Programming Forums > Application Development > C++

Eric the Red · Feb 27th, 2006, 1:13 PM

I'm new to c++. I recently read a book on it. After reading the book i just wanted to get some practical experience. Now i'm trying to get "operator overloading" to work. I have a Car class and all i want to do is increment the Honda's speed by 1, (from the Car class). Doing "++Honda" (Honda is the object). However, Instead of increamenting by 1 it increments by approx ONE HUNDRED MILLION. What am i doing wrong??

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#include <iostream>
#include <stdlib.h>

class Car
{
public:
    Car(); 
    ~Car();
    int GetSpeed() {return *itsSpeed;}
    void SetSpeed (int speed) { *itsSpeed = speed;}
    void Speeding();
    void Speed() { *itsSpeed = *itsSpeed + 1;}
    const Car& operator++ ();
private:
    int * itsSpeed;
    int * itsFuel;
};
    
Car::Car()
{
    itsSpeed = new int(10);
    itsFuel = new int(20);
}

Car::~Car()
{
    delete itsSpeed;
    delete itsFuel;
}

const Car& Car::operator++()
{
    * ++itsSpeed;
}

void Car::Speeding()
{
    if (* itsSpeed > (30))
    {
        std::cout << "You are over the speed limit, please slow down. Your going " << *itsSpeed << " KM/H \n";
    }
    else 
    {
        std::cout << "Your under the speed limit. Your at " << *itsSpeed << " KM/H at the moment.\n"; 
    }
}
int main()
{
    //int CarSpeed;
  
    Car * Honda = new Car;
    Honda->Speeding();    
    Honda->SetSpeed(45);
    Honda->Speeding();
    Honda->Speed();
    Honda->Speeding();
    ++Honda;
    Honda->Speeding();
    ++Honda;
    Honda->Speeding();
           
    delete Honda;
    system("Pause");
    return 0;
}

Kaja Fumei · Feb 27th, 2006, 1:33 PM

With unary operators, they are evaluated right to left, not left to right, so in your code, the ++ operation is done to itsSpeed before *. This causes the pointer to be incremented, rather than the value stored at the pointer's address, which is what you meant to do.

Simple fix:

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++*itsSpeed;

Eric the Red · Feb 27th, 2006, 1:43 PM

Thanks for your help. Unfortunately, this doesn't seem to fix the problem. I tried

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const Car& Car::operator++()
{
    ++ (* itsSpeed); 
}

// and 

const Car& Car::operator++()
{
    ++ * itsSpeed;
}

The result, still give me a value in the millions when incremented. Just try running the program and you'll see.

Klarre · Feb 27th, 2006, 1:56 PM

You also have to change on the Honda object.

Kaja Fumei · Feb 27th, 2006, 2:02 PM

I see the problem now, I wasn't paying too much attention before. Overloaded operators can only be used on the the object "value". If you declare a pointer to an object, any operators you do on that pointer will be done to the pointer, not to the data. So you must declare the car on the stack or if you have a pointer to a Car (call the pointer variable Honda), you can use ++*Honda.

Method one:

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    Car Honda;
    Honda.Speeding();    
    Honda.SetSpeed(45);
    Honda.Speeding();
    Honda.Speed();
    Honda.Speeding();
    ++Honda;
    Honda.Speeding();
    ++Honda;
    Honda.Speeding();

Method 2:

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    Car * Honda2 = new Car;
    Honda2->Speeding();    
    Honda2->SetSpeed(45);
    Honda2->Speeding();
    Honda2->Speed();
    Honda2->Speeding();
    ++*Honda2;
    Honda2->Speeding();
    ++*Honda2;
    Honda2->Speeding();
    
    delete Honda2;

I ran your code on both of these and it worked fine.

Eric the Red · Feb 27th, 2006, 2:02 PM

Yeah ur right.. and wow can these pointers can get tedius.

I'm using pointers in this class to get practice with it. However, is there really a point to be using pointers in this program?

Polyphemus_ · Feb 27th, 2006, 2:05 PM

Quote:

Originally Posted by Eric the Red

However, is there really a point to be using pointers in this program?

Well, there is always a point, but in this case no need. Since you exactly know how much memory you are using (basically a car object, and two integers) there is indeed no need for them.
It is good to practice them though, because practice makes perfect (it is said)

.

