Re: what is the one base class subobject in virtual inheritance?
Am 05.11.2010 23:22, schrieb muler:
Does the most derived object from virtual inheritance take the single
base class subobject depending on the order of inheritance? The below
example appears to show this but couldn't find the rule that governs
this behavior in the C++98 standard doc.
I don't think that this example answers your question (or I'm misunderstanding
your question). The standard does not specify, in which part of the memory of
the most derived object virtual base classes are located. Your example does only
demonstrate the observable effect of the normative construction order of
non-virtual base classes.
class L {
public:
int x_;
};
class A : virtual public L {
public:
A()
{
x_ = -1;
}
};
class B : virtual public L {
public:
B()
{
x_ = 1;
}
};
class C : public A, public B {
};
class D : public B, public A {
};
int main(int argc, char* argv[])
{
A a;
B b;
C c;
D d;
std::cout<< c.x_<< std::endl; // prints 1
std::cout<< d.x_<< std::endl; // prints -1
return 0;
}
The standard requires that virtual base classes are constructed first, this
means that the default constructor of L is called (which is a noop), see 12.6.2
p. 5:
"Initialization shall proceed in the following order:
? First, and only for the constructor of the most derived class as described
below, virtual base classes shall be initialized in the order they appear on a
depth-first left-to-right traversal of the directed acyclic graph of base
classes, where "left-to-right" is the order of appearance of the base class
names in the derived class base-specifier-list."
After this the non-virtual base-class constructors are called in the order of
declaration, see bullet 2 of the same paragraph:
"? Then, direct base classes shall be initialized in declaration order as they
appear in the base-specifier-list (regardless of the order of the
mem-initializers)."
For C this calls first A::A() setting L::x_ to -1 and second B::B() setting
L::x_ to 1. Vice versa for D. Nothing more can be concluded from this example.
HTH & Greetings from Bremen,
Daniel Kr?gler
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