Copy construction between Span of compatible type is allowed to fulfil the use
case Span<T> -> Span<const T>. This is achieved by a templated copy constructor
like constructor.
In p0122, the overload is discarded from the constructor set if the ElementType
of the Span in input is not convertible into the ElementType of the Span being
constructed.
To discard function overload, SFINAE has to be used which polutes the documentation
and make the code harder to read and maintain.
Unlike p0122, our Span class doesn't exposes (yet) functions with default argument
or functions that convert container in input into span the only overload with the
a single parameter that we exposes are:
- template<size_t N> Span(ElementType (&element)[N])
- Span(const Span& other): <- generated by the compiler.
For both of this functions we expect exact match and their resolution should not
interfere with the constructor that converts from another type of Span.
As a result it is possible to rely solely on C++ default resolution rules as we
won't hit cases were constructors convert from another type (std::array, std
container, span) and raise an error with a static assert if the element type
can't be converted.
If another copy - conversion - constructor is added then SFINAE has to be
reintroduced.
This commit aims to make Span implementation more in line with what is present in N4762:
- use appropiate index types where applicable.
- use typedefed type inside the class (index_type, reference, pointer, element_type)
- assertion where applicable
- restrict default construction to Span with extent == 0 or extent == dynamic.
- construct span from a range of pointer
- remove non const overload of the subscript operator
- remove non const overload of the data function
- implement subspan function
- implement missing first and last function of dynamic span
The Span class allows the creation of views over contiguous memory. The view
do not own memory, is typed and has a length. It can be used as a replacement of
the traditional pair of pointer and size in parameters or class fields.
Main operations:
- size(): return the lenght of the memory viewed
- empty(): return if the memory viewed is empty
- [index]: access elements viewed
- data(): return a pointer to the memory viewed.
- first(count): Create a subview from the first count elements.
- last(count): Create a subview from the last count elements.
- == and !=: compare two views or a view to array and return if they are equal or
not.
The Span class came in two flavors:
- Static size: The size is encoded in the Span type and it is as lightweitgh as
a single pointer,
- Dynamic size: The object can store arbitrary views and it costs one pointer
and the size of the view.
Vincent Coubard
Amanda Butler