Merge 2018-11 LWG Motion 24

P1165R1 Make stateful allocator propagation more consistent for operator+(basic_string)

Fixes #2430

Author: zygoloid

source/strings.tex

@@ -3143,12 +3143,20 @@
   basic_string<charT, traits, Allocator>
     operator+(const basic_string<charT, traits, Allocator>& lhs,
               const basic_string<charT, traits, Allocator>& rhs);
+template<class charT, class traits, class Allocator>
+  basic_string<charT, traits, Allocator>
+    operator+(const basic_string<charT, traits, Allocator>& lhs, const charT* rhs);
 \end{itemdecl}
 
 \begin{itemdescr}
 \pnum
-\returns
-\tcode{std::move(basic_string<charT, traits, Allocator>(lhs).append(rhs))}.
+\effects
+Equivalent to:
+\begin{codeblock}
+basic_string<charT, traits, Allocator> r = lhs;
+r.append(rhs);
+return r;
+\end{codeblock}
 \end{itemdescr}
 
 \indexlibrarymember{operator+}{basic_string}%
@@ -3157,26 +3165,19 @@
   basic_string<charT, traits, Allocator>
     operator+(basic_string<charT, traits, Allocator>&& lhs,
               const basic_string<charT, traits, Allocator>& rhs);
-\end{itemdecl}
-
-\begin{itemdescr}
-\pnum
-\returns
-\tcode{std::move(lhs.append(rhs))}.
-\end{itemdescr}
-
-\indexlibrarymember{operator+}{basic_string}%
-\begin{itemdecl}
 template<class charT, class traits, class Allocator>
   basic_string<charT, traits, Allocator>
-    operator+(const basic_string<charT, traits, Allocator>& lhs,
-              basic_string<charT, traits, Allocator>&& rhs);
+    operator+(basic_string<charT, traits, Allocator>&& lhs, const charT* rhs);
 \end{itemdecl}
 
 \begin{itemdescr}
 \pnum
-\returns
-\tcode{std::move(rhs.insert(0, lhs))}.
+\effects
+Equivalent to:
+\begin{codeblock}
+lhs.append(rhs);
+return std::move(lhs);
+\end{codeblock}
 \end{itemdescr}
 
 \indexlibrarymember{operator+}{basic_string}%
@@ -3189,106 +3190,92 @@
 
 \begin{itemdescr}
 \pnum
-\returns
-\tcode{std::move(lhs.append(rhs))}.
+\effects
+Equivalent to:
+\begin{codeblock}
+lhs.append(rhs);
+return std::move(lhs);
+\end{codeblock}
+except that both \tcode{lhs} and \tcode{rhs}
+are left in valid but unspecified states.
+\begin{note}
+If \tcode{lhs} and \tcode{rhs} have equal allocators,
+the implementation may move from either.
+\end{note}
 \end{itemdescr}
 
 \indexlibrarymember{operator+}{basic_string}%
 \begin{itemdecl}
 template<class charT, class traits, class Allocator>
   basic_string<charT, traits, Allocator>
-    operator+(const charT* lhs, const basic_string<charT, traits, Allocator>& rhs);
-\end{itemdecl}
-
-\begin{itemdescr}
-\pnum
-\returns
-\tcode{basic_string<charT, traits, Allocator>(lhs) + rhs}.
-
-\pnum
-\remarks
-Uses
-\tcode{traits::length()}.
-\end{itemdescr}
-
-\indexlibrarymember{operator+}{basic_string}%
-\begin{itemdecl}
+    operator+(const basic_string<charT, traits, Allocator>& lhs,
+              basic_string<charT, traits, Allocator>&& rhs);
 template<class charT, class traits, class Allocator>
   basic_string<charT, traits, Allocator>
     operator+(const charT* lhs, basic_string<charT, traits, Allocator>&& rhs);
 \end{itemdecl}
 
