Class design

Hi all, I have this class in my code:
class Mystring {
public:
char *s;
Mystring(const char *);
Mystring();
~Mystring(){}

Mystring operator=(const char* str)
{

strcpy (s, str);
return *this;
}
operator char *()//Conversion operator
{
return this->s;
}
//...
};

1) I would like to convert from Mystring* to Mystring** using a '=' operator. for this line to work:"
Mystring* p = new Mystring;
Mystring** pp = p;

2) And to convert from Mystring* to char* using a conversion operator (I already have one in my class).

I'm having difficulties with both issues, Thanks for any help!!
BanKuZ

The first doesn't make a whole lot of sense.

The second is easy; just use the MyString object anywhere a char* is expected, or do an explicit static_cast. However, it's worth noting that implicit conversions are a controversial subject; some say they're a bad idea.

Thank you,
But that's exactly my problem,
For the first one I'm trying to avoid changing any lines in the main function (since it was given to me).
And for the second one I can't use the MyString object, just a pointer for the object for the same reason.

Quote:

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Originally Posted by BanKuZ

Thank you,
But that's exactly my problem,
For the first one I'm trying to avoid changing any lines in the main function (since it was given to me).

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Show the main function then so we can figure out what that line is trying to do.

Quote:

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And for the second one I can't use the MyString object, just a pointer for the object for the same reason.

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So dereference the pointer.

Here is the relevant part of the main function:

int main (void) {

Mystring* p = new Mystring();

*p = "aaa";
Mystring** pp =p;
Print(*pp);//defined as Print(char*);
//...
Print(*p); // this line currently work
//...

I tried to dereference the pointer in several ways - none worked, can you show me how?

I'm not following why you think you need a **

I'm not seeing where you're allocating memory for your internal char* either.

It's not clear what name is.

Your char* operator should return s.

Please use code tags when you post code.

Quote:

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Originally Posted by Lindley

Show the main function then so we can figure out what that line is trying to do.

So dereference the pointer.

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Quote:

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Originally Posted by GCDEF

I'm not following why you think you need a **

I'm not seeing where you're allocating memory for your internal char* either.

It's not clear what name is.

Your char* operator should return s.

Please use code tags when you post code.

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HI GCDEF,

The main function is a given, that's the only reason I need a **.
I didn't have any problem for not having a memory allocation for the internal char* why is it important? should I add it to the constructors?
You are right about my char* operator, it is now fixed. thanks!

Quote:

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Originally Posted by BanKuZ

HI GCDEF,

I didn't have any problem for not having a memory allocation for the internal char* why is it important? should I add it to the constructors?

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You're writing to unallocated memory. The most likely result of that is a crash.

The whole pointer pointer thing makes no sense in that program, but why not just do it the normal way?

Mystring** pp = &p;

Also, your assignment operator should return a MyString reference, not a MyString.

Code:

---

Mystring& operator=(const char* str)
{
// you still need to free existing memory and allocate new memory here.
    strcpy (s, str);
    return *this;
}

---

Quote:

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Originally Posted by BanKuZ

HI GCDEF,

The main function is a given, that's the only reason I need a **.
I didn't have any problem for not having a memory allocation for the internal char* why is it important? should I add it to the constructors?
You are right about my char* operator, it is now fixed. thanks!

---

The obvious question is why you just don't use std::string? All of this work is taken care of already for you.

If you had this trouble in figuring out how to properly handle dynamically allocated memory, then it is more of a reason to use standard classes instead of trying to code something that is buggy.

Secondly, your code will fail here:

Code:

---

int main()
{
    Mystring s("abc");
    Mystring s2 = s;
}

---

Just that simple two line program, given what you've coded, has an error when main() exits. You need an assignment operator and copy constructor. Again, more of a reason to use std::string.

Edit: I don't see your class doing any dynamic allocation whatsoever, so what is the purpose of this class? How about posting your class implementation, so we know what you're trying to do.

Regards,

Paul McKenzie

Code:

---

class Mystring {

public:
   
    char s[MAX_SIZE];

    Mystring(){}
   
    Mystring (const char* str)
    {
        strcpy (s, str);
    }

    Mystring (const Mystring & Astr)
        {
                strcpy (s, Astr.s);
        }
    ~Mystring(){}
   
    Mystring operator=(const char* str)
    {
        strcpy (s, str);
        return *this;
    }

    Mystring** operator=( Mystring* Astr) /// didn't solve
    {
                Mystring** ppTemp= &Astr;
                return ppTemp;
    }
 
    Mystring operator+(Mystring CMy)
    {
        Mystring temp;
        strcpy (temp.s, s);
        strcat (temp.s, CMy.s);
        return temp;
    }
   
    Mystring operator += (const char* str)
    {
        strcat (s, str);
        return *this;
    }

    Mystring operator [] (const int n)
    {
        Mystring temp;
        temp.s[0]=s[n];
        temp.s[1]='\0';
       
        return temp;
    }
   
 
    operator char *()//Conversion operator
        {

       
        return this->s;
    }
        operator Mystring **()//Conversion operator - didn't solev
        {
        return *this;
    }
    int len ()
    {
        int tmp_len;
        tmp_len = strlen(s);
        return tmp_len;
    } 
           
};

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That's what I wrote so far, I have problem with the lines that I mentioned before. As I said the MAIN is given and I trying to avoid changing it's syntax.
I'm more then happy to here remarks about how to make my code better.
I didn't fully understand your last remark about how I should use memory allocation, and of course std::string is not an option for me. The main idea is for me to learn how to code it correctly so next time is won't be "something that is buggy",
I really appreciated your help,
BanKuZ

Again, anywhere where you're returning *this, your return type needs to be a MyString&.

