  Let $W$ be a finite reflection group. A $W$-invariant function of
class~$C^{\infty}$ may be expressed as a functions of class $C^{\infty}$ of the
basic invariants. In finite class of differentiability, the situation is not
this simple. Let~$h$ be the greatest Coxeter number of the irreducible
components of $W$ and $P$ be~the Chevalley mapping, if $f$ is an invariant
function of class $C^{hr}$, and $F$ is the function of invariants associated by
$f=F\circ P$, then $F$ is of class $C^r$. However if~$F$ is of class $C^r$, in
general $f=F\circ P$ is of class $C^r$ and not of class $C^{hr}$. Here we
determine the space of $W$-invariant functions that may be written as functions
of class $C^r$ of the polynomial invariants and the subspace of functions $F$
of class $C^r$ of the invariants such that the invariant function $f=F\circ P$
is of class $C^{hr}$.
