Nice trick or illegal construct?

[rmilh]

As you can see in this example the coder did't think it is necessary
to have an instance of a Derived to be able to set Base member prot.
Is it legal?

class Base
{
protected:

int prot;
};

class Derived : public Base
{
public:

using Base::prot;
};

int main()
{
Base base;
//instance of derived not needed to access protected member?
static_cast<Derived&>(base).prot = 1;
return 0;
}

Robert Milharcic

[Ivan Vecerina]

"rmilh" <rm***@mailaddr.com> wrote in message
news:uq********************************@4ax.com...

As you can see in this example the coder did't think it is necessary
to have an instance of a Derived to be able to set Base member prot.
Is it legal? .... Base base;
//instance of derived not needed to access protected member?
static_cast<Derived&>(base).prot = 1;

Not a good idea.
Formally, casting a base class instance as a derived type triggers
undefined behavior (even though the above code is likely to work
"as expected" on many platforms).
Cheers, Ivan
--
http://ivan.vecerina.com/contact/?subject=NG_POST <- email contact form
Brainbench MVP for C++ <> http://www.brainbench.com

[Tom|420]

rmilh wrote:

static_cast<Derived&>(base).prot = 1;

dynamic_cast is more appropriate for classes

[Andre Caldas]

Hello!

Is it legal?

class Base *snip*
class Derived : public Base *snip*
int main()
{
Base base;
//instance of derived not needed to access protected member?
static_cast<Derived&>(base).prot = 1;
return 0;
}

This only works because Base starts at the "begining" of Derived. If you
had:
class Derived: public A, public Base, public C
{
...
}

Then I think it would not work.

Andre Caldas.

[Clark S. Cox III]

On 2005-04-09 02:22:37 -0400, rmilh <rm***@mailaddr.com> said:

As you can see in this example the coder did't think it is necessary
to have an instance of a Derived to be able to set Base member prot.
Is it legal?
No.
class Base;
class Derived : public Base; Base base;
static_cast<Derived&>(base)

Your cast is undefined behavior.

--
Clark S. Cox, III
cl*******@gmail.com