Question

I understand that dynamic/static polymorphism depends on the application design and requirements. However, is it advisable to ALWAYS choose static polymorphism over dynamic if possible? In particular, I can see the following 2 design choice in my application, both of which seem to be advised against:

1. Implement Static polymorphism using CRTP: No vtable lookup overhead while still providing an interface in form of template base class. But, uses a Lot of switch and static_cast to access the correct class/method, which is hazardous

2. Dynamic Polymorphism: Implement interfaces (pure virtual classes), associating lookup cost for even trivial functions like accessors/mutators

My application is very time critical, so am in favor of static polymorphism. But need to know if using too many static_cast is an indication of poor design, and how to avoid that without incurring latency.

EDIT: Thanks for the insight. Taking a specific case, which of these is a better approach?

class IMessage_Type_1
{
  virtual long getQuantity() =0;
...
}
class Message_Type_1_Impl: public IMessage_Type_1
{
  long getQuantity() { return _qty;}
...
}

OR

template <class T>
class TMessage_Type_1
{
  long getQuantity() { return static_cast<T*>(this)->getQuantity(); }
...
}
class Message_Type_1_Impl: public TMessage_Type_1<Message_Type_1_Impl>
{
  long getQuantity() { return _qty; }
...
}

Note that there are several mutators/accessors in each class, and I do need to specify an interface in my application. In static polymorphism, I switch just once – to get the message type. However, in dynamic polymorphism, I am using virtual functions for EACH method call. Doesnt that make it a case to use static poly? I believe static_cast in CRTP is quite safe and no performance penalty (compile time bound) ?

Answer 1

A switch is nothing more than a sequence of jumps that -after optimized- becomes a jump to an address looked-up by a table. Exactly like a virtual function call is.

If you have to jump depending on a type, you must first select the type. If the selection cannot be done at compile time (essentially because it depends on the input) you must always perform two operation: select & jump. The syntactic tool you use to select doesn’t change the performance, since optimize the same.

In fact you are reinventing the v-table.