c++ inheritence base and sub class - c++

Question

This is my base class Shape.h
#ifndef Shape_H
#define Shape_H
#include <iostream>
using namespace std;
class Shape
{
protected:
string name;
bool containsObj;
public:
Shape();
Shape(string, bool);
string getName();
bool getContainsObj();
double computeArea();
};
#endif
Shape.cpp
#include "Shape.h"
Shape::Shape(string name, bool containsObj)
{
this -> name = name;
this -> containsObj = containsObj;
}
string Shape:: getName()
{
return name;
}
bool Shape::getContainsObj()
{
return containsObj;
}
and this is my sub class. Cross.h
#ifndef Cross_H
#define Cross_H
#include <iostream>
#include "Shape.h"
using namespace std;
class Cross: public Shape
{
protected:
int x[12];
int y[12];
public:
Cross();
double computeArea();
};
#endif
Cross.cpp
#include "Cross.h"
Cross::Cross()
{
for (int i = 0; i < 12; i++)
{
this -> x[i] = 0;
this -> x[0] = 0;
}
}
Shape and Cross are in different files, but inside the same folder. The weird thing is when i compile this, errors that i have never seen before came up such as "In function 'ZN5CrossC1Ev', undefined reference to Shape::Shape(),'ZN5CrossC1Ev', undefined reference to Shape::Shape(), undefined reference to WinMain#16".
I tried to do some debugging myself. When i remove the Cross constructor, it works fine. But i definitely need it. Can anyone explain this to me?

Answer 1

You didn't define the default constructor but you declared it Shape();. The only constructor you defined is the one with string and bool parameters Shape(string, bool);.
adding
Shape::Shape()
{
}
or removing
Shape();
will fix it.
For future debugging read the error more carefully, it explains exactly whats wrong:
undefined reference to Shape::Shape()

Answer 2

You've declared a default constructor for Shape, but not defined it anywhere. The default constructor for Cross uses it implicitly to initialise its base class.
You options are:
Define the constructor, if you want Shape to be default-constructible;
Otherwise, remove the declaration and get Cross to initialise Shape with the other constructor.