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How to define an object with different subclasses in an if-statement?
Note: I asked this question on TopAnswers a couple weeks ago, but didn't get any response, so I figured I'd ask it here.
I am currently learning C++. I have a parent class (Vehicle) and two subclasses (Car and Boat). I am wondering how to create an object that will either be Car or Boat, depending on what the user specifies.
Below is my best attempt. It compiles and runs, but doesn't do what I want. It is supposed to make *vh a Car or Boat, but *vh always remains a Vehicle.
#include <iostream>
using namespace std;
class Vehicle
{
public:
void print(void)
{
cout<<"this is a vehicle\n";
}
};
class Car: public Vehicle
{
public:
void print(void)
{
cout<<"this is a car\n";
}
};
class Boat: public Vehicle
{
public:
void print(void)
{
cout<<"this is a boat\n";
}
};
int main()
{
Vehicle *vh = new Vehicle();
Car *cr = new Car();
Boat *bt = new Boat();
int x;
cout<<"type 0 for car and 1 for boat: ";
cin>>x;
if(x==0)
{
cout<<"you chose car\n";
vh = cr;
}
else if(x==1)
{
cout<<"you chose boat\n";
vh = bt;
}
vh->print();
}
1 answer
The following users marked this post as Works for me:
| User | Comment | Date |
|---|---|---|
| Trevor | (no comment) | Feb 7, 2023 at 04:19 |
What's going on is that the compiler is deciding on what function to call at compile time rather than runtime. Since the type of vh is Vehicle *, it is essentially creating this call:
vh->Vehicle::print();
There are a couple of different solutions to this, but the simplest is probably just to make the function virtual. This says that the function should be determined at runtime instead. (This is usually accomplished through virtual tables.)
class Vehicle
{
public:
virtual void print(void)
{
std::cout << "this is a vehicle\n";
}
};
class Car: public Vehicle
{
public:
void print(void) override
{
std::cout << "this is a car\n";
}
};
class Boat: public Vehicle
{
public:
void print(void) override
{
std::cout << "this is a boat\n";
}
};
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