How expensive are dynamic casts in C++?

Question

For my GUI API which works with a variety of backends (sdl, gl, d3d, etc) I want to dynamically cast the generic type image to whatever it may happen to be.

So the bottom line is, I would be doing around 20 * 60fps dynamic casts per second.

How expensive is a dynamic cast? Will I notice that it has a noticeable negative impact on performance? What alternatives do I have that still maintain an acceptable level of performance?

Answer 1

1200 dynamic_casts per second is not likely to be a major performance problem. Are you doing one dynamic_cast per image, or a whole sequence of if statements until you find the actual type?

If you're worried about performance, the fastest ways to implement polymorphism are:

• --- fastest ---
• Function overloading (compile-time polymorphism only)
• CRTP (compile-time polymorphism only)
• Tags, switches and static casts (brittle, doesn't support multi-level inheritance, a maintenance headache so not recommended for unstable code)
• Virtual functions
• Visitor pattern (inverted virtual function)
• --- almost as fast ---

In your situation, the visitor pattern is probably the best choice. It's two virtual calls instead of one, but allows you to keep the algorithm implementation separate from the image data structure.