The whole ** thing just doesn't make sense. As far as I know, there's no way to do it. When you're operating with pointers such as
MyString** pp = p, you're not even dealing with your MyString object. You're just assigning one memory address to another in a way that doesn't make sense and won't work.

You're telling the compiler that pp will point to a MyString pointer, then assigning it to an address that points to a MyString object. It's a nonsensical concept that you should strop trying to implement.

Quote:

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Originally Posted by GCDEF

Again, anywhere where you're returning *this, your return type needs to be a MyString&.

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Can you specify what to write instead?

Quote:

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Originally Posted by GCDEF

The whole ** thing just doesn't make sense. As far as I know, there's no way to do it.

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O.K I appreciate it, I don't know either that's why I asked.

Is there a way to use conversion cast for casting from Mystring* to char*?
Like if I have a function defined as: func (char*) and I would like to use it this way:

Code:

---

 
Mystring** pp =&p;
    Print(*ppr);

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Just change the return type to MyString&

Your double pointer code should work. Did you try it?

One other thought. Your operator [] should probably just return a char, not a MyString and it should check that the int passed in is valid.

Quote:

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Originally Posted by BanKuZ

I didn't fully understand your last remark about how I should use memory allocation

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Your present approach of using a fixed-size char array is better than what you had in the first post, but it's not ideal. You haven't done any checks for whether the passed string will fit in the array you have available, and your object will be much larger than necessary for small strings.

What you really want to do is dynamically allocate the memory necessary to copy the passed in string. However, doing this brings along several additional responsibilities:
1) You must release the allocated memory in the destructor.
2) You must ensure you allocate a different block of memory when copying, not just copy the pointer. (Unless you want to do reference counting, but that's even more difficult.)
3) If your string is reassigned to be longer, you must modify your allocation safely to be able to handle the new value.

Quote:

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Originally Posted by GCDEF

Just change the return type to MyString&

Your double pointer code should work. Did you try it?

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Tried it, didn't work. Can you please write the code lines so I'll find out if I did it the way you meant?

Quote:

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Originally Posted by GCDEF

One other thought. Your operator [] should probably just return a char, not a MyString and it should check that the int passed in is valid.

Also, you're creating a temporary MyString, but not assigning it a value, then attempting to access elements of s, which is meaningless since it has no value yet.

---

Changed, thanks.

Quote:

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Originally Posted by Lindley

Your present approach of using a fixed-size char array is better than what you had in the first post, but it's not ideal. You haven't done any checks for whether the passed string will fit in the array you have available, and your object will be much larger than necessary for small strings.

What you really want to do is dynamically allocate the memory necessary to copy the passed in string. However, doing this brings along several additional responsibilities:
1) You must release the allocated memory in the destructor.
2) You must ensure you allocate a different block of memory when copying, not just copy the pointer. (Unless you want to do reference counting, but that's even more difficult.)
3) If your string is reassigned to be longer, you must modify your allocation safely to be able to handle the new value.

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Thank you, I will make those changes as soon as my program runs on a simple input.

Quote:

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Originally Posted by BanKuZ

The main idea is for me to learn how to code it correctly so next time is won't be "something that is buggy",

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The issue is that you do not have a full grasp of pointers and how to properly handle dynamically allocated memory. Without this, you cannot create a string class.

Secondly, there shouldn't be a "next time" once you can code some string class. Unless there is a compelling reason to do so, you should use what is known to work correctly, and that is std::string. The std::string class and other C++ library classes have been written by some of the best C++ programmers in the business, so there is little (to be honest, no) chance you're going to do better than their version.

Also, I'm being very frank -- I have never seen a beginner programmer ever get a string class correct. That's why it takes (at the very least) an intermediate C++ programmer who is well-grounded with issues such as pointer manipulation, memory management, and other aspects to properly code a string class that actually works reasonably well with little, to no issues.

