Re: Question From a Brainbench exam

From:

Alberto Ganesh Barbati <AlbertoBarbati@libero.it>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 23 May 2007 18:12:34 CST

Message-ID:

<0m15i.16973$%k.83461@twister2.libero.it>

Marcel ha scritto:

class Foo {};

class Bar: public Foo {};

class Bar2: public Foo {};

class FooBar: public Bar {};

class FooBar2: public Bar2{};

Referring to the code above, which one of the following is valid C++
and why are the others invalid?

a. FooBar2 *fb3 = new Foo;

Wrong. Conversion from pointer-to-base-class to pointer-to-derived-class
is never implicit.

b. FooBar2 &fb2 = dynamic_cast<Foo&>(new FooBar2);

Wrong. "new FooBar2" is an rvalue of pointer type and you can't convert
it to a reference to a non-pointer type.

c. Foo &foo = dynamic_cast<Foo&>(*(new FooBar2));

Correct. Notice that it's actually an upcast (conversion from derived
class to base class), in this special case, dynamic_cast doesn't require
types to be polymorphic, as it would be required for a downcast or a
crosscast (see 5.2.7/5).

d. Foo &foo = static_cast<Foo&>(FooBar2 f);

Wrong. This one contains a syntax error: "FooBar2 f" is not a valid
expression.

e. Foo &foo2 = new FooBar;

Wrong. Similar to b, you can't bind an rvalue of pointer type to a
reference to a non-pointer type.

HTH,

Ganesh

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