Feb 27th, 2006, 1:13 PM	#1
Eric the Red Hobbyist Programmer Join Date: Feb 2006 Posts: 214 Rep Power: 0	A class with operator overloading I'm new to c++. I recently read a book on it. After reading the book i just wanted to get some practical experience. Now i'm trying to get "operator overloading" to work. I have a Car class and all i want to do is increment the Honda's speed by 1, (from the Car class). Doing "++Honda" (Honda is the object). However, Instead of increamenting by 1 it increments by approx ONE HUNDRED MILLION. What am i doing wrong?? (Toggle Plain Text) #include <iostream> #include <stdlib.h> class Car { public: Car(); ~Car(); int GetSpeed() {return itsSpeed;} void SetSpeed (int speed) { itsSpeed = speed;} void Speeding(); void Speed() { itsSpeed = itsSpeed + 1;} const Car& operator++ (); private: int * itsSpeed; int * itsFuel; }; Car::Car() { itsSpeed = new int(10); itsFuel = new int(20); } Car::~Car() { delete itsSpeed; delete itsFuel; } const Car& Car::operator++() { * ++itsSpeed; } void Car::Speeding() { if (* itsSpeed > (30)) { std::cout << "You are over the speed limit, please slow down. Your going " << itsSpeed << " KM/H \n"; } else { std::cout << "Your under the speed limit. Your at " << itsSpeed << " KM/H at the moment.\n"; } } int main() { //int CarSpeed; Car * Honda = new Car; Honda->Speeding(); Honda->SetSpeed(45); Honda->Speeding(); Honda->Speed(); Honda->Speeding(); ++Honda; Honda->Speeding(); ++Honda; Honda->Speeding(); delete Honda; system("Pause"); return 0; } #include <iostream> #include <stdlib.h> class Car { public: Car(); ~Car(); int GetSpeed() {return itsSpeed;} void SetSpeed (int speed) { itsSpeed = speed;} void Speeding(); void Speed() { itsSpeed = itsSpeed + 1;} const Car& operator++ (); private: int * itsSpeed; int * itsFuel; }; Car::Car() { itsSpeed = new int(10); itsFuel = new int(20); } Car::~Car() { delete itsSpeed; delete itsFuel; } const Car& Car::operator++() { * ++itsSpeed; } void Car::Speeding() { if (* itsSpeed > (30)) { std::cout << "You are over the speed limit, please slow down. Your going " << itsSpeed << " KM/H \n"; } else { std::cout << "Your under the speed limit. Your at " << itsSpeed << " KM/H at the moment.\n"; } } int main() { //int CarSpeed; Car * Honda = new Car; Honda->Speeding(); Honda->SetSpeed(45); Honda->Speeding(); Honda->Speed(); Honda->Speeding(); ++Honda; Honda->Speeding(); ++Honda; Honda->Speeding(); delete Honda; system("Pause"); return 0; }

Feb 27th, 2006, 1:33 PM	#2
Kaja Fumei Hobbyist Programmer Join Date: Oct 2005 Posts: 134 Rep Power: 3	With unary operators, they are evaluated right to left, not left to right, so in your code, the ++ operation is done to itsSpeed before . This causes the pointer to be incremented, rather than the value stored at the pointer's address, which is what you meant to do. Simple fix: (Toggle Plain Text) ++itsSpeed; ++*itsSpeed;

Feb 27th, 2006, 1:43 PM	#3
Eric the Red Hobbyist Programmer Join Date: Feb 2006 Posts: 214 Rep Power: 0	Thanks for your help. Unfortunately, this doesn't seem to fix the problem. I tried (Toggle Plain Text) const Car& Car::operator++() { ++ (* itsSpeed); } // and const Car& Car::operator++() { ++ * itsSpeed; } const Car& Car::operator++() { ++ (* itsSpeed); } // and const Car& Car::operator++() { ++ * itsSpeed; } The result, still give me a value in the millions when incremented. Just try running the program and you'll see.

Feb 27th, 2006, 2:02 PM	#5
Kaja Fumei Hobbyist Programmer Join Date: Oct 2005 Posts: 134 Rep Power: 3	I see the problem now, I wasn't paying too much attention before. Overloaded operators can only be used on the the object "value". If you declare a pointer to an object, any operators you do on that pointer will be done to the pointer, not to the data. So you must declare the car on the stack or if you have a pointer to a Car (call the pointer variable Honda), you can use ++Honda. Method one: (Toggle Plain Text) Car Honda; Honda.Speeding(); Honda.SetSpeed(45); Honda.Speeding(); Honda.Speed(); Honda.Speeding(); ++Honda; Honda.Speeding(); ++Honda; Honda.Speeding(); Car Honda; Honda.Speeding(); Honda.SetSpeed(45); Honda.Speeding(); Honda.Speed(); Honda.Speeding(); ++Honda; Honda.Speeding(); ++Honda; Honda.Speeding(); Method 2: (Toggle Plain Text) Car Honda2 = new Car; Honda2->Speeding(); Honda2->SetSpeed(45); Honda2->Speeding(); Honda2->Speed(); Honda2->Speeding(); ++Honda2; Honda2->Speeding(); ++Honda2; Honda2->Speeding(); delete Honda2; Car * Honda2 = new Car; Honda2->Speeding(); Honda2->SetSpeed(45); Honda2->Speeding(); Honda2->Speed(); Honda2->Speeding(); ++Honda2; Honda2->Speeding(); ++Honda2; Honda2->Speeding(); delete Honda2; I ran your code on both of these and it worked fine.

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Feb 27th, 2006, 1:56 PM	#4
Klarre Game engine designer Join Date: May 2005 Location: Sweden Posts: 301 Rep Power: 4	You also have to change on the Honda object.

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Programming Forums > Application Development > C++

A class with operator overloading