Re: Performance cost of RTTI vs programmed type system

From:

blargg.h4g@gishpuppy.com (blargg)

Newsgroups:

comp.lang.c++.moderated

Date:

Fri, 1 Aug 2008 03:11:03 CST

Message-ID:

<blargg.h4g-0108080148070001@192.168.1.4>

In article
<41e77b2b-83a6-4f69-8c85-50ab59855fd2@w7g2000hsa.googlegroups.com>,
Sebastian Karlsson <Sebastian.Karlsson@mmorpgs.org> wrote:

I'm kind of comparing RTTI vs a programmed type sytem of the type:

enum Type { blah, etc }

class Base
{
virtual Type GetType() const = 0;
}

If speed is most important, you could even store the type as a field in
the object, making GetType() a single inline memory fetch and nothing
more:

     class Base {
         Type const type;
     public:
         Base( Type t ) : type( t ) { }
         Type GetType() const { return type; }
     };

If you're comparing to something like

     class Base2 {
     public:
         virtual ~Base2() { }
         std::type_info const& GetType() const { return typeid (*this); }
     };

On most compilers I'd expect two memory fetches to get the type: one to
read the vtable pointer, and another to get the pointer to the type_info
object. A smart compiler could perhaps store the type_info object in the
vtable, perhaps at a negative offset, eliminating one of the memory
fetches.

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