Error when I try to create array of object

Question

I am getting quite a bit confused, to me I think the following two implementation are valid (but my syntax has error), but I can not get either one of them to work. This is a year 1 C++ intro class homework, we can not use Vector (or else it will be done a long time ago...)

I already finished my class Student, but now I have to create a container to store n students. I was thinking the following two option.

1) Create an array of student objects: Student arrayOfStudents[20];

My student class is very simple (It is modified different because too many lines). But the idea is like this...

class Student
{
    public:
    // Constructor for the Book class, take 6 arguments 
    Student(string myUID, string myLastName, string myFirstName, 
         string major, string age, string homeState, bool isWorking);
    ...
};

But when I try to create an array of students, I get this error message.

cs114p2main.cpp: In function ‘int main(int, char**)’:
cs114p2main.cpp:103: error: no matching function for call to ‘Student::Student()’
cs114p2Student.h:14: note: candidates are: Student::Student(std::string*)
cs114p2Student.h:11: note:                 Student::Student(std::string, std::string, std::string, std::string, std::string, bool)
cs114p2Student.h:7: note:                 Student::Student(const Student&)

2) Also tried: Create an array of student pointer that points to each student. Student* ArrayOfStudents = Student[20];

My question is why doesn't the method 1 work? And do I have to use pointers in this case (something along the line of method 2).

Because I need to pass this array into a function that going to set up each student. (I do not know how many students are going to be there). So does that mean I have to pass a pointer to the array of students? In that case I really hope method 1 can work, I can't imagine dealing with pointer to array of student pointers. I was thinking have to return an array of data[numOfData][numOfStudnet], and do it in the main, but either way I would like to figure out why this gives me error.. thank you.

Edit: Follwo up question : I know through research, I must declare the size of the array of Student as the parameter, but what if I only know the size after it is running? In that case, what are my options?

You need a default constructor which is called to construct every element of the array. — Cosyn
– Cosyn, Commented Apr 12, 2012 at 15:20

Community · Accepted Answer · 2017-11-11 02:08:18Z

3

Your Student class has only one constructor, namely

Student(string myUID, string myLastName, string myFirstName,  
     string major, string age, string homeState, bool isWorking);

Because of this, the default constructor does not get generated by the compiler. Since you also don't provide one (default ctor: one callable with no arguments), the compiler cannot create the array -- it would need default-constructable objects to place in the elements (initialize the array with).

To fix, you will need to provide a default ctor:

Student();

the behavior of which you will need to find out yourself based on how Student is assumed to behave.

You could also provide default arguments to all parameters of the listed Student ctor, so it is callable without explicitly specifying any of the parameters -- thus qualifies as a default ctor

If a default ctor is not feasable, you will need to become more creative and dynamically allocate the array, copy all existing elements to the new one and assign the given new element to it when needed. Note that with this you are essentially re-creating the functionality of the std::vector container. Note also, that this approach is really hard to get it right, so I advise against it, especially since you are just learning the basics.

Yet another approach is to create an array of pointers to Student, so you can assign each as you create them dynamically. You will have to test whether the element is a null pointer, before you attempt to access the Student object pointed to.

Student* students[20];

students[0] = new Student(/*list of actual parameter values*/);

if (student[0] != NULL) {
  students[0]->getName(); // assuming you have a member function getName()
}

Note: the index 0 is used only for illustrative purposes, you can use a variable as well (as long as you ensure it stays within 0 and 19 (inclusive) -- note even though you have 20 elements, the largest index is 19 as the counting starts from 0

edited Nov 11, 2017 at 2:08

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answered Apr 12, 2012 at 15:19

Attila

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3 Comments

George Over a year ago

Do I just create an Empty constructor? That doesn't do anything? Or it is recommended I do give them some value, say null?

Attila Over a year ago

not sure what you mean by Empty ctor. Also not sure where you want to use null

George Over a year ago

Thank you for the very detailed explanation, but my head is still spinning a little bit. The pointer situation gets even worst when I have to pass them into a function.

hmjd · Accepted Answer · 2012-04-12 15:37:16Z

2

My question is why doesn't the method 1 work?

Because Student does not have a default constructor (a constructor with no arguments). A default constructor is generated by the compiler only if no constructors are defined. As constructors have been defined, which take at least one argument no default constructor exists. When you declare the array:

Student arrayOfStudents[20];

Twenty instances of Student are being created, using the default constructor which does not exist.

You could define a default constructor which would allow use of the array, but does it make sense for a Student to exist without a name, age, etc?

Or, use an array of Student*:

Student* arrayOfStudents[20] = {}; // The '{}' will set all pointers to NULL.

EDIT:

If the number of students is not known until runtime then:

// Get number of students somehow.
int numberOfStudents = ...;

// The '()' will set all pointers to NULL.
Student** arrayOfStudents = new Student*[numberOfStudents]();

Remember to delete each entry in arrayOfStudents and to delete[] the arrayOfStudents itself.

edited Apr 12, 2012 at 15:37

answered Apr 12, 2012 at 15:19

hmjd

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4 Comments

George Over a year ago

But what about function's parameter? I tried using global numberOfStudents, but I don't think it worked, it seems to want const only.

George Over a year ago

I have to write a function set up the Students base on a text value. I am creating this array of Students in my main, but I don't know the size until I "run" this program.

hmjd Over a year ago

As this is homework I don't want to write it in full for you, which would be pointless for you. You need to convert the text value to an int (see std::ostringstream for that). Pass that int as the single argument to the create array function and have the create array function return a Student**.

George Over a year ago

I already have the function written, it is about passing the array of student. But I will ask another question if I can't figure it out... Thank you hmjd.

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