Can virtual functions be inlined

Question

If I define a class like this:

class A{
public:
    A(){}
    virtual ~A(){}
    virtual void func(){}
};

Does it mean that that the virtual destructor and func are inlined

Answer 1

Whether the compiler chooses to inline a function which is defined inline is entirely up to the compiler. In general, virtual functions can only be inlined when the compiler can either prove that the static type matches the dynamic type or when the compiler can safely determine the dynamic type. For example, when you use a value of type A the compiler knows that the dynamic type cannot be different and it can inline the function. When using a pointer or a reference the compiler generally cannot prove that the static type is the same and virtual functions generally need to follow the usual virtual dispatch. However, even when a pointer is used, the compiler may have enough information from the context to know the exact dynamic type. For example, MatthieuM. gave the following exmaple:

A* a = new B;
a->func();

In this case the compiler can determine that a points to a B object and, thus, call the correct version of func() without dynamic dispatch. Without the need for the dynamic dispatch, func() could then be inlined. Of course, whether compilers do the corresponding analysis depends on its respective implementation.

As hvd correctly pointed out, the virtual dispatch can be circumvented by calling a virtual function will full qualification, e.g., a->A::func(), in which case the virtual function can also be inlined. The main reason virtual functions are generally not inlined is the need to do a virtual dispatch. With the full qualification the function to be called is, however, known.