The object qualifer that are not compatible with the member functions [closed] - c++

Question

Closed. This question needs debugging details. It is not currently accepting answers.
Edit the question to include desired behavior, a specific problem or error, and the shortest code necessary to reproduce the problem. This will help others answer the question.
Closed 6 years ago.
Improve this question
I am trying to change a subclass values Capacity, RentRate and RentMin using operation overloading. I'm newish to c++, come from java.
I want to create the objects
VanIn Van7("Large", 200, 2.0);
ManVanIn ManVan8("Abc", 99999, "Medium", 100, 1.0);
ManVan8 = Van7;
Making ManVan8 values change from "Medium, 100, 1.0" to "Large, 200, 2.0" but I keep getting a object qualifer error at the operations overload method
using namespace std;
class AbstractVan {
private:
int RentMin;
string Drivername;
long DLno;
string Capacity;
float RentRate;
public:
AbstractVan(string Drivername, long DLno, string Capacity, int RentMin, float RentRate) : Capacity(Capacity), RentMin(RentMin), RentRate(RentRate), DLno(DLno), Drivername(Drivername) {}
void setCapacity(string cap) { Capacity = cap; }
void setRentRate(float rate) {RentRate = rate;}
void setRentMin(int min) {RentMin = min;}
string getCapacity() { return Capacity; }
float getRentRate() { return RentRate; }
int getRentMin() { return RentMin; }
virtual void print() = 0;
};
Derived class from AbstractVan
class VanIn : public AbstractVan {
public:
VanIn(string Capacity, int RentMin, float RentRate) : AbstractVan(Capacity, RentMin, RentRate) {}
AbstractVan(string Drivername, long DLno, string Capacity, int RentMin, float RentRate) : Capacity(Capacity), RentMin(RentMin), RentRate(RentRate), DLno(DLno), Drivername(Drivername) {}
Derived class from VanIn
class ManVanIn : public VanIn {
private:
string Drivername;
int DLno;
public:
ManVanIn(string Drivername, long DLno, string Capacity, int RentMin, float RentRate) : VanIn(Drivername, DLno, Capacity, RentMin, RentRate){}
void print() { cout << "Drivername " << this->Drivername << " Registration " << this->DLno << " - " << getCapacity() << endl; }
~ManVanIn() {cout << "Destroy ManVanIn" << endl;}
void operator = (const VanIn &D) {
setCapacity(D.getCapacity());
setRentRate(D.getRentRate());
setRentMin(D.getRentMin());
}
};
Entry
int main()
{
VanIn Van7("Large", 200, 2.0);
ManVanIn ManVan8("Abc", 99999, "Medium", 100,1.0);
ManVan8 = Van7;
ManVan8.print();
system("pause");
return 0;
};

Answer 1

First of all things, as you will see later on, it's good practice to define getters with const qualifier. Otherwise it cannot be called on const object - I will get into that later.
string getCapacity() const { return Capacity; }
float getRentRate() const { return RentRate; }
int getRentMin() const { return RentMin; }
By using const qualifier you declare, that these methods only read from object and they don't change anything within the object. By following this 'rule' print() should be declared with const qualifier as well:
virtual void print() const = 0;
Second thing is if you have at least one virtual method, destructor should be virtual as well.
virtual ~AbstractVan() = default;
Next problem is in your VanIn class. Definition of constructor is wrong. VanIn is derived class from AbstractVan, therefore before creating VanIn, base class (in this case AbstractVan) must be created. Since AbstractVan doesn't have default constructor you must call parametric one (which accepts 5 arguments) in initialization section. Like this:
VanIn(string Capacity, int RentMin, float RentRate)
: AbstractVan(/* 5 parameters MUST be here */) { }
Don't forget what order of parameters is in AbstractVan constructor(e.g. it won't accept float if it expects string).
Note: you might want to use const string& in this constructor instead of string. const string& means that it is read-only reference (no unnecessary copying).
Next issue is in ManVanIn class. I don't see use of private variables. Same thing will be saved in AbstractVan after its constructor is called. Also when you call constructor of ManVanIn you try to call VanIn constructor with invalid number of arguments. Your declared version expects 3, and you give 5.
Next one is not a issue but is a good practice. When you override virtual functions use override specifier, like this:
void print() const override { /* ... */ }
It is good practice because if you try to override function which is not virtual your program won't compile (you avoid a lot of mistakes by using this). For example if you declared print function in Abstract as I did and you try override function like this:
double print() override { /* ... */ }
or even like this
void print() override { /* ... */ }
compiler will warn you that you are overriding function which is not virtual. In first case it should be clear, you didn't declare print() member function returning double. In second case it is because of missing const qualifier.
The reason why getters should be const lies here:
void operator = (const VanIn &D) {
setCapacity(D.getCapacity());
setRentRate(D.getRentRate());
setRentMin(D.getRentMin());
}
Your operator = overload accepts one parameter which is const reference to VanIn object. What you say here is that you won't change VanIn object within the body of this function. Therefore compiler cannot call non-const methods on const objects. If you miss const qualifier in these cases your program won't even compile (it should give error about discarding cv-qualifier).