Use attribute from derived class while using method from the base class? - c++

Question

I have the following code:
#include <map>
class vehicle {
private:
int id = 0;
std::map<std::pair<int, char>, int> seats{};
public:
void displaySeats();
};
class Bus : public vehicle {
private:
std::string type = "Bus";
public:
std::map<std::pair<int, char>, int> seats{ {{1,'A'},0 }, {{1,'B'},0 }, {{ 1,'C' }, 0}, {{ 1,'D' }, 0 }, {{ 1,'E' }, 0 },
{{2,'A'},0 }, {{2,'B'},0 }, {{ 2,'C' }, 0}, {{ 2,'D' }, 0 }, {{ 2,'E' }, 0 },
{{3,'A'},0 }, {{3,'B'},0 }, {{ 3,'C' }, 0}, {{ 3,'D' }, 0 }, {{ 3,'E' }, 0 },
{{4,'A'},0 }, {{4,'B'},0 }, {{ 4,'C' }, 0}, {{ 4,'D' }, 0 }, {{ 4,'E' }, 0 },
{{5,'A'},0 }, {{5,'B'},0 }, {{ 5,'C' }, 0}, {{ 5,'D' }, 0 }, {{ 5,'E' }, 0 }};
};
class MiniVan : public vehicle {
private:
std::string type = "MiniVan";
public:
// Seats map. <int, char> represents seats id(1A 1B 1C) the <int> will be either 0 or 1, representing if the seat is taken(1) or free(0).
std::map<std::pair<int, char>, int> seats{ {{1,'A'},0 }, {{1,'B'},0 }, {{ 1,'C' }, 0},
{{2,'A'},0 }, {{2,'B'},0 }, {{ 2,'C' }, 0},
{{3,'A'},0 }, {{3,'B'},0 }, {{ 3,'C' }, 0},
{{4,'A'},0 }, {{4,'B'},0 }, {{ 4,'C' }, 0} };
};
void vehicle::displaySeats()
{
std::pair<int, char> seat{ 1, 'E'};
int columns = 5?this->seats.count(seat)>0:3;
int displayRow = 0;
for (const auto& p : this->seats) {
if (displayRow == columns) {
std::cout << std::endl;
displayRow = 0;
}
displayRow++;
std::cout << p.first.first << p.first.second << "\t ";
}
};
In main() i have:
MiniVan testMiniVan;
testMiniVan.displaySeats();
It displays the empty seats map attribute from the base class.
I am new to c++, but in other languages, it takes the derived class attribute. How can I solve this problem? Will I have to create displaySeats for each child class ? If yes, then why do I need base class in the first place?

Answer 1

This is something I've done fairly often as well, the issue is how things are constructed and which functions have access to which variables. Maybe you could do something like:
class Vehicle {
private:
int id = 0;
std::map<std::pair<int, char>, int> seats{};
public:
// Add a constructor that takes in the `seats` map
Vehicle(std::map<std::pair<int, char>, int> s) : seats(std::move(s)) {}
void displaySeats();
};
and then for each child class:
class MiniVan : public Vehicle {
private:
std::string type = "MiniVan";
public:
// call the parent constructor passing in the seat map
MiniVan() : Vehicle({{1,'A'},0 }, ...}) {}
};
Another approach would be to make a virtual function on the base class like:
class Vehicle {
...
protected:
...
const std::map<std::pair<int, char>, int>& get_seats() const = 0;
};
that the displaySeats function would call instead of directly loading this->seats. Each child class would need to define get_seats(), but that would be pretty easy (pretty much just return seats;). Hopefully that makes sense, let me know if not!