Returns: An object of type real-type
where the ith element is initialized to the result of
cmplx-func(operator[](i)) for all i in the range
[0, size()), where cmplx-func is the corresponding function
from <complex>.
Effects: Replaces each element of the basic_vec object such that the
ith element is replaced with value_type(v[i],
operator[](i).imag()) or value_type(operator[](i).real(), v[i])
for real and imag respectively, for all i in the range [0, size()).