makes sense to add the
c'tors in *all* classes. There is one situation, where this
requirement really annoys and that are *abstract* base classes
(which can never be instantiated). here is still an active
issue lurking around:
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#257
which attempts to solve this problem for this special use-case
(which is not relevant for your example).
HTH & Greetings form Bremen,
Daniel Kr?gler
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