Re: Cost of dynamic_cast
Ole Nielsby wrote:
I just happened to F11 into a dynamic_cast in a disassember
window, and I was stunned by the complexity of this operation.
I have used it in quite a few places, assuming the operation
would be fast.
In single-inheritance systems like Java and .NET, the operation
is - or ought to be - pretty fast. I know multiple inheritance is
bound to complicate matters a bit, but C++ is supposed to be
a langage where you don't pay for what you don't use, so I
expected the cast would be fast when no multiple inheritance
is involved.
This performance also depends on the compiler, in particular the VC6
compiler is way outdated. In case you hit performance problems, upgrade the
compiler first, later versions are generally much faster.
class A;
class B;
class Base {
public:
virtual *A as_A() {return NULL;}
virtual *B as_B() {return NULL;}
};
class A {public: *A as_A() {return this;}};
class B {public: *B as_B() {return this;}};
(and accept a vtable bloat of order N^2)
or is there a better way???
Well, first thing is that you measure, which is preliminary for performance.
Second thing is to reduce the number of dynamic_casts, in case those are
really critical. You can e.g. cache results or maybe move the dynamic
nature of your program to a more abstract level, keeping the low-level code
more concrete (and free of such casts).
Sometimes, the so-called visitor pattern (a.k.a. double dispatch) also
helps. In particular when you need to add a virtual function externally
without intruding into the hierarchy of classes this is a good replacement.
Also, but with a slightly different behaviour, you can compare the typeid()
of different objects, which is generally faster and also helps.
Uli