 \begin{itemdescr}
 \pnum
-\returns
-\tcode{std::move(rhs.insert(0, lhs))}.
-
-\pnum
-\remarks
-Uses
-\tcode{traits::length()}.
-\end{itemdescr}
-
-\indexlibrarymember{operator+}{basic_string}%
-\begin{itemdecl}
-template<class charT, class traits, class Allocator>
-  basic_string<charT, traits, Allocator>
-    operator+(charT lhs, const basic_string<charT, traits, Allocator>& rhs);
-\end{itemdecl}
-
-\begin{itemdescr}
-\pnum
-\returns
-\tcode{basic_string<charT, traits, Allocator>(1, lhs) + rhs}.
+\effects
+Equivalent to:
+\begin{codeblock}
+rhs.insert(0, lhs);
+return std::move(rhs);
+\end{codeblock}
 \end{itemdescr}
 
 \indexlibrarymember{operator+}{basic_string}%
 \begin{itemdecl}
 template<class charT, class traits, class Allocator>
   basic_string<charT, traits, Allocator>
-    operator+(charT lhs, basic_string<charT, traits, Allocator>&& rhs);
+    operator+(const charT* lhs, const basic_string<charT, traits, Allocator>& rhs);
 \end{itemdecl}
 
 \begin{itemdescr}
 \pnum
-\returns
-\tcode{std::move(rhs.insert(0, 1, lhs))}.
+\effects
+Equivalent to:
+\begin{codeblock}
+basic_string<charT, traits, Allocator> r = rhs;
+r.insert(0, lhs);
+return r;
+\end{codeblock}
 \end{itemdescr}
 
 \indexlibrarymember{operator+}{basic_string}%
 \begin{itemdecl}
 template<class charT, class traits, class Allocator>
   basic_string<charT, traits, Allocator>
-    operator+(const basic_string<charT, traits, Allocator>& lhs, const charT* rhs);
+    operator+(charT lhs, const basic_string<charT, traits, Allocator>& rhs);
 \end{itemdecl}
 
 \begin{itemdescr}
 \pnum
-\returns
-\tcode{lhs + basic_string<charT, traits, Allocator>(rhs)}.
-
-\pnum
-\remarks
-Uses
-\tcode{traits::length()}.
+\effects
+Equivalent to:
+\begin{codeblock}
+basic_string<charT, traits, Allocator> r = rhs;
+r.insert(r.begin(), lhs);
+return r;
+\end{codeblock}
 \end{itemdescr}
 
 \indexlibrarymember{operator+}{basic_string}%
 \begin{itemdecl}
 template<class charT, class traits, class Allocator>
   basic_string<charT, traits, Allocator>
-    operator+(basic_string<charT, traits, Allocator>&& lhs, const charT* rhs);
+    operator+(charT lhs, basic_string<charT, traits, Allocator>&& rhs);
 \end{itemdecl}
 
 \begin{itemdescr}
 \pnum
-\returns
-\tcode{std::move(lhs.append(rhs))}.
-
-\pnum
-\remarks
-Uses
-\tcode{traits::length()}.
+\effects
+Equivalent to:
+\begin{codeblock}
+rhs.insert(rhs.begin(), lhs);
+return std::move(rhs);
+\end{codeblock}
 \end{itemdescr}
 
 \indexlibrarymember{operator+}{basic_string}%
@@ -3300,8 +3287,13 @@
 
 \begin{itemdescr}
 \pnum
-\returns
-\tcode{lhs + basic_string<charT, traits, Allocator>(1, rhs)}.
+\effects
+Equivalent to:
+\begin{codeblock}
+basic_string<charT, traits, Allocator> r = lhs;
+r.push_back(rhs);
+return r;
+\end{codeblock}
 \end{itemdescr}
 
 \indexlibrarymember{operator+}{basic_string}%
@@ -3313,8 +3305,12 @@
 
 \begin{itemdescr}
 \pnum
-\returns
-\tcode{std::move(lhs.append(1, rhs))}.
+\effects
+Equivalent to:
+\begin{codeblock}
+lhs.push_back(rhs);
+return std::move(lhs);
+\end{codeblock}
 \end{itemdescr}
 
 \rSec3[string.comparison]{Non-member comparison functions}