A “core::Functoid” in Clasp is a base class for an object that can be called from Common Lisp. It implements a method that is the calling convention from Common Lisp. It can wrap an environment and whatever it needs to call some code.
It is the base class of core::Closure which also wraps an environment.
I have core::InterpretedClosure that also wraps a List_sp which is an S-expression that is walked to interpret it.
I have CompiledClosure that wraps an LLVM function pointer that can be called.
I have BuiltInClosure that wraps a pointer to a C++ function that can be called.
There are also VariadicMethoid objects that call C++ class instance methods with different numbers of arguments.
In many cases these classes are generated at C++ compile time using template code that generates everything needed to convert Common Lisp types to C++ types and back again. All that needs to be specified to expose a C++ function to Clasp Common Lisp is a string that becomes the name of the symbol that binds the C++ function and a pointer to the C++ function. Since Common Lisp types in Clasp are all implemented as C++ types the overhead of these conversions should be pretty low. You can write from_object and to_object (template classes) translators that do the translation back and forth.