switch meta-function among four types.
switch
Declared in <bslmf_switch.h>
<bslmf_switch.h>
template< size_t t_SWITCH_SELECTOR, class t_T0, class t_T1, class t_T2, class t_T3> using Switch4 = Switch<t_SWITCH_SELECTOR, t_T0, t_T1, t_T2, t_T3>;