homework question on class...multiple choice

[jack223]

I have no clue what the answer is.

Which of the following dynamically allocates an object of type ourClass?

a) ourClass ourPtr;
ourPtr = new ourClass;

b) ourClass ourClassObj = new ourClass;

c) typedef ourClass* ourClassPtrType;
ourClassPtrType ourClassPtr;
ourClassPtr = new ourClassPtrType;

d) typedef ourClass* ourClassPtrType;
ourClassPtrType ourClassPtr;
ourClassPtr = new ourClassPtrType;

The class Derived is publicly derived from a class Base. The Base class has a public member function mem() that is not virtural. The member function mem() is redefined in class Derived.

Base *dPtr = new Derived;
dptr ->mem();

In the second line, which version of mem() is accessed?

I think the aswer is "b" but i'm not sure.

a) Base::mem()
b) Derived::mem()
c) This code causes an error
d) Both Base::mem() and Derived::mem()

any help will be appreciated.

[Narue]

>Which of the following dynamically allocates an object of type ourClass?
e, none of the above.

>In the second line, which version of mem() is accessed?
c, this code causes an error.

[Bench]

Compile this code.. then try changing bits of it... what happens when you modify it to some of the scenarios given in your homework assignment..?

#include <iostream>
using namespace std;

class duck
{
public:
    virtual void speak() { cout << "quack"; }
};

class swan : public duck
{
public:
    void speak() { cout << "This is a swan"; }
};

class heron : public duck
{
public:
    void speak() { cout << "This is a heron"; }
};

int main()
{
    duck* myDuck = new swan;
    myDuck->speak();
    cin.get();
    return 0;
}

[SpS]

c, this code causes an error.

Why is that?

[Bench]

Why is that?

I think Narue is nitpicking a bit... nevertheless its the right answer according to the text in the post - The OP has probably made a typo (assuming he copied it from a book or sheet).. it took me a few reads to work out what the error was :)
(Compiler error would go something like "dptr: undeclared variable")

[jack223]

oops~ sorry.

i made a typo on the first question....is that mean the answer is "C"

c) typedef ourClass* ourClassPtrType;
ourClassPtrType ourClassPtr;
ourClassPtr = new ourClass;

d) typedef ourClass* ourClassPtrType;
ourClassPtrType ourClassPtr;
ourClassPtr = new ourClassPtrType;

[Narue]

>i made a typo on the first question
You made a typo on both questions.

>is that mean the answer is "C"
Yes, that's more like it.

[jack223]

no, i don't think i made any mis typo on the second question...i double checked with my homework sheet...maybe the professor made a mistake.

Base *dPtr = new Derived;
dPtr -> mem();

In the second line, which version of mem() is accessed?

a) Base::mem()
b) Derived::mem()
c) This code causes an error.
d) Both Base::mem() and Derived::mem()

[Narue]

>i don't think i made any mis typo on the second question
Then the worksheet had a typo. dptr is not the same as dPtr, and the example would fail to compile.

Base *dPtr = new Derived;
dPtr -> mem();

A pointer has a real type and a virtual type. The real type of dPtr is Base (the type it was declared as), and the virtual type is Derived (the actual object type pointed to). Unless a function is declared as virtual, the real type will be used, so dPtr->mem() will call Base::mem since mem isn't virtual.

[jack223]

if you look at my original question, i stated that "The Base class has a public member function mem() that is not virtural. The member function mem() is redefined in class Derived."

the base class mem() is not virtual so dPtr -> mem(); will call Base::mem??

[Narue]

>if you look at my original question, i stated that
Yes, I read it, but thanks for the reminder anyway.

>the base class mem() is not virtual so dPtr -> mem(); will call Base::mem??
Seeing as how this is a variation of what I said:

so dPtr->mem() will call Base::mem since mem isn't virtual.

It seems like you understand. The real type of dPtr is Base because it was declared as a pointer to Base. mem isn't a polymorphic function, and dPtr has no way of knowing that it actually points to an object of Derived. Therefore, Base::mem is used instead of Derived::mem.

[jack223]

Thanks for your help Narue...you have a great weekend.
thanks again.