c++ class instance error lnk2019

Question

I searched for my problem for more than 3 hours but couldn't find any good articles about so decided to post a question here to ask for your advice. My problem is I don't understand how class works. It is just a simple class but in one case it doesn't work. First, below is the code that works

class R{
    int x, y;
public:
    R();
    R(int a, int b) : x(a), y(b) {};
    void setX(int a){ x = a; }
    void setY(int b){ y = b; }
    int getX(){ return x; }
    int getY(){ return y; }
    void display();
};
void R::display(){
// displaying x and y I removed the function because it somehow remove the rest of the code after arrow operator
}
int main(){
    R r(2,3);
    r.setX(1);
    r.setY(2);
    r.display();
}

but when I change the main like this it doesn't work

int main(){
    R r;
    r.setX(1);
    r.setY(2);
    r.display();
}

I first thought the reason might be that I didn't actually created an instance but found a code that works fine from a website. The code was like this:

// overloading operators example
#include 
using namespace std;

class CVector {
  public:
    int x,y;
    CVector () {};
    CVector (int a,int b) : x(a), y(b) {}
    CVector operator + (const CVector&);
};

CVector CVector::operator+ (const CVector& param) {
  CVector temp;
  temp.x = x + param.x;
  temp.y = y + param.y;
  return temp;
}

int main () {
  CVector foo (3,1);
  CVector bar (1,2);
  CVector result;
  result = foo + bar;
// displaying x and y I removed the function because it somehow remove the rest of the code after arrow operator
  return 0;
}

It also just made an instance "CVector temp" not "CVector temp(2,3)" but it works fine while mine is not working.

If anyone knows what I am missing and the problem of my code, could you please give me your advice?

Thanks for reading and your time

Answer 1

From your comments, I can surmise that the error is related to the missing definition of R:: R ().

MSVC's Error LNK2019 refers to an unresolved symbol. A symbol is any function, object, variable, or type which has been declared but not defined.

In this case, I'm guessing that you have never provided a definition for R:: R (), but you wrote it in the class definition, which tells the compiler that it's there somewhere -- that's the declaration.

But, apparently, you never defined it. Adding {} provides the body of the function, which is why it works with those braces.

You could also define R:: R() as you did R:: display (), externally, or, if your compiler supports C++11, write explicitly R:: () = default.

Once you give the linker a body of the function, your code should link fine.

Answer 2

When you write it like

R();

it is a Declaration of the constructor. You have to give each method you use a Definition also by giving it a body (even if the body is empty like here). For class method, either you provide the body as you did here inside the class, or you can define it separately in a source (.cpp) file like

R::R(){/*your code*/}

it is similar to the code for CVector CVector::operator+ as you posted.

Answer 3

You NEED to have a default constructor.