Problem converting one user-defined class to another user-defined class

Question

I just get introduced to C++ OOP.
I created two classes name Integer and Float and defined conversion constructor for every class.
I have no problem converting Float to Integer, as shown below.
The real problem comes when I remove the comment symbol to convert from Integer to Float.

#include <iostream>
using namespace std;

class Float
{
    float x, y;
public:
    Float() : x(0),y(0) {}
    Float(float a, float b) : x(a),y(b) {}
//  Float(const Integer&);
    float getX() const { return this->x; }
    float getY() const { return this->y; }
    float summa()
    {
        return this->x + this->y;
    }
};

class Integer
{
    int v, w;
public:
    Integer() : v(0),w(0) {}
    Integer(int a, int b) : v(a),w(b) {}
    Integer(const Float&);
    int getV() const { return this->v; }
    int getW() const { return this->w; }
    int summa()
    {
        return this->v + this->w;
    }
};

int main()
{
//  Integer i(5, 2);
//  Float f = i;
    Float F(3.23, 5.78);
    Integer I = F;
//  cout << f.summa();
    cout << I.summa();
    return 0;
}
/*
Float::Float(const Integer& i)
{
    this->x = (float)(i.getV());
    this->y = (float)(i.getW());
}
*/
Integer::Integer(const Float& f)
{
    this->v = (int)(f.getX());
    this->w = (int)(f.getY());
}

You need a [forward declaration](https://stackoverflow.com/questions/553682/when-can-i-use-a-forward-declaration) It also means you have to pass `Integer` as a pointer. — lakeweb, Sep 17 '20 at 00:06
Just declare `class Integer;` before you define the `Float` class. — Elliott, Sep 17 '20 at 00:06
Because the compiler needs to know the structure of the object to pass as a reference. You might also consider a free function to couple the classes. — lakeweb, Sep 17 '20 at 00:09
@lakeweb _Because the compiler needs to know the structure of the object to pass as a reference._ Actually, [that's not true](https://wandbox.org/permlink/tuzaejKPK0PEdoGq) — Paul Sanders, Sep 17 '20 at 00:14
Hi @Paul Sanders I just tested, and no it doesn't! Old habit from c++98?? — lakeweb, Sep 17 '20 at 00:16
@ Paul Sanders I've been carrying that around in my head forever. Now I feel dumb :) — lakeweb, Sep 17 '20 at 00:21

score 0 · Accepted Answer · answered Sep 17 '20 at 00:11

When Float(const Integer&); is declared, the Integer class has been not declared yet.

This should work for what you want:

#include <iostream>
using namespace std;

class Integer;

class Float
{
    float x, y;
public:
    Float() : x(0),y(0) {}
    Float(float a, float b) : x(a),y(b) {}
    Float(const Integer&);
    float getX() const { return this->x; }
    float getY() const { return this->y; }
    float summa()
    {
        return this->x + this->y;
    }
};

class Integer
{
    int v, w;
public:
    Integer() : v(0),w(0) {}
    Integer(int a, int b) : v(a),w(b) {}
    Integer(const Float&);
    int getV() const { return this->v; }
    int getW() const { return this->w; }
    int summa()
    {
        return this->v + this->w;
    }
};

int main()
{
    Integer i(5, 2);
    Float f = i;
    Float F(3.23, 5.78);
    Integer I = F;
    cout << f.summa();
    cout << I.summa();
    return 0;
}

Float::Float(const Integer& i)
{
    this->x = (float)(i.getV());
    this->y = (float)(i.getW());
}

Integer::Integer(const Float& f)
{
    this->v = (int)(f.getX());
    this->w = (int)(f.getY());
}

Problem converting one user-defined class to another user-defined class

1 Answers1