Initialize class variable by passing a reference to it - c++

Question

I have a quite interesting and funny C++ exercise:
This the imposed main:
int main(void)
{
Exam e = Exam(&Exam::cheat);
e.kobayashiMaru = &Exam::start;
(e.*e.kobayashiMaru)(3);
Exam::cheat = true;
if (e.isCheating())
(e.*e.kobayashiMaru)(4);
return (0);
}
And this is the asked output:
[The exam is starting]
3 Klingon vessels appeared out of nowhere.
You lost again.
[The exam is starting]
4 Klingon vessels appeared out of nowhere.
Win !
You now need to create the Exam class to get the correct output.
Here is what I've done:
class Exam
{
public:
Exam(bool *_cheat);
typedef void (Exam::*func)(int);
void start(int);
bool isCheating();
static bool cheat;
func kobayashiMaru;
};
I encounter problems with the Exam(&Exam::cheat) thing. What I understand so far is that Exam is taking the address of it's own cheat variable. When entering the Exam's constructor cheat is uninitialized. So for me I would initialize it here with false.
Exam::Exam(bool * _cheat)
{
*_cheat = false;
}
But by doing this I get a multiple definition with Exam(&Exam::cheat). I'm not sure about my reflection, can maybe someone enlighten me on what's really happening here?

Answer 1

Change:
static bool cheat;
to:
bool cheat;
inside your Exam class to allow every new object handle its own cheat variable with its own value.
Constructor will initialize your cheat variable by given value while you creating new object with constructor like this:
Exam::Exam(bool isCheating)
{
this->cheat = isCheating;
}
Or if you want to initialize cheat variable as false/true by default you can make constructor like this:
Exam::Exam()
{
cheat = false;
//or cheat = true;
}
You can handle multiple constructors too.
Now to create new object from Exam class use:
Exam *exam1 = new Exam(true); //Will initialize cheat as false
Exam *exam2 = new Exam(false); //Will initialize cheat variable as true
Then access your methods inside exam1 and exam2 object like this:
exam1->methodName(1243);
exam2->secondMethodName(true);
exam2->thirdMethodName();
exam3->addFriend(&exam1);
Those ones are just examples.
Hope I understood your problem. :)

Answer 2

You have a few issues (described in comments below)
class Exam
{
public:
// If cheat is supposed to be a member variable assigned to each instance, passing a pointer to itself is silly and unnecessary
// If cheat is supposed to be shared between all instances of exam, passing a pointer to itself is silly and unnecessary
Exam(bool *_cheat);
static bool cheat;
// these two declarations are unnecessary. They do nothing but add complexity where it is not needed
typedef void (Exam::*func)(int);
func kobayashiMaru;
void start(int);
bool isCheating();
};
I encounter problems with the Exam(&Exam::cheat) thing. What I
understand so far is that Exam is taking the address of it's own cheat
variable. When entering the Exam's constructor cheat is uninitialized.
So for me I would initialize it here with false.
Static variables are initialized before the creating of the first instance of a class. The problem you have is two-fold: 1) you haven't initialized your static member variable, 2) you aren't treating it as a static member variable.
Declaring a variable as static means it will be shared between all instances of the class (e.g. there will be 1 memory location for cheat). Unless you want everyone taking the Exam to cheat as soon as 1 person cheats, this is not likely what you want.
To fix it, you would want your Exam class to look more like this:
class Exam
{
public:
Exam(bool cheat) : m_cheat(cheat) {}
// don't expose member variables directly - use accessor functions
void setCheating(bool cheat) { m_cheat = cheat; }
bool isCheating() const { return m_cheat; }
void start(int);
private:
bool m_cheat;
};
The corresponding changes to your main function
int main(void)
{
Exam e(false);
e.start(3);
e.setCheating(true);
if (e.isCheating())
e.start(4);
return 0;
}