Why do we need two constructors while doing operator overloading?

Question

I saw a code in sololearn platform(C++ course) and it is about defining overloading for operator +. you can see the code below:

#include <iostream>
using namespace std;

class Account {
    private:
        int balance=0;
        int interest=0;
    public:
        Account() {}
        Account(int a): balance(a) 
        {
            interest += balance/10;
        }
        int getTotal() {
            return balance+interest;
        }
        Account operator+(Account &obj){
            Account OBJ2;
            OBJ2.balance=this->balance + obj.balance;
            OBJ2.interest=this->interest +obj.interest;
            return OBJ2;
        }
        

};

int main() {
    int n1, n2;
    cin >> n1 >> n2;
    Account a(n1);
    Account b(n2);
    Account res = a+b;

    cout << res.getTotal();
}

My question is: why do we need to keep two constructors in class "Account"? I don't understand the reason of having the default constructor( the first one without any parameter). I tried to delete it but then the code does not work.

Thank you

If the default constructor did not exist, which constructor would you expect the following line in the shown code: `Account OBJ2;` -- which constructor do you expect to be used here? — Sam Varshavchik, Oct 17 '22 at 02:13
C++ **must** be learnt using a [good c++ book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) and not by some online *"sololearn platform(C++ course)"*. These **trivial** things are explained in any of the [beginner c++ books](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) which are also available as PDFs for free. **Stackoverflow is not an introduction to C++**. For learning the basics one should refer to a book first. — Jason, Oct 17 '22 at 03:38
Didn't the compiler produce an error message? That message should be in your question. Didn't the error message identify a line where the default constructor is needed? — JaMiT, Oct 17 '22 at 06:37
Find a different source for learning C++. The code in the question is poorly written; definitely not something beginners should be using as a base for future work. — Pete Becker, Oct 17 '22 at 12:33
Kindly introduce the source you have in mind for learning C++.Thank you. — sm75, Oct 18 '22 at 04:23

score 2 · Accepted Answer · answered Oct 17 '22 at 02:13

Look at your operator+. The first line is:

Account OBJ2;

Without a default constructor, that line is illegal. You could avoid the need for the default constructor there by using the other constructor:

    Account operator+(const Account &obj) const { // Added const for correctness
        Account OBJ2(this->balance + obj.balance);  // Create with constructor taking int
        OBJ2.interest = this->interest + obj.interest;  // May be avoidable if constructor calculation known to be valid
        return OBJ2;
    }

Why do we need two constructors while doing operator overloading?

1 Answers1