Thread: why base ptr cannot refer the virtual funtion in derived class

  1. #1
    Registered User
    Join Date
    Oct 2006

    Red face why base ptr cannot refer the virtual funtion in derived class

    I cannot compile this code.
    From my understanding about virtual function, the compiler should have no problem to de-referencing the f4 in its derived object.

    Since the virtual ptr is already in the derived object, I guess even though this object is passed to the base ptr as in the code, when de-referencing f4 in the object, the v-ptr should know the position without problem.

    currently I got compilation failed.
    Any one can explain it reasonably?

    ~/private/test>more bv.C
    #include <iostream>
    using namespace std;
    class base {
         base() {}
         virtual void f3() { cout<<"base::f3()"<<endl;}
    class derived: public base {
         void f3(){cout<<"derived::f3()"<<endl;}
         virtual void f4(){cout<<"derived::f4()"<<endl;}
    int main(){
    base* p = new derived;
    return 0;
    ~/private/test>g++ bv.C
    bv.C: In function `int main()':
    bv.C:24: error: `f4' undeclared (first use this function)
    bv.C:24: error: (Each undeclared identifier is reported only once for each
    function it appears in.)

  2. #2
    Registered User
    Join Date
    Jun 2004
    The actual type of the instance is base * and thats what the compiler sees.
    base doesnt contain a function f4 so it doesnt compile.

    ((derived *)p)->f4(); would compile but its probably not what is wanted

  3. #3
    Registered User
    Join Date
    Jan 2005
    The v-ptr is used at runtime. That code still must be compiled. At compile time, how does the compiler know that f4 exists? What if someone else created another derived class and used that instead. The compiler can't tell what derived class type the instance actually has, it only knows the static type of the pointer, so it wouldn't know if that pointer really pointed to a class that had an f4 or not.

  4. #4
    Hurry Slowly vart's Avatar
    Join Date
    Oct 2006
    Rishon LeZion, Israel
    Add pure virtual f4 to the base class

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