Quadratic roots shows NaN - c++

Question

I referred to this post for complex number roots of a quadratic equation. Solve Quadratic Equation in C++
So, I wrote something similiar in C++ using OpenCV and std libraries, but I am always getting NaN and dont know why.
cv::Vec3f coefficients(1,-1,1);
cv::Vec<std::complex<float>,2> result_manual = {{0,0},{0,0}};
float c = coefficients.operator()(0);
float b = coefficients.operator()(1);
float a = coefficients.operator()(2);
std::cout << "---------manual method solving quadratic equation\n";
double delta;
delta = std::pow(b,2)-4*a*c;
if ( delta < 0) {
result_manual[0].real(-b/(2*a));
result_manual[1].real(-b/(2*a));
result_manual[0].imag((float)std::sqrt(delta)/(2*a));
result_manual[1].imag((float)-std::sqrt(delta)/(2*a));
}
else {
result_manual[0].real((float)(-b + std::sqrt(delta))/2*a);
result_manual[1].real((float)(-b - std::sqrt(delta))/2*a);
}
std::cout << result_manual[0] << std::endl;
std::cout << result_manual[1] << std::endl;
Result
---------manual method solving quadratic equation
(0.5,-nan)
(0.5,nan)

Answer 1

Answering myself just for the completion, after many useful comments.
The link in the question is a wrong implementation as the sqrt of a negative number is not defined. The correct implementation would be
result_manual[0].imag((float)-std::sqrt(std::abs(delta))/(2*a));
result_manual[1].imag((float)std::sqrt(std::abs(delta))/(2*a));