Update: Replaced the destructor example with a straight up method call example.
Hi,
If I have the following code:
class a
{
public:
virtual void func0(); // a has a VTable now
void func1();
};
class b : public a
{
public:
void func0() { a::func0(); }
void func2();
};
Thanks
VTable structure used by the compiler to keep track of the virtual functions associated with a class. There is one instance of a VTable for every class containing virtual functions. All instances of a given class point to the same VTable.
The vtable contains all inherited virtual functions and any newly introduced one.
There is only one VPTR for each object when using simple inheritance like this. The VPTR must be initialized to point to the starting address of the appropriate VTABLE. (This happens in the constructor, which you'll see later in more detail.)
Client applications and service providers written in C define MAPI objects by creating a data structure and an array of ordered function pointers known as a virtual function table, or vtable. A pointer to the vtable must be the first member of the data structure.
If you declare virtual functions you should also declare your destructor virtual ;-).
B has a virtual table, because it has a virtual function, namely func0()
. If you declare a function (including a destructor) virtual in a base class, all its derived classes will have the function with same signature virtual as well. And it will cause them to have a vtable. Moreover, B would have the vtable even if you didn't declare func0
in it explicitly.
Non-virtual functions are not referenced through vtables.
See 2.
No. Classes' vtables are constructed based on class declarations. The bodies of class' functions (let alone other functions) are not taken into account. Therefore, B has a vtable, because its function func0()
is virtual.
There also is a tricky detail, although it's not the gist of your question. You declared your function B::func0()
as inline. In gcc
compiler, if a virtual function is declared inline, it retains its slot in virtual table, the slot pointing to a special function emitted for that inline one (that counts as taking its address, which makes the inline emitted). That means, whether the funciton is inline doesn't influence amount of slots in vtable and its necessity for a class.
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