Throwing exceptions handler

[firstlast]

I'm pretty new to the concept of throwing and catching exception handlers. I originally thought, that in the event of an exception to a code (in this case of the quadratic formula if a<0, the code would throw a and the project would break. What else do I need to do in order to complete the exception handler?

// Test Case: Solve quadratic equations 
// And throws variables that are either a is less than 0 or b2 is less than 0 
#include <iostream> 
#include <cstdlib> 
#include <cmath>
using namespace std;

int main( ) 
{
//Preconditions: a must be positive while b^2 exceeds the product 4ac
//Postconditions: solves quadratic equation 

	double a, b, c; // coefficient of ax^2 + bx + c = 0 
	double x1, x2; // The two roots
    double temp;

cout << "Enter the three coefficients \n"; 
cin >> a >> b >> c;
if(a != 0)
{
	temp = b*b - 4*a*c;
	if(temp >= 0) 
	{ // Two roots 
		x1 = ( -b + sqrt(temp))/2*a;
		x2 = ( -b - sqrt(temp))/2*a; 
		cout << "The two roots are: " << x1 << " and " << x2 << endl;
	}
	else 
	{
		cout << "Square root of negative values is not defined \n"; 
		exit(1); 
	}
}else
{
	cout << "Division by zero, not defined \n";
	exit(1);
}
if (a<0) //throw if a is less then 0
throw a;

if (b*b < 4*a*c) //throw if b^2 is less than 4ac
throw temp;

system ("pause");
return 0;
}

[cire]

This is not a good candidate for exception handling, but here is how it might look:

// Test Case: Solve quadratic equations 
// And throws variables that are either a is less than 0 or b2 is less than 0 
#include <string>
#include <iostream> 
#include <cstdlib> 
#include <cmath>
#include <stdexcept>

void check_precondition(double a, double b, double c)
{
    std::string errors;

    if (a == 0)
        errors += "a cannot be 0.";
    else if (a < 0)
        errors += "a cannot be negative.";

    if (b*b <= 4 * a*c)
    {
        if (!errors.empty())
            errors += '\n';

        errors += "b squared must be greater than 4ac\n";
    }

    if (!errors.empty())
        throw std::runtime_error(errors);
}

int main()
{
    double a, b, c;

    try {
        std::cout << "Enter the three coefficients \n";
        std::cin >> a >> b >> c;

        check_precondition(a, b, c);

        double discriminant = b*b - 4 * a*c;

        std::cout << "The two roots are: " << ((-b + std::sqrt(discriminant)) / (2 * a));
        std::cout << " and " << ((-b + std::sqrt(discriminant)) / (2 * a)) << '\n';
    }

    catch (std::exception& ex)
    {
        std::cout << "Problem encountered:\n" << ex.what() << '\n';
        std::cout << "With a = " << a << ", b = " << b << ", c = " << c << '\n';
        return 1;
    }
}

http://ideone.com/5PUt0q