'class MainClass' has no member named 'procent - c++

Question

#include <iostream>
#include <vector>
using namespace std;
class MainClass{
public:
virtual void getData() = 0;
virtual void printData() = 0;
};
class Car : public MainClass{
private:
int hp;
int motor;
public:
Car(int _hp = 0, int _motor = 0){
hp = _hp;
motor = _motor;
}
~Car(){
cout << "The program is over";
}
void getData(){
cout << "HorsePower = ";
int _hp;
cin >> _hp;
hp = _hp;
cout << "Motor = ";
int _motor;
cin >> _motor;
motor = _motor;
}
void printData(){
cout << "HorsePower = " << hp << '\n';
cout << "Motor = " << motor << '\n';
}
float procent(){
return float(motor) / hp;
}
};
int main()
{
MainClass *p = new Car;
p -> getData();
p -> printData();
cout << p -> procent();
return 0;
}
I'm pretty new with oop concepts so why do I get the error from title and how can i solve it? I tried to put virtual float procent() = 0 in MainClass, but then i should use this function in every class that i would create next and I dont want this.

Answer 1

procent() is not a member of class MainClass.
The pointee type of p is MainClass.
Hence, you can only access what is known in MainClass.
That the pointer p actually holds an address of a Car instance doesn't count for this.
getData() and printData() are known but procent() not. (It's part of the derived class Car.)
That you can access Cars overridden getData() and printData() by the MainClass pointer p is a result of the (wonderful world of) polymorphy i.e. they are virtual.
So, this is a potential solution – to make procent() a virtual member function of MainClass as well.
There is still another solution but it should be used as last resort only:
You can use dynamic_cast() to check whether/that p holds actually a pointer to an instance of class Car:
#include <iostream>
#include <vector>
using namespace std;
class MainClass{
public:
virtual void getData() = 0;
virtual void printData() = 0;
};
class Car : public MainClass{
private:
int hp;
int motor;
public:
Car(int _hp = 0, int _motor = 0){
hp = _hp;
motor = _motor;
}
~Car(){
cout << "The program is over";
}
void getData(){
cout << "HorsePower = ";
int _hp;
cin >> _hp;
hp = _hp;
cout << "Motor = ";
int _motor;
cin >> _motor;
motor = _motor;
}
void printData(){
cout << "HorsePower = " << hp << '\n';
cout << "Motor = " << motor << '\n';
}
float procent(){
return float(motor) / hp;
}
};
int main()
{
MainClass *p = new Car;
p -> getData();
p -> printData();
if (Car *pCar = dynamic_cast<Car*>(p)) {
cout << pCar -> procent();
}
return 0;
}
Output:
HorsePower = 300
Motor = 6
HorsePower = 300
Motor = 6
0.02
Live Demo on coliru
The solution with dynamic_cast may be preferable for large class inheritance trees where it's merely impractical to add virtual member functions to the base class for any (exotic) specific feature of derived classes.

Answer 2

The error happens because your trying to access the procent() method in the MainClass object. To resolve this, you need to add it as a virtual method in the MainClass and override it in the Car object.
Plus make sure to delete the allocated memory.
#include <iostream>
#include <vector>
using namespace std;
class MainClass{
public:
virtual void getData() = 0;
virtual void printData() = 0;
virtual void procent() = 0;
};
class Car : public MainClass {
private:
int hp;
int motor;
public:
Car(int _hp = 0, int _motor = 0){
hp = _hp;
motor = _motor;
}
~Car(){
cout << "The program is over";
}
void getData() override {
cout << "HorsePower = ";
int _hp;
cin >> _hp;
hp = _hp;
cout << "Motor = ";
int _motor;
cin >> _motor;
motor = _motor;
}
void printData() override {
cout << "HorsePower = " << hp << '\n';
cout << "Motor = " << motor << '\n';
}
float procent() override {
return float(motor) / hp;
}
};
int main()
{
MainClass *p = new Car;
p -> getData();
p -> printData();
cout << p -> procent();
delete p;
return 0;
}
The reason for the error is that when you assign the data of the derived class to the base class pointer, the data accessed from the base class pointer is treated as if its from the base class. And since the method procent() is not available in it, the error is flagged.
TIP:
Even though stating override in the overridden method is not necessary, it'll definitely increase the readability of your code.