Return type based on input value

Question

Is there a way in C ++ 11 to implement a function sqrtthat works for both positive and negative input values double? I would like the return type to std::complex<double>be if the input is negative, and a doubleif it is positive. I understand that the simple solution is to always return std::complex<double>, but this is not what I am looking for.

The following is an example of my first attempt, however this will not compile due to the presence ain the return type:

inline decltype((a > 0)?(double):(std::complex<double>)) sqrt(const double& a)
{
    if(a > 0)
    {
        return std::sqrt(a);
    }
    else
    {
        return ((std::complex<double>(0.0,1.0))*std::sqrt(-a));
    }
}

Answer 1

No, It is Immpossible.

Types are compile-time constructs, and your function type is fixed at compile time.

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Answer 2

boost::variant<double, std::complex<double>> ( ), .

Answer 3

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