Bjarne Stroustrup “Programming Principles and Practice Using C++” Chapter 4 Exercise 18
Using std_lib_facilities.h by Bjarne Stroustrup.

// Philipp Siedler
// Bjarne Stroustrup's PP
// Chapter 4 Exercise 18
#include "std_lib_facilities.h"
int main()
try
{
float a, b, c, x1, x2, determinant, realNum, imagineNum;
cout << "Enter a value for 'a', 'b' and 'c' to solve a quadratic equation.\n";
if(cin >> a >> b >> c){}
else {
error("Bad input for a, b or c");
}
determinant = pow(b, 2) - 4 * a * c;
if (determinant > 0) {
x1 = (-b + sqrt(determinant)) / (2 * a);
x2 = (-b - sqrt(determinant)) / (2 * a);
cout << "Roots are real and different.\n";
cout << "x1 = " << x1 << "\n";
cout << "x2 = " << x2 << "\n";
}
else if (determinant == 0) {
x1 = (-b + sqrt(determinant)) / (2 * a);
cout << "Roots are real and same.\n";
cout << "x1 = x2 " << x1 << "\n";
}
else {
realNum = -b / (2 * a);
imagineNum = sqrt(-determinant) / (2 * a);
cout << "Roots are complex and different.\n";
cout << "imagined part + real part: x1 = " << realNum + imagineNum << "\n";
cout << "imagined part - real part: x2 = " << realNum - imagineNum << "\n";
}
keep_window_open(".");
}
catch (runtime_error e) {
cout << e.what() << "\n";
keep_window_open(".");
}
catch (...) {
cout << "Extiting" << "\n";
keep_window_open(".");
}

Output:
Enter a value for 'a', 'b' and 'c' to solve a quadratic equation.
13 6 9
Roots are complex and different.
imagined part + real part: x1 = 0.568639
imagined part - real part: x2 = -1.03018
Please enter . to exit