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### 1042. Provide `ContiguousStorage` concept and apply it to corresponding containers

Section: 24.3 [sequences] Status: NAD Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 244 [CD1]

The validity of the expression `&a[n] == &a[0] + n` is contingent on `operator&` doing the "right thing" (as captured by the `CopyConstructible` requirements in table 30 in C++2003). However this constraint has been lost in the Concepts of C++0x. This applies to `vector` and `array` (it actually applies to `string` also, but that's a different chapter, so I'll file a separate comment there and cross-reference).

Suggested solution:

Define a `ContiguousStorage` and apply it to `vector`, `array` and `string`.

[ Summit: ]

> Agree with the issue but not the details of the proposed solution. Walter to provide wording for the new concept.

[ Post Summit Alisdair adds: ]

> Another LWG subgroup wondered if this concept should extend to `complex<T>`, and so not be built on the container concept at all?

[ 2009-07 post-Frankfurt: ]

> Leave Open, pending a post-Concepts Working Draft.

[ 2009-10 Santa Cruz: ]

> Mark issue 1042 as NAD, in rationale state that this was solved by removal of concepts.

Proposed resolution:

Add to `<container_concepts>` synopsis in [container.concepts]

>     concept< typename C > ContiguousStorageContainer see below;

Add a new section to the end of [container.concepts]

> 23.1.6.x ContiguousStorageContainer concept [container.concepts.contiguous]

>     concept ContiguousStorageContainer< typename C >
>       : Container<C>
>     {
>       value_type* data(C&);

>       axiom Contiguity(C& c, size_type i) {
>         if( i < size(c) ) {
>              addressof( * (data(c) + i) )
>           == addressof( * advance(data(c), i) );
>         }
>       }
>     }

> The `ContiguousStorageContainer` concept describes a container whose elements are allocated in a single region of memory, and are stored sequentially without intervening padding other than to meet alignment requirements. For example, the elements may be stored in a single array of suitable length.

>     value_type * data( C& );

> > Returns: a pointer to the first element in the region of storage. Result is unspecified for an empty container.

Change 24.3.7 [array] p1:

> -1- The header `<array>` defines a class template for storing fixed-size sequences of objects. An `array` supports random access iterators. An instance of `array<T, N>` stores `N` elements of type `T`, so that `size() == N` is an invariant. The elements of an `array` are stored contiguously, meaning that if `a` is an `array<T, N>` then it obeys the identity `&a[n] == &a[0] + n` for all `0 <= n < N` satisfies the concept `ContiguousStorageContainer< array<T, N>>`.

Add to the synopsis in 24.3.7 [array]:

>         ...
>         T * data();
>         const T * data() const;
>       };

>       template< typename T, size_t N >
>         concept_map ContiguousStorageContainer< array<T, N>> {};
>     }

Change 24.3.11 [vector] p1:

> A `vector` is a sequence container that supports random access iterators. In addition, it supports (amortized) constant time insert and erase operations at the end; insert and erase in the middle take linear time. Storage management is handled automatically, though hints can be given to improve efficiency. The elements of a vector are stored contiguously, meaning that if `v` is a `vector<T, Alloc>` (where `T` is some type other than `bool`), then it obeys the identity `&v[n] == &v[0] + n` for all `0 <= n < v.size()` satisfies the concept `ContiguousStorageContainer< vector< T, Alloc>>`.

Add at the end of the synopsis in 24.3.11 [vector] p2:

>     template< typename T, typename A >
>       requires !SameType< T, bool >
>       concept_map ContiguousStorageContainer< vector<T, A>> {};

Rationale:

Solved by removal of concepts.