How can I create a vector of virtual class? - c++

Question

I'm new to C++, so I decided to work on some little project to improve myself. I try to write a simple chess program with class Unit, and class King which is inherited from Unit
#include <list>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <stdlib.h> /* abs */
using namespace std;
// Each unit class represent a chess unit
class Unit{
protected:
int currentX;
int currentY;
string side;
public:
Unit();
Unit(string sideplay, int Xpos,int Ypos)
{
currentX=Xpos; currentY= Ypos;side=sideplay;
}
int getX()
{
return currentX;
}
int getY()
{
return currentY;
}
string getside()
{
return side;
}
void setpos(int newX,int newY) //set new position
{
currentX=newX;
currentY=newY;
}
bool validmove(vector<Unit> unitlist ,string sidepick,int Xpos,int Ypos)
{ int i=0;
while(i != 3)
{ int X=unitlist[i].getX();
int Y=unitlist[i].getY();
string sidetemp= unitlist[i].getside();
if ((X==Xpos)&&(Y==Ypos)&&(sidetemp==sidepick))
{
return false;
}
else if ((X==Xpos)&&(Y==Ypos)&&(sidetemp!=sidepick))
{ //unitlist[i]=NULL;
return true;
}
i++;
}
return true;
}
virtual void moveunit(vector<Unit> unitlist ,int nextX,int nextY);
};
class King: public Unit{
public:
King(string sideplay, int Xpos,int Ypos):Unit(sideplay,Xpos,Ypos)
{}
void moveunit(vector<Unit> unitlist ,int nextX,int nextY){
int diffX=abs(nextX-currentX);
int diffY=abs(nextY-currentY);
if ((diffX==1)||(diffY==1))
{ if (validmove(unitlist,side,nextX,nextY))
{
setpos(nextX,nextY);}
}
}
};
and here is my main:
int main()
{
vector<Unit> chessunit;
chessunit.push_back(King("white",3,1));
chessunit.push_back(King("black",3,2));
chessunit.push_back(King("white",4,1));
if (chessunit[0].validmove(chessunit,"white",3,2))
{
cout<<"hehe"<<endl;
}
chessunit[0].moveunit(chessunit,3,2);
int k= chessunit[0].getY();
cout<<k<<endl;
return 0;
}
I keep getting LNK 2001 error: Unresolved external symbol for my virtual method "moveunit". How can I fix that bug ?

Answer 1

The easiest way of fixing your problem is using pointers or smart pointers: Store vector<Unit*>, vector<std::shared_ptr<Unit>> or vector<std::unique_ptr<Unit>> (thanks #rubenvb) instead of vector<Unit> and then add your kings like so:
myVector.push_back(new King...); // or
myVector.push_back(std::shared_ptr<King>(new King...)); // or
myVector.push_back(std::unique_ptr<King>(new King...));
Why?
If you allocate an object of a virtual class (e.g. Unit unit) and you want to assign an object of an implementation of that class to it, e.g.:
Unit unit;
unit = King(...);
Then you will get an error, or at least run into trouble, unless you provide a constructor for Unit that takes King as an argument or provide a sufficient move operator. That is because if you try to assign an object of a type that is not Unit to unit, the compiler and/or run-time (depending on what the back-end of your compiler is) will have a tough time figuring out how compatible the types are and what to do if things "don't fit" memory-wise and how to cope with memory layout issues.
Further Reading
For more on pointers vs. smart pointers, consider this thread. Also here is a related Stackoverflow question and an article on using shared_ptr with STL collections
More information on trying to "squeeze" an object of one type into another (called slicing) can be found in this thread.

Answer 2

The problem you are facing right now is due to slicing: when you add a King to the vector, it gets sliced into an instance of Unit.
One way to fix this is to turn chessunit into a vector of std::shared_ptr<Unit> and allocate units on the heap.
P.S. Since you are not defining Unit::moveunit(), make it pure virtual:
virtual void moveunit(vector<Unit> unitlist ,int nextX,int nextY) = 0;
^^^