Re: some puzzles

"Daniel T." <>
Fri, 22 Feb 2008 07:19:59 -0500
thomas <> wrote:

class base{
    virtual void func();
class derive:public base{
    virtual void func(); //1
As we know that Line 1 implements the function overloading,
what's the difference between "virtual void func();" and "void
func();" in L1?

There is none. At line 1, the 'virtual' keyword is optional.

2.why people sometimes define constructor/destructor virtual?

Nobody ever defines a constructor as virtual. Destructors are defined as
virtual so the base destructor will be called when a pointer to derived
is deleted.

what if a pure virtual constructor/destructor?

Destructors must be defined. You can have this:

class Foo {
   virtual ~Foo() = 0;

but you still have to define Foo::~Foo() somewhere. What the above does
is make the class abstract, even if all other member-functions are

        int *x = new int[0];
the result is not 0, what happened?

'new' is guaranteed to return memory or throw. In the above, you newed a
block of memory of unspecified size, that can't be accessed for any
reason. "*x" would be undefined behavior.

4. when calling "delete []p;", how does the program know how many
elements should be destroyed?

How the system does unspecified by the language.

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