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huixie90 committed Jun 9, 2023
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90 changes: 64 additions & 26 deletions concat.md
Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,7 @@ toc: true

- Fixed conversions of `difference` types
- Redesign `iter_swap`
- Relax `random_access_range` constraints
- Various wording fixes

## R2
Expand Down Expand Up @@ -351,8 +352,11 @@ went with the second constraint. In this paper, both are supported.

## Random Access Range

`concat_view` can be `random_access_range` if all the underlying ranges model
`random_access_range` and `sized_range`.
`concat_view` can model `random_access_range` if the underlying ranges satisfy
the following conditions:

- Every underlying range models `random_access_range`
- All except the last range model `sized_range`

## Sized Range

Expand Down Expand Up @@ -632,7 +636,7 @@ concept @_concatable_@ = requires { // exposition only
```cpp
template <bool Const, class... Rs>
concept @_concat-is-bidirectional_@ = @*see below*@; // exposition only
concept @_concat-is-random-access_@ = @*see below*@; // exposition only
```

:::bq
Expand All @@ -643,9 +647,18 @@ concept @_concat-is-bidirectional_@ = @*see below*@; // exposition only
template <bool Const, class... Rs>
concept @_concat-is-random-access_@ = // exposition only
(@*all-random-access*@<Const, Rs> && ...) &&
(sized_range<@*maybe-const*@<Const, Fs>> && ...)
(sized_range<@*maybe-const*@<Const, Fs>> && ...);
```
:::
```cpp
template <bool Const, class... Rs>
concept @_concat-is-bidirectional_@ = @*see below*@; // exposition only
```

:::bq

[4]{.pnum} Let `V` be the last element of `Rs`, and `Fs` be the pack that consists of all elements of `Rs` except `V`, then `@_concat-is-bidirectional_@` is equivalent to:

```cpp
Expand Down Expand Up @@ -834,10 +847,10 @@ namespace std::ranges{
requires @_concat-is-random-access_@<Const, Views...>;

friend constexpr difference_type operator-(const @_iterator_@& x, default_sentinel_t)
requires @_concat-is-random-access_@<Const, Views...>;
requires @*see below*@;

friend constexpr difference_type operator-(default_sentinel_t, const @_iterator_@& x)
requires @_concat-is-random-access_@<Const, Views...>;
requires @*see below*@;

friend constexpr decltype(auto) iter_move(const iterator& it) noexcept(@*see below*@);

Expand Down Expand Up @@ -1318,7 +1331,7 @@ denote `y.@*it_*@.index()`

```cpp
friend constexpr difference_type operator-(const @_iterator_@& x, default_sentinel_t)
requires @_concat-is-random-access_@<Const, Views...>;
requires @*see below*@;
```

:::bq
Expand All @@ -1336,21 +1349,33 @@ otherwise, equivalent to
return -(static_cast<difference_type>(@*d~x~*@) + static_cast<difference_type>(s));
```

[37]{.pnum} *Remarks*: Let `V` be the last element of Pack `Views`, the expression in the requires-clause is equivalent to:

```cpp
(@_concat-is-random-access_@<Const, Views...> && sized_range<@*maybe-const*@<Const, V>>)
```

:::

```cpp
friend constexpr difference_type operator-(default_sentinel_t, const @_iterator_@& x)
requires @_concat-is-random-access_@<Const, Views...>;
requires @*see below*@;
```

:::bq

[37]{.pnum} *Effects*: Equivalent to:
[38]{.pnum} *Effects*: Equivalent to:

```cpp
return -(x - default_sentinel);
```

[39]{.pnum} *Remarks*: Let `V` be the last element of Pack `Views`, the expression in the requires-clause is equivalent to:

```cpp
(@_concat-is-random-access_@<Const, Views...> && sized_range<@*maybe-const*@<Const, V>>)
```

:::

```cpp
Expand All @@ -1359,9 +1384,9 @@ friend constexpr decltype(auto) iter_move(const iterator& it) noexcept(@*see bel
:::bq
[38]{.pnum} *Preconditions*: `it.@*it_*@.valueless_by_exception()` is `false`.
[40]{.pnum} *Preconditions*: `it.@*it_*@.valueless_by_exception()` is `false`.
[39]{.pnum} *Effects*: Equivalent to:
[41]{.pnum} *Effects*: Equivalent to:
```cpp
return std::visit(
Expand All @@ -1372,7 +1397,7 @@ return std::visit(
it.@*it_*@);
```

[40]{.pnum} *Remarks*: The exception specification is equivalent to:
[42]{.pnum} *Remarks*: The exception specification is equivalent to:

```cpp
((is_nothrow_invocable_v<decltype(ranges::iter_move),
Expand All @@ -1391,29 +1416,42 @@ friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(@
:::bq
[41]{.pnum} *Preconditions*: `x.@*it_*@.valueless_by_exception()` and
[43]{.pnum} *Preconditions*: `x.@*it_*@.valueless_by_exception()` and
`y.@*it_*@.valueless_by_exception()` are each `false`.
[42]{.pnum} *Effects*: Equivalent to:
[44]{.pnum} *Effects*: Equivalent to:
```cpp
std::visit(ranges::iter_swap, x.@*it_*@, y.@*it_*@);
std::visit(
[&](const auto& it1, const auto& it2) {
if constexpr (is_same_v<decltype(it1), decltype(it2)>) {
ranges::iter_swap(it1, it2);
} else {
ranges::swap(*x, *y);
}
},
x.@*it_*@, y.@*it_*@);
```

[43]{.pnum} *Remarks*: The exception specification is `true` if and
only if: For every combination of two types `X` and `Y` in the set of all types
in the parameter pack `iterator_t<@_maybe-const_@<Const, Views>>>...`,
`is_nothrow_invocable_v<decltype(ranges::iter_swap), const X&, const Y&>` is true.
[45]{.pnum} *Remarks*: Let `N` be the logcial `AND` of the following expressions:

[44]{.pnum} *Remarks*: The expression in the requires-clause is `true` if and
only if: For every combination of two types `X` and `Y` in the set of all types
in the parameter pack `iterator_t<@_maybe-const_@<Const, Views>>>...`,
`indirectly_swappable<X, Y>` is modelled.
```cpp
noexcept(ranges::iter_swap(std::get<@*i*@>(x.@*it_*@), std::get<@*i*@>(y.@*it_*@)))
```
requires(indirectly_swappable<iterator_t<__maybe_const<Const, Views>>> && ... &&
swappable_with<xo::concat_reference_t<__maybe_const<Const, Views>...>,
xo::concat_reference_t<__maybe_const<Const, Views>...>>)
for every integer 0 <= @*i*@ < `sizeof...(Views)`, the exception specification is equavalent to
```cpp
noexcept(ranges::swap(*x, *y)) && N
```

[46]{.pnum} *Remarks*: The expression in the requires-clause is equivalent to

```cpp
(indirectly_swappable<iterator_t<@*maybe-const*@<Const, Views>>> && ... &&
swappable_with<@*concat-reference-t*@<@*maybe-const*@<Const, Views>...>,
@*concat-reference-t*@<@*maybe-const*@<Const, Views>...>>)
```

:::

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