exc_arithmetic error, c++ xcode 4.62 - c++

Question

I'm a beginner of c++. I want know why the exception is thrown at this line:
operator double() const
{
return this->denominator/this->numerator;
}
I found numerator == 0,but I have given default values in the ctor. Why? Please help me.
class Rational {
public:
int numerator;
int denominator;
Rational(int numerator = 2, int denominator = 1)
{
numerator = 2;
denominator = 1;
};
operator double() const
{
return this->denominator/this->numerator;
};
};
int main(int argc, const char * argv[])
{
Rational r(1, 2);
cout << r;
// double d = 0.5*r;
return 0;
}

Answer 1

First, this line is very wrong:
Rational(int numerator = 2, int denominator = 1){ numerator = 2; denominator = 1;};
The arguments numerator and denominator are in the stack frame of your ctor thus hiding your class's members Rational::numerator and Rational::denominator and secondly, you are completely ignoring what the caller actually sends as its arguments (not because you specify the default values, but because you explicitly set numerator=2; denominator=1. This line should actually be:
Rational(int numerator = 2, int denominator = 1){ this->numerator = numerator; this->denominator = denominator;};
OR you could use an initializer list (and overloaded constructors):
Rational() : numerator(2), denominator(1) {};
Rational(int numerator, int denominator) : numerator(numerator), denominator(denominator) {};
Now, the second problem is that int/int => int :: dividing an integer by another integer yields integer division, you need to cast one of your int's to double so that both int's are upconverted to double before calculating the division, like so:
operator double() const{ return ((double)this->denominator)/this->numerator;};

Answer 2

Rational(int numerator = 2, int denominator = 1)
{
numerator = 2;
denominator = 1;
};
I would change this to
Rational(int numerator, int denominator)
{
this->numerator = numerator;
this->denominator = denominator;
};
I don't know exactly if this is causing your problem, but I think it's contributing to it. Your call to Rational(int,int) will still work fine and add your specified values into the Object you're creating.
The issue with your code, is scope. I believe you were just re-assigning the arguments/parameters instead of actually re-assigning your instance variables, which I'm assuming was your goal.