Making integer caculator using operator overloading but not work

Question

I'm making integer calculator using C++. Why is the following not working?

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

class Number {
    int i;
  public:
    Number(int i) {
        this->i = i;
    }
    void show();
    Number operator+(Number op2);
};

void Number::show() {
    cout << "result is" << i << endl;
}

Number Number::operator+(Number op2) {
    Number tmp;
    tmp.i = this->i + op2.i;
    return tmp;
}

int main() {
    Number real1, real2, lastsum;

    cout << "first integer" << endl;
    getline(cin, real1.i);

    cout << "second integer" << endl;
    getline(cin, real2.i);

    lastsum = real1 + real2;
    lastsum.show();
}

I intended in this way.

1. First, "first integer" sentence appear on console. Type first integer what i want.
2. Second, "second integer" sentence appear on console. Type second integer what i want.
3. cuz of operator overloading, real1.i and real2.i added each other(like 1+2=3) and the result copy in lastum.i
4. "result is (sum)"appears at last cuz of lastsum.show();

And here is the error message (I'm using onlinegdb but imma text only red sentences appeared.)

main.cpp:23:12:error: no matching function for call to ‘Number::Number()
   23 |     Number tmp;
      |            ^~~

main.cpp:32:12: error: no matching function for call to ‘Number::Number()’
   32 |     Number real1, real2, lastsum;
      |            ^~~~~ 

main.cpp:32:19: error: no matching function for call to ‘Number::Number()’
   32 |     Number real1, real2, lastsum;
      |                   ^~~~~

main.cpp:35:24: error: ‘int Number::i’ is private within this context
   35 |     getline(cin, real1.i);
      |                        ^

main.cpp:35:12: error: no matching function for call to ‘getline(std::istream&, int&)’
   35 |     getline(cin, real1.i);
      |     ~~~~~~~^~~~~~~~~~~~~~

main.cpp:38:24: error: ‘int Number::i’ is private within this context
   38 |     getline(cin, real2.i);
      |                        ^

main.cpp:38:12: error: no matching function for call to ‘getline(std::istream&, int&)’
   38 |     getline(cin, real2.i);
      |     ~~~~~~~^~~~~~~~~~~~~~

Answer 1

There are a number of issues with the code's design, but putting those aside, and just fixing the code so it compiles and runs as expected...

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

class Number{
public:
    int i;


public:
 Number() {
    this->i = 0;
 }

 Number(int i) {
     this->i=i;
 }
 void show();
 Number operator+ (Number op2);
};


 void Number::show(){
    cout<<"result is "<<i<<endl;
}

 Number Number::operator+(Number op2){
    Number tmp;
    tmp.i=this->i+op2.i;
    return tmp;

}



int main(){
    Number real1, real2, lastsum;

    cout<<"first integer"<<endl;
    cin >> real1.i;

    cout<<"second integer"<<endl;
    cin >> real2.i;

    lastsum=real1+real2;

    lastsum.show();


}

The issues:

1. Your original code getline(cin, real1.i) is reading into real1.i, but the member variable i is private. I made it public in the "fixed" version.
2. In the Number::operator+() function you have the statement Number tmp with the intention of making a temporary Number object. However, the C++ doesn't know how to construct this object (it doesn't match your constructor). So, I added a default constructor for this. This fixes several errors.
3. I also didn't like the readline() statement so I changed to cin >> x.

With those changes it should compile and work (did so with me, using g++ compiler on Linux).

There are a number of issues of course, like why you would do it this way in the first place, exposing i as public, etc. But I assume you are doing this just as way to experiment with C++ and operator overloading.

Try and get a good book or online course on C++.

Answer 2

Let me just do a quick refactor of your code,

#include <iostream>
#include <string>

// FIRST: Stop using namespace, especially namespace std
// using namespace std;

class Number{
    int i;      // this is private, so you get a getter and setter for it
public:
    // this is a constructor
    Number(int i = 0) {      // i added the '=0' so it can simulate a constructor with no parameters
        this->i=i;
    }
    void show();
    Number operator+(Number op2); 

    // you need a setter for the private atribute i
    void setI(int number) {
        this->i = number;
    }
    // you need a getter for your private atribute i
    int getI() {
        return i;
    }
};


void Number::show(){
    std::cout<<"result is "<<i<<std::endl;
}

Number Number::operator+(Number op2){
    // get the values of i from getters, insted of this->i and op2.i
    return Number(getI() + op2.getI());
    // this is not necesary now
    // Number tmp;
    // tmp.i=this->i+op2.i;
    // return tmp; 
}



int main(){
    // okay, now this does't work because you dont have a constructor with no parameters
    // you either have to add a default value to the existing one
    // or make a new with no parameters
    Number real1, real2, lastsum; 
    
    std::cout<<"first integer"<<std::endl;

    // dont do this, just... why
    // getline(cin, real1.i);
    // do this instead
    int i;
    std::cin>>i;
    real1.setI(i);
    // same for real2, you can use the same variable i
    std::cout<<"second integer"<<std::endl;
    std::cin>>i;
    real2.setI(i);
    
    lastsum=real1+real2;
    
    lastsum.show();

    // always add return 0; at the end of yout main() funtion
    return 0;
    
}