Re: some puzzles
thomas <FreshThomas@gmail.com> wrote:
As we know that Line 1 implements the function overloading,
what's the difference between "virtual void func();" and "void
func();" in L1?
That's the same difference as with 'final' methods in java.
I guess you mean that this method cannot be overloaded by child
classes if not defined virtual.
That is incorrect. It was defined virtual in the base class, so it can
be overridden by any derived class, even if some derived class in the
chain doesn't use the virtual keyword. For example:
class A { public: virtual int foo(); };
class B : public A { public: int foo(); };
class C : public B { public: int foo(); };
int main() {
C c;
A* a = &c;
a->foo(); // C::foo() will be called
}
constructor can't be defined as virtual AFAIK. ?destructurs must be
defined as virtual as soon as there is a virtual method, to allow
calling the right destructor (that is, the one of the exact class of
the object), when delete is called with a pointer to the object typed
as a superclass:
int main(){
? ? A* obj=new B();
? ? delete obj; ?// <-- we want ~B to be called too here!
? ? return 0;
}
I think I got it!
another problem:
--code--
class A{
virtual int func();
};
class B{
virtual double func(); //is this overloading?
};
No. "Two functions that appear in the same scope are overloaded if they
have the same name but have different parameter list." The above two
functions are not in the same scope and don't have a different parameter
list, so this is not an example of overloading.
3.
--code--
? ?int *x = new int[0];
? ?cout<<x<<endl;
--code--
the result is not 0, what happened?
There is only one null pointer. ?
So the x will always get one piece memory allocated?
Correct. 'new' always returns memory, or throws.
If "delete []x" is never called, will it never be freed?
Correct.
4. when calling "delete []p;", how does the program know how many
elements should be destroyed?
new[] stores the size allocated in the allocated memory block.
How is the memory organized?
However the system wants. The organization isn't specified by the
standard.
Is the first element any different(store
the size) with others?
No.
Why calling "delete p" will not free all the memory if size is known?
Calling delete on memory that was allocated using new[] is undefined. It
might release all the memory, it might not. It might call all the
destructors, it might not. It might crash the program, it might not...
&c.