Merge pull request #19 from steve-downey/origin/PeterSommerlad-patch-4

Origin/peter sommerlad patch 4

presentation/optional_ref.md

@@ -0,0 +1,182 @@
+# `std::optional<T&>`
+
+## we want it
+
+* for a large number of people as "we"
+  * including me(Peter): [P2070](https://wg21.link/P2070)
+  * JeanHeyd Meneide: [P1175](https://wg21.link/P1175)[P1683](https://wg21.link/P1683)
+    * [SanDiego1175](https://wiki.edg.com/bin/view/Wg21sandiego2018/P1175)
+  * plus numerous supporters on the mailing list giving a +1 or +100
+
+* any non-standard C++ optional implementation supports lvalue references
+
+> please standardize existing practice
+
+## Why it isn't in the standard yet
+
+> Removed, because couldn't agree on assignment
+
+* Hawaiian sun in Kona?
+* Frozen brains in meeting room?
+* Design by committee?
+* FUD?
+
+
+
+see also https://thephd.dev/to-bind-and-loose-a-reference-optional
+
+# What I (Peter) teach
+
+> Get rid of plain pointers
+
+* eliminates chances of pointer arithmetic
+* eliminates confusion with plain array
+* eliminates confusion about references
+
+> requires an optional for references
+
+> IMO, the Core Guidelines are wrong in their use of "single object pointer" meaning an optional reference.
+
+## Pointer replacement overview
+
+<table style="font-size:60%">
+<colgroup>
+<col span="3" style="font-size:80%;width: 10%;">
+<col span="1" style=";">
+<col span="1" style="font-size:60%;width: 35%">
+</colgroup>
+
+<tbody>
+<tr class="fragment">
+<td rowspan="4" style="vertical-align:middle;">
+owning `T`{.cpp}
+</td>
+<td rowspan="2"  style="vertical-align:middle;">non-null</td>
+<td style="background-color: #73FA79A0">value</td>
+<td style="background-color: #73FA79A0">
+`T`{.cpp}
+</td>
+<td style="font-size:80%; background-color: #73FA79A0">most safe and useful</td>
+</tr>
+<tr class="fragment">
+<td style="vertical-align:middle;">heap</td>
+<td  style="vertical-align:middle;">
+`unique_ptr<T> const `{.cpp style="font-size:80%"}
+</td>
+<td style="font-size:80%">
+
+must be init with<br/>`make_unique<T>`{.cpp}
+
+</td>
+</tr>
+<tr class="fragment">
+<td rowspan="2" style="vertical-align:middle; background-color: lightpink;">
+nullable
+<p style="size:200%;">🦸‍♀️</p>
+</td>
+<td style="background-color: #EAFDC7A0; vertical-align:middle;">value</td>
+<td style="font-size:80%; background-color: #EAFDC7A0; vertical-align:middle;">
+`optional<T>`{.cpp}
+</td>
+<td style="font-size:80%; background-color: #EAFDC7A0">
+to denote missing value<br/>
+best for return values 
+</td>
+</tr>
+<tr class="fragment">
+<td style="vertical-align:middle;">heap</td>
+<td style="vertical-align:middle;">
+`unique_ptr<T> const `{.cpp style="font-size:80%"}
+</td>
+<td style="font-size:80%">
+`T`{.cpp} can be base class<br/>with `make_uniqe<Derived>`{.cpp}
+</td>
+</tr>
+<tr class="fragment">
+<td rowspan="4"  style="vertical-align:middle; background-color: lightpink;">
+
+referring `T`{.cpp}
+<p style="size:200%">🦸‍♀️</p>
+</td>
+<td rowspan="2" style="vertical-align:middle;">non-null</td>
+<td  style="background-color: #EAFDC7A0">fixed</td>
+<td  style="background-color: #EAFDC7A0">
+`T &`{.cpp}
+</td>
+<td style="font-size:100%; background-color: #EAFDC7A0; color:red">
+can dangle
+</td>
+</tr>
+<tr class="fragment">
+<td>rebind</td>
+<td>
+`reference_wrapper<T>`{.cpp}
+</td>
+<td style="font-size:70%">
+assignability with a reference member
+</td>
+</tr>
+<tr class="fragment">
+<td rowspan="2" style="vertical-align:middle; background-color: lightpink;">
+nullable
+<p style="size:200%">🦸‍♀️</p>
+</td>
+<td  style="background-color: #F9DE96A0; vertical-align:middle;">fixed</td>
+<td  style="background-color: #F9DE96A0; font-size: 70%; vertical-align:middle;">
+
+`jss::object_ptr<T> const`{.cpp}<br/>
+`boost::optional<T&> const`{.cpp}
+
+</td>
+<td rowspan="2" style="vertical-align:middle;font-size:70%;background-color: #F9DE96A0">
+
+missing in std<br/>
+`std::optional`{.cpp} can not do this<br/>
+`boost::optional`{.cpp} can<br/>
+`object_ptr<T>`{.cpp} by A. Williams
+
+</td>
+</tr>
+<tr class="fragment">
+<td  style="background-color: #F9DE96A0; vertical-align:middle;">rebind</td>
+<td style="background-color: #F9DE96A0; font-size: 70%; vertical-align:middle;">
+`jss::object_ptr<T>`{.cpp}<br/>
+`boost::optional<T&>`{.cpp}<br/>
+`optional<reference_wrapper<T>>`{.cpp}
+</td>
+</tr>
+</tbody>
+</table>
+
+# Handling Assignment Controversy
+
+> cross-type assignment is overused
+
+* general remark by me, old days as an "optimization"
+* in the same realm as non-`explicit` constructors or conversion operators
+
+> assigning a reference to an `optional<T&>` rebinds.
+
+* no other semantics is actually reasonable. 
+
+## Why the mess wrt assignment ?
+
+:::{style="font-size:80%;text-align:left;"}
+
+N3406 says it at all:
+
+> These entities [optional refs] are surprising to many people because they do not appear to add any more functionality than pointers do.
+
+I say:
+
+> Pointers have a too wide interface (arithmetic!, array semantics, `void*` conversion), optional references remove unsafe operations and provide semantic meaning lacking in a pointer, where it is only given by convention, as in the Core Guidelines
+
+`optional<T&>` 
+
+* type says what it is
+* monadic API makes usage superior to `T*`
+
+:::
+
+# [P2988](https://wg21.link/P2988)
+