C++ How to return array of objects from a function

Question

recently I have started learning c++, so I'm trying to make a simple grade calculator using my base knowledge (I'm already good with Javascript so i know the basics of programming).

So in this case, I had to return an array of objects from a function call so i can use it later in the program, but I just couldn't find the right way to do so.

So basically i want to return subArr from the getInput function, but I couldn't do that due to my basic knowledge in the language. I tried googling but didn't find any simple solution.

Here is the code and hope it is simple:

//the Subject class:
class Subject {
    public:
        string name;
        float grade;
        int factor;
        
        Subject(){};
        
        Subject(string x, float y, int z){
            name = x;
            grade = y;
            factor = z;
        }
};

//get Input function declaration
Subject getInput(int num){
    
    //array of objects of type "Subject"
    Subject subArr[num];
    
    //a for loop to assign the array's elements
    for(int i = 0; i < num; i++){
        string name;
        float grade;
        int factor;
        
        cout << "what is the name of subject " << i+1 <<"? "<<endl;
        cin >> name;
        
        cout << "what is the grade of subject " << i+1 << "? "<<endl;
        cin >> grade;
        
        cout << "what is the factor of subject " << i+1 << "? "<<endl;
        cin >> factor;
        
        subArr[i]=Subject(name, grade, factor);
    };
    
    //trying to return the subArr at last
    return subArr;
};

//main function
int main(){
    //get the number of subjects
    int numOfSubjects;
    cout << "how many subjects are there? ";
    cin >> numOfSubjects;
    
    //trying to receive the subArr from getInput call
    Subject subArr = getInput(numOfSubjects);
    
};

Subject subArr[num]; is not valid c++, since num is a run-time value. Just use a std::vector instead. — cigien
– cigien, Commented Oct 15, 2020 at 14:19
@cigien but i need the num to initialize the array's size, also can u give me a code example of using std::vector? i'm a newbie, thanks — Salah Eddine Makdour
– Salah Eddine Makdour, Commented Oct 15, 2020 at 14:20
Thousands on the net. This one is from a solid documentation site you should bookmark. — WhozCraig
– WhozCraig, Commented Oct 15, 2020 at 14:24
@SalahEddineMakdour -- You should initialize your member variables in the Subject default constructor. Subject s; std::cout << s.grade; would produce undefined behavior, due to s.grade not being initialized. — PaulMcKenzie
– PaulMcKenzie, Commented Oct 15, 2020 at 14:30

777moneymaker · Accepted Answer · 2020-10-15 14:27:37Z

Use std::vector:

#include <iostream>
#include <vector>

using namespace std;

// the Subject class
class Subject {
    public:
        string name;
        float grade;
        int factor;
        
        Subject(){};
        
        Subject(string x, float y, int z){
            name = x;
            grade = y;
            factor = z;
        }
};

// get Input function declaration
vector<Subject> getInput(int num){
    
    // array of objects of type "Subject"
    vector<Subject> subArr;
    
    // a for loop to assign the array's elements
    for(int i = 0; i < num; i++){
        string name;
        float grade;
        int factor;
        
        cout << "what is the name of subject " << i+1 <<"? "<<endl;
        cin >> name;
        
        cout << "what is the grade of subject " << i+1 << "? "<<endl;
        cin >> grade;
        
        cout << "what is the factor of subject " << i+1 << "? "<<endl;
        cin >> factor;
        
        subArr.push_back(Subject(name, grade, factor));
    };
    
    // trying to return the subArr at last
    return subArr;
};

// main function
int main(){
    // get the number of subjects
    int numOfSubjects;
    cout << "how many subjects are there? ";
    cin >> numOfSubjects;
    
    // trying to receive the subArr from getInput call
    vector<Subject> subArr = getInput(numOfSubjects);
};

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