C++ – Performance hit from C++ style casts


I am new to C++ style casts and I am worried that using C++ style casts will ruin the performance of my application because I have a real-time-critical deadline in my interrupt-service-routine.

I heard that some casts will even throw exceptions!

I would like to use the C++ style casts because it would make my code more "robust". However, if there is any performance hit then I will probably not use C++ style casts and will instead spend more time testing the code that uses C-style casts.

Has anyone done any rigorous testing/profiling to compare the performance of C++ style casts to C style casts?

What were your results?

What conclusions did you draw?

Best Answer

If the C++ style cast can be conceptualy replaced by a C-style cast there will be no overhead. If it can't, as in the case of dynamic_cast, for which there is no C equivalent, you have to pay the cost one way or another.

As an example, the following code:

int x;
float f = 123.456;

x = (int) f;
x = static_cast<int>(f);

generates identical code for both casts with VC++ - code is:

00401041   fld         dword ptr [ebp-8]
00401044   call        __ftol (0040110c)
00401049   mov         dword ptr [ebp-4],eax

The only C++ cast that can throw is dynamic_cast when casting to a reference. To avoid this, cast to a pointer, which will return 0 if the cast fails.