Dynamic Object Name

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March 9th, 2003, 10:48 AM

etones

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Dynamic Object Name

Hi all, looking for a little help with dynamic object names.

Eg, i have a class for students. When the system is run, the user will be asked how many students they require, the program can then create X instances of the student class,

student1, student2... studentX etc.

Im just wondering how todo this. Is Student student[i](); correct for a constructor? If not, how do I do it ?

Cheers
Taz

March 9th, 2003, 10:56 AM

M.Hirsch

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the constructor should remain "standard":
Student();

to create the objects:

Code:

Student student[];
...
for (i=0;i<students_number;i++) {
  student[i]=new Student();
}

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March 9th, 2003, 11:02 AM

etones

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thank you very much young sir, will report back when i balls it up

Cheers
Taz

March 9th, 2003, 11:07 AM

etones

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one thing, I need to pass 2 arguments to my constructor.

So does...

PHP Code:


		
			
 Student student[];

...

for (i=0;i<students_number;i++) {

  student[i]=new Student();

}

become

PHP Code:


		
			
 Student student[];

...

for (i=0;i<students_number;i++) {

  student[i]=new Student([b]MY ARGUMENTS?[/b]);

}

March 9th, 2003, 11:16 AM

etones

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right, on compilng the following code I get the following errors..

Quote:

int num_students = 2;
Students students[];

// Generate required number of monkey objects
int i;
for(i = 0; i < num_students; i++)
{
students[i]=new Students(5,3);
}

Errors:

Quote:

stage3.cc: In function `int main()':
stage3.cc:24: array size missing in `students
stage3.cc:24: no matching function for call to `Students::Students()'
monkey.h:14: candidates are: Students::Students(const Students&)
students.h:25: Students::Students(int, int)
stage3.cc:30: no match for `Students& = Students*&' operator
students.h:14: candidates are: Students& Students::operator=(const Students&)

Side note. The class works when calling normally, eg Students students(5,3);

March 9th, 2003, 01:34 PM

Scorpions4ever

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Try declaring your array like this:

Code:

int num_students = 2;
Student *students = new Student[num_students](5,3);
... 
... do your stuff
...
delete [] students;

March 9th, 2003, 01:36 PM

7stud

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From your error message it looks like you don't have a constructor defined that takes two arguments, but that is the way you would call the constructor. A constructor is like any other function except it doesn't and can't have a return type, and when you call any function, the parameters must match up with a function definition.

student[i]=new Student(MY ARGUMENTS?);
---------------

Student student[];******what size array did you create here?

for (i=0;i<students_number;i++) {
student[i]=new Student();
}

Last edited by 7stud : March 9th, 2003 at 02:06 PM.

March 9th, 2003, 02:25 PM

etones

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Hi guys, the following fixed it, required pointers. Note, student is now monkey

Regards

Quote:

// Create pointer array
int num_monkeys = 2;
Monkey *monkey[num_monkeys];

// Generate required number of monkey objects
for(int i = 0; i < num_monkeys; i++)
{
monkey[i] = new Monkey(5,3);
}

March 9th, 2003, 03:05 PM

Gmorphus

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7stud was right. some clarifications.
What you are trying to do, as you probably know, is to dynamically allocate an array of

Student.
You have to use pointers since you don't know the size of the array.

before that you have some problems with the constructor.
let's take the following class:

PHP Code:


		
			
class Student

{

public:

    Student(int i, int j)

    {

        nClassID = i;

        nStudentID = j;

    }

private:

    int nClassID;

    int nStudentID;

}

let's say you want to retrieve the number of the students, as well as the IDs, from the user

and than construct these instances.
This is the wrong way:

PHP Code:


		
			
 int nNumberOfStudents;

Student *pStudentsArray;

// get the number of students from the user and put it in nNumberOfStudents

// than dynamically allocate the array

pStudentsArray = new Student[nNumberOfStudents];

why? because when you try to allocare the "new Student[..]" the program tries to construct

these instances.

you have to possibilities as far as I know:
1. Use pointer to pointer (I will explain)
2. Add a function "Create(int i, int j)" and empty the constructor.

Explaination:
1. Instead of allocating an array of students we allocate an array of pointers to Student

PHP Code:


		
			
 typedef Student *PStudent;

// get number of students from the user

PStudent *ppStudentsArray = new PStudent[nNumberOfStudents];

for (int i=0; i<nNumberOfStudents; i++)

{

    int nClsID, nStudentID;

    //get values from the user to nClsID and nStudentID

    ppStudentsArray[i] = new Student(nClsID, nStudentID);

}

2. Add Create(int, int) that does the same thing as the constructor. make the constructor do

NOTHING and do the following:

PHP Code:


		
			
// get number of students from the user

Student *pStudentsArray = new Student[nNumberOfStudents];

for (int i=0; i<nNumberOfStudents; i++)

{

    int nClsID, nStudentID;

    //get values from the user to nClsID and nStudentID

    pStudentsArray[i].Create(nClsID, nStudentID);

}

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#10

March 9th, 2003, 03:06 PM

Gmorphus

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Your solution is good only if the number of monkeys is constant - but if you want to get this and the other values from the user it wont work i suspect.

#11

March 9th, 2003, 03:09 PM

Scorpions4ever

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>> 2. Add Create(int, int) that does the same thing as the constructor. make the constructor do nothing
Or, you could use my example posted above and use the constructor just fine. BTW I tested my example with g++ before posting it here

#12

March 9th, 2003, 03:18 PM

Gmorphus

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It would work - but i'm not sure it will satisfy all the requirements:
1. The number of Elements is CONSTANT and not dynamic
2. What if you want to construct the each element with different values?!

#13

March 9th, 2003, 03:27 PM

Scorpions4ever

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>> 1. The number of Elements is CONSTANT and not dynamic
Nope, you can get num_students from user input.

>> 2. What if you want to construct the each element with different values?!
Then you'd have to have a constructor with no arguments and set the values inside a for loop, like your example did

#14

March 9th, 2003, 03:47 PM

Gmorphus

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