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The tt(Multi) class template defines tt(PlainTypes) as the tt(TypeList)
holding all the types of its parameter pack. Each tt(MultiBase) derived from a
tt(UWrap) type also defines a type tt(Type) representing the policy type that
was used to define the tt(UWrap) type and a type tt(Base) representing the
type of its nested tt(MultiBase) class.
These three type definitions allow us to access the types from which the
tt(Multi) object was created as well as the values of those types.
The class template tt(typeAt), is a pure template meta program class template
(it has no run-time executable code). It expects a tt(size_t idx) template
argument specifying the index of the policy type in a tt(Multi) type object as
well as a tt(Multi) class type. It defines the type tt(Type) as the tt(Type)
defined by tt(Multi)'s tt(MultiBase<idx, ...>) base class. Example:
verb(
typeAt<0, Multi<Vector, int, double>>::Type // Type is vector<int>
)
The class template tt(typeAt) defines (and uses) a nested class template
tt(PolType) doing all the work. tt(PolType)'s generic definition specifies two
template parameters: an index used to specify the index of the requested type
and a typename initialized by a tt(MultiBase) type argument. tt(PolType)'s
recursive definition recursively reduces its index non-type parameter,
passing the next base class in tt(MultiBase)'s inheritance tree to the
recursive call. As tt(PolType) eventually defines the type tt(Type) to be the
requested policy type the recursive definition defines its tt(Type) as the
type defined by the recursive call. The final (non-recursive) specialization
defines the initial policy type of the tt(MultiBase) type as tt(Type). Here is
tt(typeAt)'s definition:
verbinclude(//TYPEAT examples/multi.h)
The types specified by tt(Multi)'s parameter pack can also be retrieved using
a second helper class template: tt(plainTypeAt). Example:
verb(
plainTypeAt<0, Multi<Vector, int, double>>::Type // Type is int
)
The class template tt(plainTypeAt) uses a comparable (but simpler)
implementation than tt(typeAt). It is also a pure template meta program class
template defining a nested class template tt(At). tt(At) is implemented like
tt(typeAt) but it visits the types of the original template pack that was
passed to tt(Multi), and made available by tt(Multi) as its tt(PlainTypes)
type. Here is tt(plainTypeAt)'s definition:
verbinclude(//PLAINAT examples/multi.h)
Arguably the neatest support template is tt(get). This is a function template
defining tt(size_t idx) as its first template parameter and tt(typename Multi)
as its second template parameter. The function template tt(get) defines one
function parameter: a reference to a tt(Multi), so it can deduct tt(Multi)'s
type by itself. Knowing that it's a tt(Multi), we reason that it is also a
tt(UWrap<nr, PolicyType>) and therefore also a tt(PolicyType), as the latter
class is defined as a base class of tt(UWrap).
Since class type objects can initialize references to their base classes the
tt(PolicyType &) can be initialized by an appropriate tt(UWrap) reference,
which in turn can be initialized by a tt(Multi) object. Since we can
determine tt(PolicyType) using tt(TypeAt) (note that evaluating tt(typename
typeAt<idx, Multi>::Type) is a purely compile-time matter), the tt(get)
function can very well be implemented em(inline) by a single tt(return)
statement:
verbinclude(//GET examples/multi.h)
The intermediate tt(UWrap) cast is required to disambiguate between
identical policy types (like two tt(vector<int>) types). As tt(UWrap) is
uniquely determined by its tt(nr) template argument and this is the number
argument that is passed to tt(get) ambiguities can easily be prevented.
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