Negative square root

Question

How do you take the square root of a negative number in C++?
I know it should return a real and a complex part, I get a NaN?
How do I take the real part?

Answer 1

#include <complex>

int main()
{
    std::complex<double> two_i = std::sqrt(std::complex<double>(-4));
}

or just

std::complex<double> sqrt_minus_x(0, std::sqrt(std::abs(x)));

Answer 2

sqrt(-x) where x is a positive number is simply 0 + sqrt(x)*i. The real part is just 0.

In general, the real part is x > 0 ? sqrt(x) : 0 and the imaginary part is x < 0 ? sqrt(x) : 0.