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Section: 22.4.5 [tuple.creation] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-88
The tuple_cat template consists of four overloads and that can concatenate only two tuples. A single variadic signature that can concatenate an arbitrary number of tuples would be preferred.
[ Resolution proposed by ballot comment: ]
Adopt a simplified form of the proposal in n2975, restricted to tuples and neither requiring nor outlawing support for other tuple-like types.
[ 2010 Rapperswil: Alisdair to provide wording. ]
[ 2010-11-06: Daniel comments and proposes some alternative wording: ]
There are some problems in the wording: First, even though the result type tuple<see below> implies it, the specification of the contained tuple element types is missing. Second, the term "tuple protocol" is not defined anywhere and I see no reason why this normative wording should not be a non-normative note. We could at least give a better approximation, maybe "tuple-like protocol" as indicated from header <utility> synopsis. Further, it seems to me that the effects need to contain a combination of std::forward with the call of get. Finally I suggest to replace the requirements Move/CopyConstructible by proper usage of is_constructible, as indicated by n3140.
[ 2010 Batavia ]
Moved to Ready with Daniel's improved wording.
Proposed resolution:
Note: This alternate proposed resolution works only if 1191 has been accepted.
namespace std {
...
// 20.4.2.4, tuple creation functions:
const unspecified ignore;
template <class... Types>
tuple<VTypes...> make_tuple(Types&&...);
template <class... Types>
tuple<ATypes...> forward_as_tuple(Types&&...);
template<class... Types>
tuple<Types&...> tie(Types&...);
template <class... TTypes, class... UTypes>
tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>&, const tuple<UTypes...>&);
template <class... TTypes, class... UTypes>
tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&&, const tuple<UTypes...>&);
template <class... TTypes, class... UTypes>
tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>&, tuple<UTypes...>&&);
template <class... TTypes, class... UTypes>
tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&&, tuple<UTypes...>&&);
template <class... Tuples>
tuple<CTypes...> tuple_cat(Tuples&&...);
...
template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>& t, const tuple<UTypes...>& u);
8 Requires: All the types in TTypes shall be CopyConstructible (Table 35). All the types in UTypes shall be CopyConstructible (Table 35).
9 Returns: A tuple object constructed by copy constructing its first sizeof...(TTypes) elements from the corresponding elements of t and copy constructing its last sizeof...(UTypes) elements from the corresponding elements of u.template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&& t, const tuple<UTypes...>& u);
10 Requires: All the types in TTypes shall be MoveConstructible (Table 34). All the types in UTypes shall be CopyConstructible (Table 35).
11 Returns: A tuple object constructed by move constructing its first sizeof...(TTypes) elements from the corresponding elements of t and copy constructing its last sizeof...(UTypes) elements from the corresponding elements of u.template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>& t, tuple<UTypes...>&& u);
12 Requires: All the types in TTypes shall be CopyConstructible (Table 35). All the types in UTypes shall be MoveConstructible (Table 34).
13 Returns: A tuple object constructed by copy constructing its first sizeof...(TTypes) elements from the corresponding elements of t and move constructing its last sizeof...(UTypes) elements from the corresponding elements of u.template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&& t, tuple<UTypes...>&& u);
14 Requires: All the types in TTypes shall be MoveConstructible (Table 34). All the types in UTypes shall be MoveConstructible (Table 34).
15 Returns: A tuple object constructed by move constructing its first sizeof...(TTypes) elements from the corresponding elements of t and move constructing its last sizeof...(UTypes) elements from the corresponding elements of u.template <class... Tuples> tuple<CTypes...> tuple_cat(Tuples&&... tpls);8 Let Ti be the ith type in Tuples, Ui be remove_reference<Ti>::type, and tpi be the ith parameter in the function parameter pack tpls, where all indexing is zero-based in the following paragraphs of this sub-clause [tuple.creation].
9 Requires: For all i, Ui shall be the type cvi tuple<Argsi...>, where cvi is the (possibly empty) ith cv-qualifier-seq, and Argsi is the parameter pack representing the element types in Ui. Let Aik be the kith type in Argsi, then for all Aik the following requirements shall be satisfied: If Ti is deduced as an lvalue reference type, then is_constructible<Aik, cvi Aik&>::value == true, otherwise is_constructible<Aik, cvi Aik&&>::value == true.
10 Remarks: The types in CTypes shall be equal to the ordered sequence of the expanded types Args0..., Args1..., Argsn-1..., where n equals sizeof...(Tuples). Let ei... be the ith ordered sequence of tuple elements of the result tuple object corresponding to the type sequence Argsi.
11 Returns: A tuple object constructed by initializing the kith type element eik in ei... with get<ki>(std::forward<Ti>(tpi)) for each valid ki and each element group ei in order.
12 [Note: An implementation may support additional types in the parameter pack Tuples, such as pair and array that support the tuple-like protocol. — end note]