Re: Future of C++
On Aug 10, 2:02 am, Bart van Ingen Schenau <b...@ingen.ddns.info>
wrote:
Why all the complication, when you can simply do
typeid(*pb); /* where pb is a pointer to the base class */
The standard guarantees that this yields the type_info for the most
derived class, if the base class is polymorphic.
True, however if the parent class has no virtual functions, it doesn't
happen. Inheritance is not synonym for polymorphism. A design may call
for just extending the base interface in derived classes. Or
inheriting only the behavior from the base class.
That said, try this and see what you get:
#include <iostream>
class Base {};
class Derived : public Base {};
int main()
{
Derived* pd = new Derived;
Base* pb = pd;
std::cout << typeid(*pb).name() << std::endl;
delete pd;
}
MSVC and gcc will print the base type name. I'd say that any compliant
compiler should do the same.
When the parent class has virtual functions, I suspect the performance
penalty of typeid(*pb) will be the same as for dynamic_cast and
virtual call returning typeid(*this) may be a cheaper way to go. The
bottom line is, the original example used to mock the code structure
could have been pulled out of context and the original author may have
had a valid case for it.
There is a subtle, but important difference in the two examples.
In the example given by Felipe, the derived class returns the type_info
of *itself*. This is completely useless, as the client code can obtain
the exact same information without a virtual function call.
In the boost::any code, the holder class returns the type_info for *the
class it contains* and is instantiated upon. This information is not
obtainable in any other way.
I agree about the subtle difference. However, Felipe has referred to
Ogre authors in a very condescending way, which has prompted me to
actually look into the code and, as it turns out, it is a copy of
boost::any. I am, BTW, in no way associated to Ogre project/code.
Alex
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