Re: Question From a Brainbench exam
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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