The jit can optimize away virtual function calls.

    ParentClass g = ....might be a subclass...

    for (int i=0;i<100000;i++) {
      g.func(i);
    }

The JIT can optimize away the vtable lookups to find func, and sometimes inline the code. Ok, maybe you could do this in C++ too.

    ParentClass[] g = ....might be a subclass...

    for (int i=0;i<100000;i++) {
      g[i].func(i);
    }

Suppose 99% of g's are the same class. The JIT can optimize away most of the virtual function lookups (particularly if this is one of your hotspots). I.e. the code becomes:

    ParentClass[] g = ....might be a subclass...

    for (int i=0;i<100000;i++) {
      if (g.class == COMMONCLASS) {
        inlined_func(i);
      else {
        g[i].func(i);
      }
    }

In the common case, a simple pointer comparison + inlined code is a lot faster than a function dereferene.