Why This doesn't work and the other one works?

Hello,
i am posting the code. Please tell me why is one of the function defintion working and one is not??even though they both look identical!

i have written the comments also like which one is working and which one is not. Please have a look at them and give me the reason!

Code:

---

#include <iostream.h>
#include <string.h>
class student
{
  char *name,roll[3];
  public:
        student()
        {
                name=new char[20];
        }
        student(char *n,char *r)
        {
                strcpy(name,n);
                strcpy(roll,r);
        }
  friend void sort(student []); //Why This Doesn't Work?

  friend void sort(student *); //This Works Perfectly??..

};
void sort(student a[])        //Why Not Working??
{
  int j;
  student temp;
  for(int i=0;i<3;i++)
  {
        for(j=0;j<2;j++)
        {
        if(strcmp(a[j].roll,a[j+1].roll)>0)
                {
                        temp=a[j];
                        a[j]=a[j+1];
                        a[j+1]=temp;
                }
        }
  }
}


int main(void)
{

 student a[3];
 a[0]=student("abc","1");
 a[1]=student("def","2");
 a[2]=student("hij","3");

 sort(a);

 return 0;
}

---

>Please tell me why is one of the function defintion working and one is not??
Please define "working" and "not working". The way I see it, your code is not working period, because you don't ever allocate memory to name and then proceed to copy a string to the memory you never allocated. This will likely cause a crash in all cases.

what exactly are you talking??
can't you see the memory allocated in default? constructor??and can't you read the //program comments written in function??which says which one is working and which one's not?
the program is running perfectly. Please dont try to teach me false things i have allocated the memory in constructor. You may see it properly

Quote:

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can't you see the memory allocated in default?

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Can't you see that you do not call a default constructor? And call the constructor that does not allocates memory?

You should learn a little respect before talking in public...

Ok you're right vart
Sorry Prelude! my tone was not proper

But what i mean to say is that won't the default constructor be called automatically?i mean it should. Shouldn't it?because i have defined default constructor??then why is it not calling?

when i create a single object the default constructor is called but when i create array of objects as i created in the above case it's not calling. Why so??

and also apart from that please tell me why is the friend void sort(student []); not working??in the above example

The default constructor is only called if you specify it or if you don't specify any constructor. If you have another constructor and call it, the default constructor is not called behind the scenes. Only the constructor that is called gets executed, in your case the one taking two parameters. And that one is not allocating any memory.

Concerning your problem, please specify what "Doesn't work" means for you. Does it compile ? Does it link ? Does it produce the desired results ? Where does it fail ? And if it fails to compile or link, what is the error message ?

The error message is "undefined structure result"

This line creates 3 students initialised with the default constructor - no problem:

Code:

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student a[3];

---

This line of code constructs another student using only the two-parameter constructor you provided. (That would normally fail, for the reasons stated). Assuming you were very lucky that this happened to not cause an access violation, but instead successfully wrote some garbage over some random memory location instead, it then copy-constructs the newly created student over top of the student at a[0], directly overwriting the value of the name pointer with the uninitialised garbage from the screwed up temporary object, leaking the memory allocated by the default constructor of the original item in a[0].

Code:

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a[0]=student("abc","1");

---

The bug has nothing to do with your sort function implementation or prototype.

Incidentally, you will be able to do what you wanted I believe, in the next revision of C++ that allows constructor delegation:

Code:

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        student(char *n, char *r) : student()
        {
                strcpy(name,n);
                strcpy(roll,r);
        }

---

But i am still not able to understand why will the default constructor not be called automatically??
if i have defined default constructor shouldn't it be called implicitly at the time of declaring objects itself?
when i create a single object like --

student b;
its calling the default constructor implicitly and when i create array of objects it doesn't call default constructor! Why??what's the reason ?Why not with array of objects?

No, it does.

student a[3];

Creates 3 student objects, and calls the default constructor for each of them.

a[0]=student("abc","1");

Creates a new temporary object student and calls the (char*, int) constructor (not the default one). Then, because they are the same type, the constructor calls the implicit copy constructor which does a shallow copy, that is, copies all of the variables from your temporary student to your a object, hence overwriting your pointer with memory.
And it's also bad in that the (char*, int) constructor never allocates any memory, so you copy into an uninitialized pointer.

Ok now things are getting clear to me!
But elysia you mean explicit call to constructor always creates a new temporary object?

I mean because you are doing
a[0] = student("abc","1");
What does this do? It calls the constructor and creates a temporary object, that's all.
Then it is assigned to the left side, which is another object of the same type.

You could also do
student my_student("abc", 1);
No temporaries because you created a new object named my_student.
It's called a temporary because its life will not last very long. Once the compiler has assigned it to a[0], it will destruct and get rid of the object. Thus, a temporary.

Ok great!!! elysia good job!
now i understood! :) you cleared all my doubts!

and now the main question. These were all problems pointed to by vart and now other doubt i have is why can't we use sort(student []); in the above example?
why can i only use only sort(student *);

These both function declarations mean the same thing. Don't they?then when i call sort(student []); i get a error like undefined structure student
WHy SO???

Nobody there to help me out?

Explain "Not working", as that can cover anything from "not compiling", "sorts in the wrong order" or "crashes when I run the code" - along with probably 100 other things.

--
Mats

Ok matsp.. the problem is that it's not compiling!

i get a compile time error. Leaving apart the logical errors like not allocating memory just i want to the syntactical error! in the definition of the 2 functions! why 1 ones and other one even though it's exactly same not working??

i get a compile time error "Undefined Structure student"

Quote:

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These both function declarations mean the same thing. Don't they?then when i call sort(student []); i get a error like undefined structure student
WHy SO???

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What do you mean by calling that function like that???

You have declared the same friend function twice, using different notation, but compilers would accept either (you shouldn't use both, though). Care to post updated code, because your code had very many problems related to memory management, any of which would be enough to make the code "not work".

Ok fine this is the new updated code

Code:

---


#include <iostream.h>
#include <string.h>
class student
{
  char *name,roll[3];
  public:
        student()
        {

        }
        student(char *n,char *r)
        {
                name=new char[20];
                strcpy(name,n);
                strcpy(roll,r);
        }
  friend void sort(student []); //Why This Doesn't Work? A compile time error like "undefined structure student"

};
void sort(student a[])        //Why Not Working??
{
  int j;
  student temp;
  for(int i=0;i<3;i++)
  {
        for(j=0;j<2;j++)
        {
                if (strcmp(a[j].roll,a[j+1].roll)>0)
                {
                        temp=a[j];
                        a[j]=a[j+1];
                        a[j+1]=temp;
                }
        }
  }
}


int main(void)
{
 student a[3];
 a[0]=student("abc","1");
 a[1]=student("def","2");
 a[2]=student("hij","3");

 sort(a);

 return 0;
}

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The code you have just posted works just fine in g++ - perhaps it's ye olde compiler you are using? - it even runs without crashing - although it produces no output (aside from the "Done" that I added to see that it finishes).

--
Mats

Quote:

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//Why This Doesn't Work? A compile time error like "undefined structure student"

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What compiler are you using? It looks correct and compiles on MSVC8, at least after I changed the pre-standard <iostream.h> to <iostream> (but it is not needed anyway). You should also change <string.h> to <cstring>, and indent your code more consistently.

OR you can get Visual Studio Express and let it automatically indent for you.

Quote:

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OR you can get Visual Studio Express and let it automatically indent for you.

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Note that other IDEs and specialised tools can also "beautify" code, but ultimately it is good practice to get into the habit of consistent formatting as you type your code.