create a vector of instances of a class in c++

Question

i created a class its name is Student as follows:

class Student
{
 private:
     unsigned int id;                                // the id of the student 
public:   
    unsigned int get_id(){return id;};   
    void set_id(unsigned int value) {id = value;};
    Student(unsigned int init_val) {id = init_val;};   // constructor
    ~Student() {};                                     // destructor
};

then after i wanted to have a container ( say a vector ) its elements are instances of class Student, but i found myself not able to understand this situation , here is my issue:

first i run this code:

#include<iostream>
#include<vector>
using namespace std;

const unsigned int N = 5;

Student ver_list[2] = {7, 9};


int main()
{

  cout<< "Hello, This is a code to learn classes"<< endl;

  cout<< ver_list[1].get_id() << endl;

return 0;
}

everything is fine and the output is :

Hello, This is a code to learn classes
9

now when i try these options:

option #1:

#include<iostream>
#include<vector>
using namespace std;

const unsigned int N = 5;

vector <Student> ver[N];             // Create vector with N elements
for(unsigned int i = 0; i < N; ++i )
ver[i].set_id(i); 


int main()
{

  cout<< "Hello, This is a code to learn classes"<< endl;

  cout<< ver[1].get_id() << endl;

return 0;
}

i got this output "error" :

test.cpp:26:3: error: expected unqualified-id before 'for'
   for(unsigned int i = 0; i < N; ++i )
   ^
test.cpp:26:27: error: 'i' does not name a type
   for(unsigned int i = 0; i < N; ++i )
                           ^
test.cpp:26:34: error: expected unqualified-id before '++' token
   for(unsigned int i = 0; i < N; ++i )
                                  ^
test.cpp: In function 'int main()':
test.cpp:43:15: error: 'class std::vector<Student>' has no member named 'get_id'

 cout<< ver[1].get_id() << endl;
               ^

option #2:

#include<iostream>
#include<vector>
using namespace std;

const unsigned int N = 5;

Student ver[N];                       // Create one dimensional array with N elements
for(unsigned int i = 0; i < N; ++i )
   ver[i].set_id(i); 


int main()
{

  cout<< "Hello, This is a code to learn classes"<< endl;

  cout<< ver[1].get_id() << endl;

return 0;
}

the output "error" was :

test.cpp:30:14: error: no matching function for call to 'Student::Student()'
Student ver[5];
             ^
test.cpp:30:14: note: candidates are:
test.cpp:14:2: note: Student::Student(unsigned int)
  Student(unsigned int init_val) {id = init_val;};   // constructor
  ^
test.cpp:14:2: note:   candidate expects 1 argument, 0 provided
test.cpp:7:7: note: Student::Student(const Student&)
 class Student
       ^
test.cpp:7:7: note:   candidate expects 1 argument, 0 provided
test.cpp:31:1: error: expected unqualified-id before 'for'
 for(unsigned int i = 0; i < N; ++i )
 ^
test.cpp:31:25: error: 'i' does not name a type
 for(unsigned int i = 0; i < N; ++i )
                         ^
test.cpp:31:32: error: expected unqualified-id before '++' token
 for(unsigned int i = 0; i < N; ++i )
                                ^

In the first try everything was looking ok , but when i tried the two next options , i received errors , i wish that i can understand what wrong i am doing.

Thanks.

Answer 1

    #include<iostream>

    using namespace std;

    class Student
    {
        private:
        int id;

        public:
        // Mutator
        void setId(int i)
        {
          id = i;
        }
        // Accessor
        int getId()const
        {
          return id;
        }
    };

    int main()
    {
        const unsigned int N = 5;
        Student ver[N];   // Define instances as an array 
        // of the Student class

        int idStudent[N] = { 11, 32, 37, 4, 50};  // Create 
        // one dimensional array with N elements of the 
        // student ID

        for(unsigned int i = 0; i < N; i++ ){ // Assign 
        //student ID for each object of the class
        ver[i].setId(idStudent[i]);}

        cout<< "Hello, This is a code to learn classes : "
        << endl << endl; // Display the student ID

        for(unsigned int i = 0; i < N; i++ ){
        cout<< "Student ID #"  << i+1 << " of "
        << N << " : " << ver[i].getId() << endl;}

        return 0;

    }

Answer 2

The solution to the question asked is to create an array of instances of the Student class as Student ver[N]. Next, using the mutator function, setID(int i) to assign the elements from a given student ID in array format ( int idStudent[N] = { 11, 32, 37, 4, 50};) to each perspective instance of the Student class: ver[i].setId(idStudent[i]) and for loop to finish the job.

