With {}, you can initialize a class without having to have a constructor.

//Operator Overloading

#include<iostream>

using namespace std;

class Complex
{
public:
    int a,b;
    void input(string s)
    {
        int v1=0;
        int i=0;
        while(s[i]!='+')
        {
            v1=v1*10+s[i]-'0';
            i++;
        }
        while(s[i]==' ' || s[i]=='+'||s[i]=='i')
        {
            i++;
        }
        int v2=0;
        while(i<s.length())
        {
            v2=v2*10+s[i]-'0';
            i++;
        }
        a=v1;
        b=v2;
    }
};


//Overload operators + and << for the class complex
//+ should add two complex numbers as (a+ib) + (c+id) = (a+c) + i(b+d)
//<< should print a complex number in the format "a+ib"

Complex operator+ (const Complex& lhs, const Complex& rhs) 
{   
    return {lhs.a + rhs.a, lhs.b + rhs.b};
}

ostream& operator<< (ostream& os, const Complex& c)
{
    int abs_b = c.b;
    string sign;

    if(c.b > 0) {
        sign = "+";
    } else {
        sign = "-";
        abs_b = -c.b;
    }
    os << c.a << sign << "i" << abs_b << endl; 
    return os;
}





int main()
{
    Complex x,y;
    string s1,s2;
    cin>>s1;
    cin>>s2;
    x.input(s1);
    y.input(s2);
    Complex z=x+y;
    cout<<z<<endl;
}

References

Following are comments in the discussion forum I found valuable.