when is a v-table created in C++?

Question

When exactly does the compiler create a virtual function table?

1) when the class contains at least one virtual function.

OR

2) when the immediate base class contains at least one virtual function.

OR

3) when any parent class at any level of the hierarchy contains at least one virtual function.

A related question to this: Is it possible to give up dynamic dispatch in a C++ hierarchy?

e.g. consider the following example.

#include <iostream>
using namespace std;
class A {
public:
  virtual void f();
};
class B: public A {
public:
  void f();
};
class C: public B {
public:
  void f();
};

Which classes will contain a V-Table?

Since B does not declare f() as virtual, does class C get dynamic polymorphism?

Answer 1

The answer is, 'it depends'. It depends on what you mean by 'contain a vtbl' and it depends on the decisions made by the implementor of the particular compiler.

Strictly speaking, no 'class' ever contains a virtual function table. Some instances of some classes contain pointers to virtual function tables. However, that's just one possible implementation of the semantics.

In the extreme, a compiler could hypothetically put a unique number into the instance that indexed into a data structure used for selecting the appropriate virtual function instance.

If you ask, 'What does GCC do?' or 'What does Visual C++ do?' then you could get a concrete answer.

@Hassan Syed's answer is probably closer to what you were asking about, but it is really important to keep the concepts straight here.

There is behavior (dynamic dispatch based on what class was new'ed) and there's implementation. Your question used implementation terminology, though I suspect you were looking for a behavioral answer.

The behavioral answer is this: any class that declares or inherits a virtual function will exhibit dynamic behavior on calls to that function. Any class that does not, will not.

Implementation-wise, the compiler is allowed to do whatever it wants to accomplish that result.

Answer 2

Answer

a vtable is created when a class declaration contains a virtual function. A vtable is introduced when a parent -- anywhere in the heirarchy -- has a virtual function, lets call this parent Y. Any parent of Y WILL NOT have a vtable (unless they have a virtual for some other function in their heirarchy).

Read on for discussion and tests

-- explanation --

When you specify a member function as virtual, there is a chance that you may try to use sub-classes via a base-class polymorphically at run-time. To maintain c++'s guarantee of performance over language design they offered the lightest possible implementation strategy -- i.e., one level of indirection, and only when a class might be used polymorphically at runtime, and the programmer specifies this by setting at least one function to be virtual.

You do not incur the cost of the vtable if you avoid the virtual keyword.

-- edit : to reflect your edit --

Only when a base class contains a virtual function do any other sub-classes contain a vtable. The parents of said base class do not have a vtable.

In your example all three classes will have a vtable, this is because you can try to use all three classes via an A*.

--test - GCC 4+ --

#include <iostream>

class test_base
{
  public:
    void x(){std::cout << "test_base" << "\n"; };
};

class test_sub : public test_base
{
public:
  virtual void x(){std::cout << "test_sub" << "\n"; } ;
};

class test_subby : public test_sub
{
public:
  void x() { std::cout << "test_subby" << "\n"; }
};

int main() 
{
  test_sub sub;
  test_base base;
  test_subby subby;

  test_sub * psub;
  test_base *pbase;
  test_subby * psubby;

  pbase = &sub;
  pbase->x();
  psub = &subby;
  psub->x();

  return 0;
}

output

test_base
test_subby

test_base does not have a virtual table therefore anything casted to it will use the x() from test_base. test_sub on the other hand changes the nature of x() and its pointer will indirect through a vtable, and this is shown by test_subby's x() being executed.

So, a vtable is only introduced in the hierarchy when the keyword virtual is used. Older ancestors do not have a vtable, and if a downcast occurs it will be hardwired to the ancestors functions.

Answer 3

C++ standards doesn't mandate using V-Tables to create the illusion of polymorphic classes. Most of the time implementations use V-Tables, to store the extra information needed. In short, these extra pieces of information are equipped when you have at least one virtual function.

Answer 4

Beyond "vtables are implementation-specific" (which they are), if a vtable is used: there will be unique vtables for each of your classes. Even though B::f and C::f are not declared virtual, because there is a matching signature on a virtual method from a base class (A in your code), B::f and C::f are both implicitly virtual. Because each class has at least one unique virtual method (B::f overrides A::f for B instances and C::f similarly for C instances), you need three vtables.

