C + + Object slicing

What's the object slicing:

C + + in the process of converting a derived class to a base class, a part of the derived class is not received by the base class, leaving only the object of the base class size.

A slice of a value call:

#include <iostream>using namespace std;class base{public:virtual void func1 () { cout <<  "Base::func1" &NBSP;&LT;&LT;&NBSP;ENDL;} VIRTUAL&NBSP;VOID&NBSP;FUNC2 () {cout <<  "Base::func2" &NBSP;&LT;&LT;&NBSP;ENDL;} private:int a;}; class derive :p ublic base{public:virtual void func1 () {cout <<  "Derive :: Func1 "&NBSP;&LT;&LT;&NBSP;ENDL;} VIRTUAL&NBSP;VOID&NBSP;FUNC3 () {cout <<  "derive::func3" &NBSP;&LT;&LT;&NBSP;ENDL;} Virtual void func4 () {cout <<  "Derive::func4" &NBSP;&LT;&LT;&NBSP;ENDL;} private:int b;}; Typedef void (*func)   (); void printvtable (int* vtable) {cout <<  "  Virtual table address > " << VTable << endl;for  (int i = 0; vtable[i)  != 0; ++i) {printf (" %d virtual function address  :0X%x,->",  i, vtable[i]); func f =  (FUNC) vtable[i];f ();} cout << endl;} Void function (base b) {b.func1 ();} Void test1 () {base b1;derive d1;int* vtable1 =  (int*) (* (int*) &b1);int*  vtable2 =  (int*) (* (int*) &d1); Printvtable (VTABLE1); Printvtable (VTable2); function (d1);} Int main () {Test1 (); System ("Pause");}

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in the process of copying a slice occurs, when the constructor is called to initialize the vptr point to the base class of vtable, and only copy the object's base class part, so finally become a base class object.

To prevent this from happening, simply define the base class as a pure virtual function.

Passing a pointer to a derived class to the base class

#include <iostream>using namespace std;class base{public:virtual void func1 () { cout <<  "Base::func1" &NBSP;&LT;&LT;&NBSP;ENDL;} VIRTUAL&NBSP;VOID&NBSP;FUNC2 () {cout <<  "Base::func2" &NBSP;&LT;&LT;&NBSP;ENDL;} private:int a;}; class derive :p ublic base{public:virtual void func1 () {cout <<  "Derive :: Func1 "&NBSP;&LT;&LT;&NBSP;ENDL;} VIRTUAL&NBSP;VOID&NBSP;FUNC3 () {cout <<  "derive::func3" &NBSP;&LT;&LT;&NBSP;ENDL;} Virtual void func4 () {cout <<  "Derive::func4" &NBSP;&LT;&LT;&NBSP;ENDL;} private:int b;}; Typedef void (*func)   (); void printvtable (int* vtable) {cout <<  "  Virtual table address > " << VTable << endl;for  (int i = 0; vtable[i)  != 0; ++i) {printf (" %d virtual function address  :0X%x,->",  i, vtable[i]); func f =  (FUNC) vtable[i];f ();} cout << endl;} Void function (base* b) {b->func1 (); B->func2 ();} Void test1 () {base b1;derive d1;int* vtable1 =  (int*) (* (int*) &b1);int*  vtable2 =  (int*) (* (int*) &d1); Printvtable (VTABLE1); Printvtable (VTable2); function ((base*) &d1);} Int main () {Test1 (); System ("Pause");}

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Explain:

When a pointer to a derived class is passed to the base class, the size of the derived class is treated as a cumulative size, so only the size of the base class can be accessed, because the constructor is not in place, so the vptr is still a derived class.

This article is from the "Fun" blog, make sure to keep this source http://10725723.blog.51cto.com/10715723/1782556

C + + Object slicing