Add slides on value categories

talk/morelanguage/move.tex

@@ -108,14 +108,146 @@
 \end{frame}
 
 \begin{frame}[fragile]
-  \frametitlecpp[11]{Move semantics}
-  \begin{block}{The idea}
+  \frametitlecpp[11]{Value categories}
+  \begin{block}{Lvalue (left value / locator value)}
+    \begin{itemize}
+      \item An expression is an lvalue
+      \begin{itemize}
+        \item If its evaluation determines identity of object/function
+        \item If the object is not expiring (``xvalue'')
+      \end{itemize}
+      \item Lvalues have a persistent address in memory
+      \item Lvalues can be assigned to unless \cppinline{const}
+    \end{itemize}
+  \end{block}
+  \begin{exampleblock}{Lvalue examples}
+    \begin{cppcode*}{}
+      int i;
+      int & j = i;
+      T t;
+      t.m = 5; // lvalue, because t is lvalue
+      ++i;     // returns underlying object
+    \end{cppcode*}
+  \end{exampleblock}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitlecpp[11]{Value Categories}
+  \begin{block}{Rvalue}
     \begin{itemize}
-      \item a new type of reference: rvalue reference
+      \item An expression is an rvalue when from these primary categories
       \begin{itemize}
-      \item used for move semantic
-      \item denoted by \cppinline{&&}
+        \item ``pure rvalue'': Result of built-in operator; initalizer of object
+        \item ``xvalue'': Expiring value, resources can ``moved''
       \end{itemize}
+      \item Has no persistent address in memory
+      \begin{itemize}
+        \item Cannot use the addressof operator
+      \end{itemize}
+      \item Cannot be used as left-hand operand of built-in assignments
+
+    \end{itemize}
+  \end{block}
+  \vspace{-6mm}
+  \begin{overprint}
+    \onslide<1>
+    \begin{exampleblock}{Examples for rvalues}
+      \begin{cppcode*}{}
+        i = 42;    // 42 is pure rv
+        i = a + b; // a+b is pure rv
+        T t = T{}; // T{} is pure rv
+        i++;       // Pure rv (post-increment returns temp.)
+        T{}.m;     // m is expiring because T{} is pure rv
+        std::move(x); // Converts lvalue into xvalue
+      \end{cppcode*}
+    \end{exampleblock}
+    \onslide<2>
+    \begin{alertblock}{Things that don't compile with rvalues}
+      \begin{cppcode*}{}
+        a + b = 2; // Cannot assign to pure rv
+        &(a+b);    // Cannot take address of pure rv
+        &i++;      // Same, post-increment returns pure rv
+        T{}.m = 2; // cannot assign if m is built-in type
+      \end{cppcode*}
+    \end{alertblock}
+  \end{overprint}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitlecpp[11]{Value Categories: Terminology}
+  \begin{block}{Source of confusion}
+    \begin{itemize}
+      \item The name ``rvalue'' originates from them \textit{mostly} being on right-hand side
+      \item But it denotes value categories of \emph{expressions}
+      \item Rvalues can be assigned to for non-builtin types
+    \end{itemize}
+  \end{block}
+
+  \begin{exampleblock}{Example: rvalue on left-hand side}
+    \begin{cppcode*}{}
+      struct T{
+        int m;
+        T& operator=(int i);
+      };
+      std::cout << (T{} = 5).m << "\n";
+    \end{cppcode*}
+  \end{exampleblock}
+
+  \begin{alertblock}{Not possible with built-in types}
+    \begin{cppcode*}{}
+      std::cout << (int{} = 5) << "\n"; // Error
+    \end{cppcode*}
+  \end{alertblock}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitlecpp[11]{Rvalue References}
+  \begin{block}{New reference type: rvalue reference}
+    \begin{itemize}
+      \item Declared with \cppinline{T&&}
+      \item Does not bind to lvalues
+    \end{itemize}
+  \end{block}
+
+  \begin{exampleblock}{Binding rules}
+    \begin{columns}[onlytextwidth]
+      \begin{column}{0.45\textwidth}
+        Lvalues
+        \begin{cppcode*}{linenos=false}
+          T lv;
+          T &     ref = lv;//OK
+          T &&  rvref = lv;//Error
+          T const& cr = lv;//OK
+        \end{cppcode*}
+      \end{column}
+      \hfil
+      \begin{column}{0.5\textwidth}
+        Rvalues
+        \begin{cppcode*}{linenos=false}
+
+          T &     ref = T{};//Error
+          T &&  rvref = T{};//OK
+          T const& cr = T{};//OK
+        \end{cppcode*}
+      \end{column}
+    \end{columns}
+  \end{exampleblock}
+
+  \begin{exampleblock}{Overload resolution prefers rvalue reference}
+    \begin{cppcode*}{}
+      void f(T const &);
+      void f(T &&);      // Selected, because one could move from T
+      f(T{});
+    \end{cppcode*}
+  \end{exampleblock}
+\end{frame}
+
+\begin{frame}[fragile]
+  \frametitlecpp[11]{Move semantics}
+  \begin{block}{The idea}
+    \begin{itemize}
+      \item use rvalue references to reuse resources
+      \item binds only to prvalues and xvalues
       \item 2 new special member functions in every class:
       \begin{description}
       \item[a move constructor] similar to copy constructor
@@ -145,14 +277,14 @@
       \end{itemize}
     \item if no move semantic is implemented, copies will be performed
     \item the language and STL understand move semantic
-    \item the compiler moves whenever possible
+    \item the compiler uses copy elision or moves whenever possible
       \begin{itemize}
       \item e.g.\ when passing temporaries or returning from a function
       \end{itemize}
     \end{itemize}
   \end{block}
   \pause
-  \begin{exampleblock}{Practically}
+  \begin{exampleblock}{Practically (in \cpp11)}
     \begin{cppcode*}{}
       T f() { T r; return r; } // move r out of f
       T v = f(); // move returned (temporary) T into v
@@ -214,7 +346,7 @@
       \item 1 swap less, separate copy assignment operator needed
       \item former content of \cppinline{*this} destroyed with caller argument
       \end{itemize}
-    \item swap, move constructor/assignment must be \cppinline{noexcept}
+    \item swap, move constructor/assignment must be \cppinline{noexcept}!
     \end{itemize}
   \end{block}
 \end{frame}
@@ -267,6 +399,33 @@
   \end{exampleblock}
 \end{frame}
 
+\begin{frame}[fragile,t]
+  \frametitlecpp[11]{Move semantics: Don't forget noexcept!}
+  \begin{alertblock}{Vector pessimisation when move constructor can throw}
+    \begin{itemize}
+      \item When a vector reallocates, it must copy/move all elements
+        \begin{itemize}
+          \item An exception during move can abort the relocation of elements
+          \item Some elements might have been moved, others in original place
+          \item Cannot recover from this state
+          \item Therefore, must copy all elements and destroy originals!
+        \end{itemize}
+      \item With \cppinline{noexcept}, the much faster move is used
+    \end{itemize}
+  \end{alertblock}
+  \begin{exampleblock}{}
+    \small
+    \begin{cppcode*}{}
+      struct Movable1 { Movable1(Movable1 &&other); };
+      struct Movable2 { Movable2(Movable2 &&other) noexcept; };
+      while (vector1.size() < 10000) {
+        vector1.push_back(Movable1{}); // Copies
+        vector2.push_back(Movable2{}); // Moves
+      }
+    \end{cppcode*}
+  \end{exampleblock}
+\end{frame}
+
 \begin{frame}[fragile]
   \frametitlecpp[11]{Move Semantic}
   \begin{exercise}{Move semantics}