You generally shouldn't worry about such details. What matters is whether you have virtual dispatch or not. You don't have to use virtual dispatch, by explicitly specifying which function to call, but this is generally only useful when implementing a virtual method (such as to call the base's method). Example:

struct B {
  virtual void f() {}
  virtual void g() {}
};

struct D : B {
  virtual void f() { // would be implicitly virtual even if not declared virtual
    B::f();
    // do D-specific stuff
  }
  virtual void g() {}
};

int main() {
  {
    B b; b.g(); b.B::g(); // both call B::g
  }
  {
    D d;
    B& b = d;
    b.g(); // calls D::g
    b.B::g(); // calls B::g

    b.D::g(); // not allowed
    d.D::g(); // calls D::g

    void (B::*p)() = &B::g;
    (b.*p)(); // calls D::g
    // calls through a function pointer always use virtual dispatch
    // (if the pointed-to function is virtual)
  }
  return 0;
}

Some concrete rules that may help; but don't quote me on these, I've likely missed some edge cases:

• If a class has virtual methods or virtual bases, even if inherited, then instances must have a vtable pointer.
• If a class declares non-inherited virtual methods (such as when it doesn't have a base class), then it must have its own vtable.
• If a class has a different set of overriding methods than its first base class, then it must have its own vtable, and cannot reuse the base's. (Destructors commonly require this.)
• If a class has multiple base classes, with the second or later base having virtual methods:
  • If no earlier bases have virtual methods and the Empty Base Optimization was applied to all earlier bases, then treat this base as the first base class.
  • Otherwise, the class must have its own vtable.
• If a class has any virtual base classes, it must have its own vtable.

Remember that a vtable is similar to a static data member of a class, and instances have only pointers to these.

Also see the comprehensive article C++: Under the Hood (March 1994) by Jan Gray. (Try Google if that link dies.)

Example of reusing a vtable:

struct B {
  virtual void f();
};
struct D : B {
  // does not override B::f
  // does not have other virtuals of its own
  void g(); // still might have its own non-virtuals
  int n; // and data members
};

In particular, notice B's dtor isn't virtual (and this is likely a mistake in real code), but in this example, D instances will point to the same vtable as B instances.

Answer 5

You made an effort to make your question very clear and precise, but there's still a bit of information missing. You probably know, that in implementations that use V-Table, the table itself is normally an independent data structure, stored outside the polymorphic objects, while objects themselves only store a implicit pointer to the table. So, what is it you are asking about? Could be:

• When does an object get an implicit pointer to V-Table inserted into it?

or

• When is a dedicated, individual V-Table created for a given type in the hierarchy?

The answer to the first question is: an object gets an implicit pointer to V-Table inserted into it when the object is of polymorphic class type. The class type is polymorphic if it contains at least one virtual function, or any of its direct or indirect parents are polymorphic (this is answer 3 from your set). Note also, that in case of multiple inheritance, an object might (and will) end up containing multiple V-Table pointers embedded into it.

The answer to the second question could be the same as to the first (option 3), with a possible exception. If some polymorphic class in single inheritance hierarchy has no virtual functions of its own (no new virtual functions, no overrides for parent virtual function), it is possible that implementation might decide not to create an individual V-Table for this class, but instead use it's immediate parent's V-Table for this class as well (since it is going to be the same anyway). I.e. in this case both objects of parent type and objects of derived type will store the same value in their embedded V-Table pointers. This is, of course, highly dependent on implementation. I checked GCC and MS VS 2005 and they don't act that way. They both do create an individual V-Table for the derived class in this situation, but I seem to recall hearing about implementations that don't.

Answer 6

The behavior is defined in chapter 10.3, paragraph 2 of the C++ language specification:

If a virtual member function vf is declared in a class Base and in a class Derived, derived directly or indirectly from Base, a member function vf with the same name and same parameter list as Base::vf is declared, then Derived::vf is also virtual ( whether or not it is so declared ) and it overrides Base::vf.

A italicized the relevant phrase. Thus, if your compiler creates v-tables in the usual sense then all classes will have a v-table since all their f() methods are virtual.