OOP Q&A

Question 12

9/19/2004

mr Steil, i just have a question, i know it is so simple that it might be ignored but i guess it i better to ask anyway, i thought NULL and '\0' were the same, after researching a bit i found out that NULL is actually 0 but i don't completely understand when should NULL be used.

is '\0' only when it refers to the null terminator in strings?

thanks in advance.

Answer:

NULL is always used to represent the address 0. That is the address that you should make a pointer point to when it is really not pointing to memory you can use.

‘\0’ is called the null terminator. It is the special character that marks the end of a c style string. It also happens to have the ASCII value 0. It should not be used as an address in memory.

Question 11

2/11/2004

I had a point deducted for using 'break' in the following code. The explanation was that this is not structured. In this case, removing it would neither affect the result nor cause any major efficiency problems, but there are certainly situations in which it would be much better to exit the loop instead of wasting time on the remaining elements. The only other solution I could think of is to set Character to (m_Value.length()) to guarantee the condition would be false. That doesn't seem any more structured though.

What do you suggest I do?

for(unsigned int Character = 0; Character < m_Value.length(); Character++)

{

//if this character is not a number

if(m_Value[Character] < 48 || m_Value[Character] > 57)

{

//if this character is not a dash in the first position

if(Character != 0 || m_Value[Character] != '-')

{

IsANumber = false;

break;

}

}

}

Answer:

I agree with the grader. Using a brake to jump out of any loop or function is unstructured and unadvisable.

Just change the choice part of the "for loop" as follows:

for(unsigned int Character = 0;

Character < m_Value.length() && IsANumber;

Character++)

{

//if this character is not a number

if(m_Value[Character] < 48 || m_Value[Character] > 57)

{

//if this character is not a dash in the first position

if(Character != 0 || m_Value[Character] != '-')

{

IsANumber = false;

}

}

}

Question 10

2/9/2003

Why would you use a const_cast?

See http://www.adtmag.com/joop/crarticle.asp?ID=1550

Question 9 Applies to Assignment 1 CString Class

1/23/2004

In the conversion constructor, am I essentially taking a char* (or a char array, too, right?) and copying its value into the member variable?

Answer:

Yes. But be sure the char* passed in is not NULL. If it is just set the m_pString to NULL.

Question 8

1/23/2004

Are classes or objects instantiated?

Answer:

Objects are instantiated. An object is an instance or instantiation of a class.

Question 7 Applies to Assignment 1 CString Class

1/23/2004

In the copy/conversion constructors, when I debug it says m_pString is a bad pointer. Should I set it to NULL before attempting to allocate memory for it and storing information in it? 

Answer:

It is not necessary to set the pointer to NULL if you are going to be setting it to some new memory directly afterward. On the other hand in both the copy and conversion constructors the new string may itself be NULL. In that case you wish you had just set the m_pString to NULL in the first place. Also, if you plan on calling any of your functions from the constructors, m_pString needs to be set to NULL first. Otherwise it is just pointing to a random location.

Question 6 Applies to Assignment 1 CString Class

1/23/2004

I was also wondering if there is a more efficient way of performing the following code for my Append function:

Answer:

I would replace the code:

m_pString = new char[size];

strcpy(m_pString, temp);

delete []temp;

with:

m_pString = temp;

Question 5

1/21/2004

What type is size_t anyway, if not int? (couldn't find this in the help)

Answer:

size_t is defined as a unsigned integer. It is declared in STDDEF.H and other include files.

On a side note, size_type is defined the same way.

Question 4 Applies to Assignment 1 CString Class

1/21/2004

It only compiles if the copy constructor is declared non-const.

Otherwise it gives me the following error message:

"error C2662: 'CString::Value': cannot convert 'this' pointer from 'const CString' to 'CString &'"

Here is the relevant code:

//this works fine without the const

CString::CString(const CString& other)

{

//I intend to fix this

m_pString = new char[10];

m_pString[0] = '\0';

//This is where the error is supposedly coming from

Set(other.Value());

}

void CString::Set(const char* const NewString)

{

Delete();

size_t length = strlen(NewString);

m_pString = new char[length+10];

strncpy(m_pString, NewString, length+1);

}

const char* const CString::Value()

{

return m_pString;

}

Any idea what is causing this odd error?

Answer:

The error message is telling you that you can not call a function that may change a member variable for an object that has been declared constant. In this case the parameter other in the copy constructor is constant. You are not allowed to call the Value member function using this object because it may change some of its member variables. Fix this problem by adding the word const to the end of the Value function. We will discuss this more in class.

Question 3 Applies to Assignment 1 CString Class

1/21/2004

Why does CString::Value need to return a *const pointer? it seems (more

than) reasonable to return a const char*, but I don't see how it is

useful to restrict what the receiver does with the pointer he's holding.

Answer:

In this case you must restrict what the receiver does with the pointer he's holding because he is pointing at your data. Typically this sort of function is used only to copy values or to pass them to other functions. In OOP the concepts of data-hiding and encapsulation encourage programmers to restrict the modification of member variables to the member functions themselves.

Question 2

1/21/2004

Why do you want us to use Hungarian notation? (CClass, m_member_variable, ppointer, etc)

Answer:

Some naming conventions need to be used for documentation purposes. You programs will be much easier to read and maintain if you stick to some standard. I am enforcing my own set of standards for multiple reasons. First, students need to become accustom to conforming to the standards of their employer/teacher. Second, when all students in my classes conform to the same standard it makes grading much easier. Third, students need to become accustom to using a standard, at this point most students have not. They need to start now and get in the habit of producing cleaner and more manageable code. Lastly, it is both beneficial and comforting to most students if they are given a clear set of coding styles to conform to.

Also, for clarification I am not asking you my students to use Hungarian notation exactly.

Question 1

1/21/2004

I'm getting a warning because it's casting size_t to int. Will you

count off for this or will I have to write code to cast it?

Answer:

Yes you need to type cast the size_t to an int to remove the compiler warning. And yes, I will "count off" for such compiler warnings. In general it is a good idea to get rid of all compiler warnings even if you think they will not cause any runtime problems. It is easier to just get rid of the compiler warnings than to read them each time you compile to see if it is one you need to worry about. When you are working on a programming team a team mate of your should not have to read through a list of compiler warnings you created to see if they need to fix them. It is a big time saver to just fix them the first time you see them.