Runtime typeswitch for typelists as a switch instead of a nested if's?
-
23-09-2019 - |
Question
This is from TTL:
////////////////////////////////////////////////////////////
// run-time type switch
template <typename L, int N = 0, bool Stop=(N==length<L>::value) > struct type_switch;
template <typename L, int N, bool Stop>
struct type_switch
{
template< typename F >
void operator()( size_t i, F& f )
{
if( i == N )
{
f.operator()<typename impl::get<L,N>::type>();
}
else
{
type_switch<L, N+1> next;
next(i, f);
}
}
};
It's used for typeswitching on a TypeList. Question is -- they are doing this via a series of nested if's. Is there a way to do this type switch as a single select statement instead?
Thanks!
Solution
You'll need the preprocessor to generate a big switch
. You'll need get<>
to no-op out-of-bound lookups. Check the compiler output to be sure unused cases produce no output, if you care; adjust as necessary ;v) .
Check out the Boost Preprocessor Library if you care to get good at this sort of thing…
template <typename L>
struct type_switch
{
template< typename F >
void operator()( size_t i, F& f )
{
switch ( i ) {
#define CASE_N( N ) \
case (N): return f.operator()<typename impl::get<L,N>::type>();
CASE_N(0)
CASE_N(1)
CASE_N(2)
CASE_N(3) // ad nauseam.
}
};
OTHER TIPS
I don't think so.
This kind of template metaprogramming is normally done with recursion. Since it all happens at compile-time, I wouldn't be surprised if there's no runtime recursion or condition-checks.
You could always use a binary search instead of a linear search. It would be more complicated and more likely to have bugs in it (binary search is surprisingly easy to mess up).
You could also manually expand out N type_switch::operator()
, where N is some reasonable upper bound on the number of typelist lengths you will have in your program.