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C++ and OOPS Sample Question

Note : All the C sample programs are tested under Turbo C/C++ compilers.
It is assumed that,

  • Programs run under DOS environment,
  • The underlying machine is an x86 system,
  • Program is compiled using Turbo C/C++ compiler.

The program output may depend on the information based on this assumptions (for example sizeof(int) == 2 may be assumed).

Following are some C sample questions.

Predict the output or error(s) for the following:

  1. class Sample
    {
    public:
            int *ptr;
            Sample(int i)
            {
            ptr = new int(i);
            }
            ~Sample()
            {
            delete ptr;
            }
            void PrintVal()
            {
            cout « "The value is " « *ptr;
            }
    };
    void SomeFunc(Sample x)
    {
    cout « "Say i am in someFunc " « endl;
    }
    int main()
    {
    Sample s1= 10;
    SomeFunc(s1);
    s1.PrintVal();
    }

    Answer:

    Say i am in someFunc
    Null pointer assignment(Run-time error)

    Explanation:

    As the object is passed by value to SomeFunc the destructor of the object is called when the control returns from the function. So when PrintVal is called it meets up with ptr that has been freed.The solution is to pass the Sample object by reference to SomeFunc:

    void SomeFunc(Sample &x)
    {
    cout « "Say i am in someFunc " « endl;
    }
    because when we pass objects by refernece that object is not destroyed. while returning from the function.
  2. Which is the parameter that is added to every non-static member function when it is called?

    Answer: 'this' pointer

  3. class base
            {
            public:
            int bval;
            base(){ bval=0;}
            };
    
    class deri:public base
            {
            public:
            int dval;
            deri(){ dval=1;}
            };
    void SomeFunc(base *arr,int size)
    {
    for(int i=0; i‹size; i++,arr++)
            cout«arr-›bval;
    cout«endl;
    }
    
    int main()
    {
    base BaseArr[5];
    SomeFunc(BaseArr,5);
    deri DeriArr[5];
    SomeFunc(DeriArr,5);
    }

    Answer:

    00000
    01010

    Explanation:

    The function SomeFunc expects two arguments.The first one is a pointer to an array of base class objects and the second one is the sizeof the array.The first call of someFunc calls it with an array of bae objects, so it works correctly and prints the bval of all the objects. When Somefunc is called the second time the argument passed is the pointeer to an array of derived class objects and not the array of base class objects. But that is what the function expects to be sent. So the derived class pointer is promoted to base class pointer and the address is sent to the function. SomeFunc() knows nothing about this and just treats the pointer as an array of base class objects. So when arr++ is met, the size of base class object is taken into consideration and is incremented by sizeof(int) bytes for bval (the deri class objects have bval and dval as members and so is of size ›= sizeof(int)+sizeof(int) ).

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