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To erase a type from a tt(TypeList) by its index we again use a
recursive template meta program. tt(EraseIdx) expects a tt(size_t) index value
and a tt(TypeList) from which its tt(idx)sup(th) type must be
erased. tt(EraseIdx) defines the type tt(List) containing the resulting
tt(TypeList). Here is the algorithm:
itemization(
it() The foundation of the algorithm consists of a struct template
tt(EraseIdx) expecting the index of the type to erase and a tt(TypeList):
verbinsert(ERASEIDX)(advancedtemplates/examples/erase.h)
it() If the typelist is empty, there's nothing to erase, and an empty
tt(TypeList) results:
verbinsert(ERASEIDXEMPTY)(advancedtemplates/examples/erase.h)
it() The recursion otherwise ends once tt(idx) becomes 0. At that point
the tt(TypeList)'s first type is ignored and tt(Type) is initialized
to a tt(TypeList) containing the types in the orginal tt(TypeList)'s
tail:
verbinsert(ERASEIDXZERO)(advancedtemplates/examples/erase.h)
it() In all other cases tt(EraseIdx) is applied to the tt(TypeList)'s
tail, providing it with a decremented value of tt(idx). To the
resulting tt(TypeList) the orginal tt(TypeList)'s head is
prefixed. The tt(TypeList) returned by the prefix operation is then
returned as tt(EraseIdx::Type):
verbinsert(ERASEIDXNEXT)(advancedtemplates/examples/erase.h)
)
Here is a statement showing how tt(EraseIdx) can be used:
verbinsert(ERASEIDX)(advancedtemplates/examples/erase.cc)
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