virtual function Implementation Mechanism of C + +

The implementation mechanism of virtual functions in C + + is mainly vtable and virtual pointers. Details are as follows:

Class A {

Public

virtual void F1 ();

virtual void F2 ();

Private

int A;

}

Class B {

Public

void F1 ();

Private

int b;

}

such as a, B, two classes, the compiler prepares a virtual table for Class A Vtablea as follows:

A::F1 's address

A::F2 's address

The compiler prepares the Vtableb for Class B as follows:

B::F1 's address

A::F2 's address

Class B overrides the F1, so the entry address of B::F1 () is recorded in the vtable of Class B, and F2 is inherited from a, so F2 still uses the A::F2 's entry address.

When you define b b = new B (), the compiler allocates space and assigns a virtual pointer vptr to the vtable that points to B.

So when using the following statement:

A *pa = &b; PA->F1 ();

The compiler knows that F1 is a virtual member function whose entry address is placed in the first item of the table, so it is converted to call* (PA->VPTR) [0]. This is the entry address where the B::F1 is placed.

The polymorphism is realized.

Copyright NOTICE: This article for Bo Master original article, without Bo Master permission not reproduced.

virtual function Implementation Mechanism of C + +