Alex Rivera | Logout

Performance of dynamic_cast?

Asked 2010-10-29T10:14:31.173
60

Before reading the question:
This question is not about how useful it is to use dynamic_cast. Its just about its performance.

I've recently developed a design where dynamic_cast is used a lot.
When discussing it with co-workers almost everyone says that dynamic_cast shouldn't be used because of its bad performance (these are co-workers which have different backgrounds and in some cases do not know each other. I'm working in a huge company)

I decided to test the performance of this method instead of just believing them.

The following code was used:

ptime firstValue( microsec_clock::local_time() );

ChildObject* castedObject = dynamic_cast<ChildObject*>(parentObject);

ptime secondValue( microsec_clock::local_time() );
time_duration diff = secondValue - firstValue;
std::cout << "Cast1 lasts:\t" << diff.fractional_seconds() << " microsec" << std::endl;

The above code uses methods from boost::date_time on Linux to get usable values.
I've done 3 dynamic_cast in one execution, the code for measuring them is the same.

The results of 1 execution were the following:
Cast1 lasts: 74 microsec
Cast2 lasts: 2 microsec
Cast3 lasts: 1 microsec

The first cast always took 74-111 microsec, the following casts in the same execution took 1-3 microsec.

So finally my questions:
Is dynamic_cast really performing bad?
According to the testresults its not. Is my testcode correct?
Why do so many developers think that it is slow if it isn't?

c++ performance dynamic-cast boost-date-time
Edit
Report

1 Answer

32

Here are a few benchmarks:
http://tinodidriksen.com/2010/04/14/cpp-dynamic-cast-performance/
http://www.nerdblog.com/2006/12/how-slow-is-dynamiccast.html

According to them, dynamic_cast is 5-30 times slower than reinterpret_cast, and the best alternative performs almost the same as reinterpret_cast.

I'll quote the conclusion from the first article:

  • dynamic_cast is slow for anything but casting to the base type; that particular cast is optimized out
  • the inheritance level has a big impact on dynamic_cast
  • member variable + reinterpret_cast is the fastest reliable way to
    determine type; however, that has a lot higher maintenance overhead
    when coding

Absolute numbers are on the order of 100 ns for a single cast. Values like 74 msec doesn't seem close to reality.

answered 2011-09-28T06:40:51.533

Your Answer