If you want to see how to put together a string class, look at the many sample programs that shows how to do it. If you want a skeleton, finish the implementation below:

Code:

---

#include <iostream>

class MyString
{
    char *m_ptr;

    public:
        MyString();
        MyString(const char* str);
        MyString(const MyString& str);   
        ~MyString();

      MyString& operator=(const MyString& rhs);
      MyString& operator=(const char* str);
      MyString& operator += (const MyString& str);
      MyString& operator += (const char * str);
      const char* getData();
};

// sample program
using namespace std;

int main()
{
  MyString s1("abc");
  MyString s2 = s1;
  MyString s3;
  s3 = s2;
  s3 += "def";
  cout << s1.getData() << "\n" << s2.getData() << "\n" << s3.getData();
}

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The output should be:

Code:

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abc
abc
abcdef

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Can you start here and implement these functions in MyClass? Once you do that, then make sure that the main() function works correctly (correct output, no memory corruption on exit of main(), etc). Then we can clearly see what you know and what you don't know.

Regards,

Paul McKenzie

I already did most of it in my code. I understand you are trying to help and it seems you have a lot of knowledge. I know the right way of doing it is what you all suggested, and I will change it.
The most helpful thing for me right now is to know if what I'm trying to do is even possible and what's the right way of doing it.
Is there a way to implement operator in Mystring so those 2 lines work properly:

Code:

---

Mystring** pp =p; // NOT: Mystring** pp =&p;

Print(*ppr); // print is defined Print(char*)

---

I really appreciate your help, it really important to me to know how to do it if it is possible.

Quote:

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Is there a way to implement operator in Mystring so those 2 lines work properly:

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There is no way, and it is a good thing that it doesn't work.

If it did work, then your code would run counter to how C++ is supposed to behave. If any C++ programmer were to see this:

Code:

---

Mystring** p;
MyString* p2;
p = p2;

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the programmer would expect a compiler error, since you cannot assign pointer types with different indirections this way. Thankfully, the compiler doesn't allow it either. To make member function calls, you need an object -- there is no object in the code above. You have a Mystring** on the left hand side, not a MyString object.

So the real question is why you want to write code that is counter to what a C++ programmer expects? Every C++ programmer knows this works:

Code:

---

Mystring** p;
MyString* p2;
p = &p2;

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So why are you trying to defeat this?

Regards,

Paul McKenzie

Quote:

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Originally Posted by BanKuZ

The most helpful thing for me right now is to know if what I'm trying to do is even possible and what's the right way of doing it.

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If it even was possible, does it make it "right"?

For example, what if I defined a class where operator + does subtraction, and operator - did addition? Could I do it? Yes. Would it be "right"? Of course not.

Class design (the title of your thread) is a big topic, and requires a lot of thought way beyond whether something is syntactically possible in C++. If creating code makes the class harder to use, or could introduce bugs, or just confuse the heck out of any C++ programmer using your class, then the class design is faulty and shouldn't even be considered.

Regards,

Paul McKenzie

I agree with your answer, but I'm trying to build a class for a given main() function.
I'm trying to make to code given to me work correctly even if there are many other and better ways of coding the same thing, I have to stay with this main() - it's not for me to choose.
Are you saying it's impossible or is it just not right to do it this way?

Print(*ppr); // print is defined Print(char*)

That still won't work because ppr is a pointer to a pointer, so *ppr is just a MyString*. You need to be working with the object, not a pointer to use your operators. You'd need to deference it twice.

Who gave you such an odd main function? Is this homework?

Quote:

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Originally Posted by GCDEF

Print(*ppr); // print is defined Print(char*)

That still won't work because ppr is a pointer to a pointer, so *ppr is just a MyString*. You need to be working with the object, not a pointer to use your operators. You'd need to deference it twice.

Who gave you such an odd main function? Is this homework?

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I tried to deference it twice, I didn't manage to do it.
And it is something like homework only it was given by a C++ professional.

Quote:

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Originally Posted by BanKuZ

I tried to deference it twice, I didn't manage to do it.
And it is something like homework only it was given by a C++ professional.

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That C++ "Professional" is in the wrong line of work. Assuming it's a test for a job, I'd just tell them that it couldn't be done and even if it could, it's a really bad idea. It's possible that's the answer they're looking for. Part of being a good programmer is being able to identify when you're headed down the wrong path and to stop.

Quote:

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Originally Posted by GCDEF

Assuming it's a test for a job, I'd just tell them that it couldn't be done and even if it could, it's a really bad idea. It's possible that's the answer they're looking for. Part of being a good programmer is being able to identify when you're headed down the wrong path and to stop.

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Well it is, and I think you're probably right. I just want to be 100% positive I'm not missing anything before I do tell them such an answer.

Thank you very much, you've been very helpful.

Quote:

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Originally Posted by GCDEF

Print(*ppr); // print is defined Print(char*)

That still won't work because ppr is a pointer to a pointer, so *ppr is just a MyString*. You need to be working with the object, not a pointer to use your operators. You'd need to deference it twice.

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Can you help me with it? I tried several ways but couldn't make it work.

Quote:

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Originally Posted by BanKuZ

Can you help me with it? I tried several ways but couldn't make it work.

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If you can't modify the main function, there's no way I know to help.

Quote:

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Originally Posted by GCDEF

If you can't modify the main function, there's no way I know to help.

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Thanks anyway, you still helped a lot.