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3.15 Typelist Quick Facts

  • Header file: Typelist.h.

  • All typelist utilities reside in namespace Loki::TL.

  • Class template Typelist<Head, Tail> is defined.

  • Typelist creation: Macros TYPELIST_1 to TYPELIST_50 are defined. They accept the number of parameters stated in their name.

  • The upper limit of macros (50) can be extended by the user. For instance:

    

#define TYPELIST_51(T1, repeat here up to T51) \


  Typelist<T1, TYPELIST_50(T2, repeat here up to T51) >

  • By convention, typelists are proper—they always have a simple type (nontypelist) as the first element (the head). The tail can only be a typelist or NullType.

  • The header defines a collection of primitives that operate on typelists. By convention, all the primitives return the result in a nested (inner) public type definition called Result. If the result of the primitive is a value, then its name is value.

  • The primitives are described in Table 3.1.

  • Synopsis of class template GenScatterHierarchy:

    

template <class TList, template <class> class Unit>


class GenScatterHierarchy;

  • GenScatterHierarchy generates a hierarchy that instantiates Unit with each type in the typelist TList. An instantiation of GenScatterHierarchy directly or indirectly inherits Unit<T> for each T in TList.

  • The structure of a hierarchy generated by GenScatterHierarchy is depicted in Figure 3.2.

  • Synopsis of class template GenLinearHierarchy:

    

template <class TList, template <class, class> class Unit>


class GenLinearHierarchy;

  • GenLinearHierarchy generates a linear hierarchy, depicted in Figure 3.6.

  • GenLinearHierarchy instantiates Unit by passing each type in the typelist TList as Unit's first template argument. Important: Unit must derive publicly from its second template parameter.

  • The overloaded Field functions provide by-type and by-index access to the nodes of the hierarchies.

  • Field<Type>(obj) returns a reference to the Unit instantiation that corresponds to the specified type Type.

  • Field<index>(obj) returns a reference to the Unit instantiation that corresponds to the type found in the typelist at the position specified by the integral constant index.

Table 3.1. Compile-Time Algorithms Operating on Typelists
Primitive Name Description
Length<TList> Computes the length of TList.
TypeAt<TList, idx> Returns the type at a given position (zero-based) in TList. If the index is greater than or equal to the length of TList, a compile-time error occurs.
TypeAtNonStrict<TList, idx> Returns the type at a given position (zero-based) in a typelist. If the index is greater than or equal to the length of TList, Null Type is returned.
IndexOf<TList, T> Returns the index of the first occurrence of type T in typelist TList. If the type is not found, the result is -1.
Append<TList, T> Appends a type or a typelist to TList.
Erase<TList, T> Erases the first occurrence, if any, of T in TList.
EraseAll<TList, T> Erases all occurrences, if any, of T in TList.
NoDuplicates<TList> Eliminates all the duplicates from TList.
Replace<TList, T, U> Replaces the first occurrence, if any, of T in TList with U.
ReplaceAll<TList, T, U> Replaces all occurrences, if any, of T in TList with U.
MostDerived<TList, T> Returns the most derived type from T in TList. If no such type is found, T is returned.
DerivedToFront<TList> Brings the most derived types to the front of TList.

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