Last, using the accessor function, ver[i].getID() and the for loop to display the info.

Answer 3

This:

vector <Student> ver[N];

Creates an array of N elements. Each element is vector<Student>. This is not you want. You were probably trying to create a vector of N elements. The syntax for this is:

vector <Student> ver(N);

But you can't use this because your class does not have a default constructor. So your next alternative is to initializae all the objects with the same element.

vector <Student> ver(N, Student(0));

You also tried to create an array of students like this:

Student ver[N];

This will not work. Because it tries to initialize every element in the array with the default constructor. But your class does not have a default constructor. So this will not work. But this is why your original code did work:

Student ver_list[2] = {7, 9};  // Here you are using the constructor for your object.
                               // It uses the normal constructor you provided not the default one.

The other issues is that you can not run code outside a function(method).
So this will not work:

for(unsigned int i = 0; i < N; ++i )
    ver[i].set_id(i); 

In C++11 you can initialize a vector the same way as an array:

vector<Student>  ver = { 0, 1, 2, 3, 4, 5};

If you don't have C++11 or initialization is more complex. Then you need to write a wrapper.

class VecWrapper
{
     public:
         std::vector<Student>   ver;
         VecWrapper()
         {
            ver.reserve(N);
            for(unsigned int i = 0; i < N; ++i )
                ver.push_back(Student(i));
         }
 };

Now You can place this in global scope and it will auto init.

 VecWrapper   myData;  // myData.vec  initializaed before main entered.

 int main()
 {}

Full solution:

Option 2:

#include<iostream>
#include<vector>
using namespace std;

const unsigned int N = 5;

// The following is not correct
// This creates an arrya of `N` elements each element is `vector <Student>`
//
// vector <Student> ver[N];             // Create vector with N elements
// 

// The following lines are not allowed.
// All code has to be inside a function.
//
// for(unsigned int i = 0; i < N; ++i )
// ver[i].set_id(i); 


// What you want is:
//    I use the following because it is unclear if you have C++11 or not.  
class VecWrapper
{
   public:
     std::vector<Student>   vec;
     VecWrapper()
     {
        vec.reserve(N);
        for(unsigned int i = 0; i < N; ++i )
            vec.push_back(Student(i));
     }
};
VecWrapper   myData;  // myData.vec 
int main()
{

  cout<< "Hello, This is a code to learn classes"<< endl;

  cout<< myData.vec[1].get_id() << endl;

return 0;
}

Answer 4

Option #1:

You should replace vector <Student> ver[N] with vector<Student> ver(N)

The std::vector is a class, that represents the vector by himself, you shouldn't create an array of vectors, you should just pass N(vector size) to it's constructor. Check this link

Option #2:

Student ver[N];

is incorrect, since the Default Constructor Student() is invoked N times, but you haven't implement it. So you have to use array initilizer Student ver[5] = {1, 2, 3, 4, 5} or implement the default constructor explicitly.

And of course - the "for" loop has to be used inside function body.

Answer 5

The main problem is you are trying to execute a for loop at global scope. It is acceptable to define and initialize variables outside of a function, but using a for loop or assignment operator is not. Put the for loop into main() (and I would recommend you also put N and the vector/student array into main() and everything should work.
Additionally, the compiler is complaining because when you declare Student array[5]; or vector<Student> ver[N]; it is looking for a default constructor for Student called Student(), which just sets default values for a class. You need to provide this inside the Student class; set the id to some value that can never be an actual student ID, something like -1.

Answer 6

This is actually not linked at all with vectors. You just need to move your "for" statement into your main