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homework question on class...multiple choice

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homework question on class...multiple choice

Mar 23rd, 2006

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.

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Re: homework question on class...multiple choice

Mar 23rd, 2006

>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.

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Re: homework question on class...multiple choice

Mar 23rd, 2006

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..?

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#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;
}
#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;
}

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Re: homework question on class...multiple choice

Mar 23rd, 2006

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Originally Posted by Narue

c, this code causes an error.

Why is that?

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Re: homework question on class...multiple choice

Mar 24th, 2006

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Originally Posted by sunnypalsingh

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")

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Re: homework question on class...multiple choice

Mar 24th, 2006

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;

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Re: homework question on class...multiple choice

Mar 24th, 2006

>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.

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Re: homework question on class...multiple choice

Mar 24th, 2006

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()

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Re: homework question on class...multiple choice

Mar 24th, 2006

>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.

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

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Re: homework question on class...multiple choice

#10

Mar 24th, 2006

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??