Empty constructor and value passing problem - c++

Question

When I create a class A object with a reference to class C, class B gives the error of:
Error C2512 'A': no appropriate default constructor available
Class A
class A
{
public:
C* cPointer;
A(C* arg)
{
cPointer = arg;
}
};
Class B
class B:
public A
{
public:
B()
{
}
};
If I add an empty constructor to class A it doesn't give errors but when I try to access cPointer it returns an empty address of 0000000000;
A()
{
}
How can I solve this problem?
Edit: example
int main()
{
C cObject;
A a(&cObject);
return 0;
}
Result: Severity Code Description Project File Line Suppression State
Error C2512 'A': no appropriate default constructor available
As I mentioned before if I put empty constructor to A it works but i cant use cPointer because it set to 0.
in class B:
cout << "Poniter: " << cPointer << endl;
result:
Pointer: 0000000000000000
Okay here is the actual code:
Skill.h (class C in example)
#pragma once
#include <iostream>
#include <vector>
#include "Unit.h"
using namespace std;
class Skill
{
public:
Unit* unitPtr;
Skill()
{
}
Skill(Unit* unit)
{
unitPtr = unit;
}
void SetUnit(Unit* unit)
{
unitPtr = unit;
}
vector <Skill *> attacks;
vector <Skill *> utilities;
vector <Skill *> movement;
};
Unit.h (class A in example)
#pragma once
#include <iostream>
#include <vector>
#include <stdlib.h>
#include <time.h>
using namespace std;
class Skill;
class Unit
{
public:
Skill* skillPtr;
Unit(Skill* skill)
{
skillPtr = skill;
}
void Setup();
string name;
vector <Unit* > heroes;
vector <Unit* > monsters;
};
Hero.h (class B in example)
#pragma once
#include "Unit.h"
class Hero :
public Unit
{
public:
Hero() : Unit(skillPtr)
{
cout << "Pointer: " << skillPtr << endl;
}
};
Main
int main()
{
Skill skill;
Unit unit(&skill);
skill.SetUnit(&unit);
unit.Setup();
return 0;
}

Answer 1

class B: public A {
B() { }
}
When a child class's constructor is called, it first initializes its parent portion by calling a parent constructor. Since you didn't specify one here, the B constructor is trying to call the default A constructor, but one doesn't exist!
To call the constructor you made explicitly, you can do:
class B: public A {
B(): A(Some_C_Pointer) { }
}