static binding of default parameter

In Effective C++, the book just mentioned one sentence wh开发者_开发知识库y default parameters are static bound:

If default parameter values were dynamically bound, compilers would have to come up with a way to determine the appropriate default values for parameters of virtual functions at runtime, which would be slower and more complicated than the current mechanism of determining them during compilation.

Can anybody elaborate this a bit more? Why it is complicated and inefficient?

Thanks so much!

Whenever a class has virtual functions, the compiler generates a so-called v-table to calculate the proper addresses that are needed at runtime to support dynamic binding and polymorphic behavior. Lots of class optimizers work toward removing virtual functions for this reason exactly. Less overhead, and smaller code. If default parameters were also calculated into the equation, it would make the whole virtual function mechanism all the more cumbersome and bloated.

Because for a function call the actual call would need to be looked up using the vtables associated with the object instance and from that the deault would need to be inferred in some manner. Meaning that the ctable would need extension or there would need to be extra administration to link the default to a vtable entry.