Is virtual function table really necessary for C + +

OOP polymorphism

In OOP languages, a Base-class object pointer can do function call according to the actual type of the object. Let's see a example in Java.

 Public classanimal{ Public voidSpark () {System.out.println ("Spark"); }  } Public classDogextendsanimal{ Public voidSpark () {System.out.println ("Wangwang"); }} Public classCatextendsanimal{ Public voidSpark () {System.out.println ("Miaomiao"); }} Public classmain{ Public Static voidMain (string[] args) {animal[] animals=NewAnimal[3]; animals[0]=NewDog (); animals[1]=NewCat (); animals[2]=NewAnimal ();  for(Animal it:animals) {It.spark () }}}

It would output

Wangwangmiaomiaospart

That ' s " polymorphism ".  

c++virtual function

Above codes is very natual for Java programmers. However in C + +, you can ' t get such "polymorphism" without the "virtual" keyword decorating the functions. Without "virtual", C + + would output

Sparksparkspark

Take a look at <the virtual table> If you don ' t yet know about virtual table. This article explains why "virtual" came out, and how virtual function was supported by virtual table.

Why virtual table

Why does C + + use virtual table?

=>because C + + compiler does not know the actual function address

--->why?

=>because C + + compiler does not know the exact type (Cat? Dog? Animal?) Of the Oject the pointer "Panimal" points to

---why? is, any of the compiler can figure out the object type?

=>yes! Using "Object type Tracking"!

Object Type Tracking

Let's consider the sources where an object is pointer gets its value. 2 sources indeed.

1. Another pointer2. Address of class instance

Where does "another pointer" get its value? Eventually, there's a pointer that gets the value from "class instance".

So, via tracking the assignment thread backwards to the original source object

= = The compiler is able the exact type of a pointer.

=>the compiler knows the address of the exact function being called

=>no virtual table is needed.

Object type tracking saves both virtual table memery and virtual table pointer of each class instances.

Where does object type tracking not work

Library linking.

If A library function returns a base-class pointer, there's no-for-the-compiler-track-to-the-original source O Bject.

Is virtual function table really necessary for C + +