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[API BREAKING] Remove conversion from weekday to unsigned

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* There has been a great deal of anguish over the encoding of
  weekdays:  whether [0, 6] maps to [Sunday, Saturday] or
  [1, 7] maps to [Monday, Sunday].  This commit attempts
  to address that issue, but will break a small amount of
  code at compile-time.  See below on how to fix that.

* The weekday constructor used to accept [0, 6] to represent
  [Sunday, Saturday].  It now accepts [0, 7] to represent
  [Sunday, Saturday] with both 0 and 7 mapping to Sunday.

* The conversion from weekday to unsigned has been removed.

* To convert a weekday to unsigned replace:

      auto u = unsigned{wd};

  with:

      auto u = (wd - Sunday).count();

  This maps [Sunday, Saturday] to [0, 6], which is the
  C/POSIX mapping.  If you prefer the ISO mapping
  ([Monday, Sunday] -> [1, 7]), then do:

      auto u = (wd - Monday).count() + 1;
This commit is contained in:
Howard Hinnant 2018-06-02 22:55:35 -04:00
parent dc217667ef
commit 4358ea7b6e
1 changed files with 19 additions and 24 deletions
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date.html
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@ -3220,7 +3220,6 @@ public:
constexpr weekday& operator+=(const days& d) noexcept;
constexpr weekday& operator-=(const days& d) noexcept;
constexpr explicit operator unsigned() const noexcept;
constexpr bool ok() const noexcept;
constexpr weekday_indexed operator[](unsigned index) const noexcept;
constexpr weekday_last operator[](last_spec) const noexcept;
@ -3249,27 +3248,27 @@ from_stream(std::basic_istream<CharT, Traits>& is, const CharT* fmt, w
std::basic_string<CharT, Traits, Alloc>* abbrev = nullptr,
std::chrono::minutes* offset = nullptr);
constexpr weekday Sunday{0};
constexpr weekday Monday{1};
constexpr weekday Tuesday{2};
constexpr weekday Wednesday{3};
constexpr weekday Thursday{4};
constexpr weekday Friday{5};
constexpr weekday Saturday{6};
constexpr weekday Sunday{7};
</pre>
<p><b>Overview</b></p>
<p>
<code>weekday</code> represents a day of the week in the Gregorian calendar. It should
only be representing values in the range 0 to 6, corresponding to Sunday thru Saturday.
However it may hold values outside this range. It can be constructed with any
<code>weekday</code> represents a day of the week in the Gregorian calendar.
It can be constructed with any
<code>unsigned</code> value, which will be subsequently truncated to fit into
<code>weekday</code>'s internal storage. <code>weekday</code> is equality comparable.
<code>weekday</code>'s internal storage. The values [1, 6] map to Monday thru Saturday.
Both 0 and 7 map to Sunday. Other values in the range [8, 255] will be stored, and will
represent values of <code>weekday</code> that are <code>!ok()</code>.
<code>weekday</code> is equality comparable.
<code>weekday</code> is not less-than comparable because there is no universal consensus
on which day is the first day of the week. This design chooses the encoding of 0 to 6 to
represent Sunday thru Saturday only because this is consistent with existing C and C++
practice. However <code>weekday</code>'s comparison and arithmetic operations treat the
on which day is the first day of the week. <code>weekday</code>'s comparison and arithmetic operations treat the
days of the week as a circular range, with no beginning and no end. One can stream out a
<code>weekday</code> for debugging purposes. <code>weekday</code> has explicit conversions
to and from <code>unsigned</code>. There are 7 <code>weekday</code> constants, one for each
@ -3301,9 +3300,14 @@ explicit constexpr weekday::weekday(unsigned wd) noexcept;
<blockquote>
<p>
<i>Effects:</i> Constructs an object of type <code>weekday</code> by constructing
<code>wd_</code> with <code>wd</code>. The value held is unspecified if <code>wd</code>
is not in the range <code>[0, 255]</code>.
<i>Effects:</i> Constructs an object of type <code>weekday</code>
which represents a day of the week according to the ISO 8601 mapping
of integers to days of the week. Additionally, if <code>wd ==
0</code>, Sunday is represented. [<i>Note:</i> Sunday can be
constructed from both 0 and 7 &mdash; <i>end note</i>]. If
<code>wd</code> is in the range [8, 255], <code>wd_</code> is
initialized with wd. If wd is not in the range [0, 255], the value
held is unspecified.
</p>
</blockquote>
@ -3421,23 +3425,14 @@ constexpr weekday&amp; weekday::operator-=(const days&amp; d) noexcept;
</p>
</blockquote>
<pre>
constexpr explicit weekday::operator unsigned() const noexcept;
</pre>
<blockquote>
<p>
<i>Returns:</i> <code>wd_</code>.
</p>
</blockquote>
<pre>
constexpr bool weekday::ok() const noexcept;
</pre>
<blockquote>
<p>
<i>Returns:</i> <code>wd_ <= 6</code>.
<i>Returns:</i> <code>true</code> if <code>*this</code> holds a value in the range
Monday thru Sunday, else returns <code>false</code>.
</p>
</blockquote>
@ -3467,7 +3462,7 @@ constexpr bool operator==(const weekday&amp; x, const weekday&amp; y) noexcept;
<blockquote>
<p>
<i>Returns:</i> <code>unsigned{x} == unsigned{y}</code>.
<i>Returns:</i> <code>x.wd_ == y.wd_</code>.
</p>
</blockquote>