Are inline virtual functions really a non-sense?

Question

Virtual functions can be inlined sometimes. An excerpt from the excellent C++ faq:

"The only time an inline virtual call can be inlined is when the compiler knows the "exact class" of the object which is the target of the virtual function call. This can happen only when the compiler has an actual object rather than a pointer or reference to an object. I.e., either with a local object, a global/static object, or a fully contained object inside a composite."

C++11 has added final. This changes the accepted answer: it's no longer necessary to know the exact class of the object, it's sufficient to know the object has at least the class type in which the function was declared final:

class A { 
  virtual void foo();
};
class B : public A {
  inline virtual void foo() final { } 
};
class C : public B
{
};

void bar(B const& b) {
  A const& a = b; // Allowed, every B is an A.
  a.foo(); // Call to B::foo() can be inlined, even if b is actually a class C.
}

There is one category of virtual functions where it still makes sense to have them inline. Consider the following case:

class Base {
public:
  inline virtual ~Base () { }
};

class Derived1 : public Base {
  inline virtual ~Derived1 () { } // Implicitly calls Base::~Base ();
};

class Derived2 : public Derived1 {
  inline virtual ~Derived2 () { } // Implicitly calls Derived1::~Derived1 ();
};

void foo (Base * base) {
  delete base;             // Virtual call
}

The call to delete 'base', will perform a virtual call to call correct derived class destructor, this call is not inlined. However because each destructor calls it's parent destructor (which in these cases are empty), the compiler can inline those calls, since they do not call the base class functions virtually.

The same principle exists for base class constructors or for any set of functions where the derived implementation also calls the base classes implementation.

I've seen compilers that don't emit any v-table if no non-inline function at all exists (and defined in one implementation file instead of a header then). They would throw errors like missing vtable-for-class-A or something similar, and you would be confused as hell, as i was.

Indeed, that's not conformant with the Standard, but it happens so consider putting at least one virtual function not in the header (if only the virtual destructor), so that the compiler could emit a vtable for the class at that place. I know it happens with some versions of gcc.

As someone mentioned, inline virtual functions can be a benefit sometimes, but of course most often you will use it when you do not know the dynamic type of the object, because that was the whole reason for virtual in the first place.

The compiler however can't completely ignore inline. It has other semantics apart from speeding up a function-call. The implicit inline for in-class definitions is the mechanism which allows you to put the definition into the header: Only inline functions can be defined multiple times throughout the whole program without a violation any rules. In the end, it behaves as you would have defined it only once in the whole program, even though you included the header multiple times into different files linked together.

Well, actually virtual functions can always be inlined, as long they're statically linked together: suppose we have an abstract class Base with a virtual function F and derived classes Derived1 and Derived2:

class Base {
  virtual void F() = 0;
};

class Derived1 : public Base {
  virtual void F();
};

class Derived2 : public Base {
  virtual void F();
};

An hypotetical call b->F(); (with b of type Base*) is obviously virtual. But you (or the compiler...) could rewrite it like so (suppose typeof is a typeid-like function that returns a value that can be used in a switch)

switch (typeof(b)) {
  case Derived1: b->Derived1::F(); break; // static, inlineable call
  case Derived2: b->Derived2::F(); break; // static, inlineable call
  case Base:     assert(!"pure virtual function call!");
  default:       b->F(); break; // virtual call (dyn-loaded code)
}

while we still need RTTI for the typeof, the call can effectively be inlined by, basically, embedding the vtable inside the instruction stream and specializing the call for all the involved classes. This could be also generalized by specializing only a few classes (say, just Derived1):

switch (typeof(b)) {
  case Derived1: b->Derived1::F(); break; // hot path
  default:       b->F(); break; // default virtual call, cold path
}