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Closed 12 years ago.
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why do I need virtual table?
What is a vtable in C++?
So far I know that vtable is a virtual table which has an array of pointers to virtual functions. Is there an article I can read with an example of a practical implementation? (Any walk through will be appreciated.)
V-tables (or virtual tables) are how most C++ implementations do polymorphism. For each concrete implementation of a class, there is a table of function pointers to all the virtual methods. A pointer to this table (called the virtual table) exists as a data member in all the objects. When one calls a virtual method, we lookup the object's v-table and call the appropriate derived class method.
vTable (virtual table) is an implementation detail of dynamic dispatch (virtual methods).
See C++-Lite-Faq for more details.
For all it's worth, it is not a standard C++ terminology. It is just an implementation detail used by the implementation to implement virtual functions/dynamic binding
Related
This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
why do I need virtual table?
What is a vtable in C++?
So far I know that vtable is a virtual table which has an array of pointers to virtual functions. Is there an article I can read with an example of a practical implementation? (Any walk through will be appreciated.)
V-tables (or virtual tables) are how most C++ implementations do polymorphism. For each concrete implementation of a class, there is a table of function pointers to all the virtual methods. A pointer to this table (called the virtual table) exists as a data member in all the objects. When one calls a virtual method, we lookup the object's v-table and call the appropriate derived class method.
vTable (virtual table) is an implementation detail of dynamic dispatch (virtual methods).
See C++-Lite-Faq for more details.
For all it's worth, it is not a standard C++ terminology. It is just an implementation detail used by the implementation to implement virtual functions/dynamic binding
How does Virtual Method Invocation work in C++?
Through virtual tables.
Read this article, http://en.wikipedia.org/wiki/Virtual_table.
I could explain it here, but the wikipedia does a better job than I could.
The C++ standard doesn't specify how the virtual function mechanism should be implemented.
That said, I think all current C++ compilers use virtual tables.
The common way to do this for classes which contain at least one virtual function to have a hidden pointer to a so-called virtual table, where the addresses of the virtual functions for a specific class are entered in compiler-specific order.
Each constructor will then set this hidden pointer to the virtual table of the class it belongs to.
Every class with at least one virtual method has it's virtual table - table of pointers to functions that are that class's methods.
It's extensively used in COM.
With VTables and function pointers. Virtual functions' function pointer will be listed in VTable
MFC is using Message Map instead of Virtual function, which reduces the size limitation. If we use several virtual function VTable will end up with big size.
How does Virtual Method Invocation work in C++?
Through virtual tables.
Read this article, http://en.wikipedia.org/wiki/Virtual_table.
I could explain it here, but the wikipedia does a better job than I could.
The C++ standard doesn't specify how the virtual function mechanism should be implemented.
That said, I think all current C++ compilers use virtual tables.
The common way to do this for classes which contain at least one virtual function to have a hidden pointer to a so-called virtual table, where the addresses of the virtual functions for a specific class are entered in compiler-specific order.
Each constructor will then set this hidden pointer to the virtual table of the class it belongs to.
Every class with at least one virtual method has it's virtual table - table of pointers to functions that are that class's methods.
It's extensively used in COM.
With VTables and function pointers. Virtual functions' function pointer will be listed in VTable
MFC is using Message Map instead of Virtual function, which reduces the size limitation. If we use several virtual function VTable will end up with big size.
How does Virtual Method Invocation work in C++?
Through virtual tables.
Read this article, http://en.wikipedia.org/wiki/Virtual_table.
I could explain it here, but the wikipedia does a better job than I could.
The C++ standard doesn't specify how the virtual function mechanism should be implemented.
That said, I think all current C++ compilers use virtual tables.
The common way to do this for classes which contain at least one virtual function to have a hidden pointer to a so-called virtual table, where the addresses of the virtual functions for a specific class are entered in compiler-specific order.
Each constructor will then set this hidden pointer to the virtual table of the class it belongs to.
Every class with at least one virtual method has it's virtual table - table of pointers to functions that are that class's methods.
It's extensively used in COM.
With VTables and function pointers. Virtual functions' function pointer will be listed in VTable
MFC is using Message Map instead of Virtual function, which reduces the size limitation. If we use several virtual function VTable will end up with big size.
This question already has answers here:
Closed 12 years ago.
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C++ Virtual/Pure Virtual Explained
What's the difference between virtual function instantiations in c++
Why pure virtual function is initialized by 0?
This is a method in some class declaration that someone gave me. And I don't know what '..=0' means. What is it?
virtual void Print() const = 0;
The = 0 makes the function pure virtual, rendering the class an abstract class.
An abstract class basically is a kind of interface, which derived classes need to implement in order to be instantiable. However, there's much more to this, and it is some of the very basics of object-oriented programming in C++. If you don't know these, you need to go back to the textbook and read up. There's no way you can advance without understanding them.
That said, see this related question for some explanations of what virtual and pure virtual functions are. And as always, the C++ FAQ is an excellent resource for such questions.
It means that the virtual function is pure, meaning that you cannot call it as such: the function doesn't have any code to it, hence the = 0. Only by deriving the class and overriding the function you can call it. The class with pure virtual functions cannot be instantiated so they are called abstract classes, interfaces in some languages.
Basically, it means the function has no code. This means that you cannot use instances of this class. Rather, it can only be a base class.