In the book 'Modern Particle Physics' byM. thomson the Higgs doublet is written as
$$\phi = \left(\begin{matrix} \phi^+ \\ \phi^0 \end{matrix}\right)=\phi = \left(\begin{matrix} \phi_1 +i\phi_2 \\ \phi_3+i\phi_4 \end{matrix}\right) $$
With the comment
Because the Higgs mechanism is required to generate the masses of the electroweak gauge bosons, one of the scalar fields must be neutral, written as $\phi^0$, and the other must be charged such that $\phi^+$ and $(\phi^+)^*$ = $\phi^-$ give the longitudinal degrees of freedom of the $W^+$ and $W^-$.
I don't understand why can we interpret these component is being charged? Ok by assuming that the degrees of freedom are being 'eaten' by the weak-bosons corresponding to $\phi_i$, $i\in \lbrace 1,2,3\rbrace$. Which, from the argument above, the first two should be eaten by the charged $W$'s and the last one by the neutral $Z$. However this seems to be somewhat ad hoc...
I don't see why the fact that their degree of freedom is transferred to the bosons is a longitudinal polarization implies that these components must be charged...