title: Can you cache a virtual function lookup in C++?
body: <p>Say I have a virtual function call foo() on an abstract base class pointer, mypointer->foo(). When my app starts up, based on the contents of a file, it chooses to instantiate a particular concrete class and assigns mypointer to that instance. For the rest of the app's life, mypointer will <em>always</em> point to objects of that concrete type. I have no way to know what this concrete type is (it may be instantiated by a factory in a dynamically loaded library). I only know that the type will stay the same after the first time an instance of the concrete type is made. The pointer may not always point to the same object, but the object will always be of the same concrete type. Notice that the type is technically determined at 'runtime' because it's based on the contents of a file, but that after 'startup' (file is loaded) the type is fixed.</p>
  
  <p>However, in C++ I pay the virtual function lookup cost every time foo is called for the entire duration of the app. The compiler can't optimize the look up away because there's no way for it to know that the concrete type won't vary at runtime (even if it was the most amazing compiler ever, it can't speculate on the behavior of dynamically loaded libraries). In a JIT compiled language like Java or .NET the JIT can detect that the same type is being used over and over and do <a href="http://en.wikipedia.org/wiki/Inline_caching" rel="noreferrer">inline cacheing</a>. I'm basically looking for a way to manually do that for specific pointers in C++.</p>
  
  <p>Is there any way in C++ to cache this lookup? I realize that solutions might be pretty hackish. I'm willing to accept ABI/compiler specific hacks if it's possible to write configure tests that discover the relevant aspects of the ABI/compiler so that it's "practically portable" even if not truly portable.</p>
  
  <p>Update: To the naysayers: If this wasn't worth optimizing, then I doubt modern JITs would do it. Do you think Sun and MS's engineers were wasting their time implementing inline cacheing, and didn't benchmark it to ensure there was an improvement?</p>