P2752R3 Static storage for braced initializers

Author: jensmaurer

source/compatibility.tex

@@ -1,6 +1,36 @@
 %!TEX root = std.tex
 \infannex{diff}{Compatibility}
 
+\rSec1[diff.cpp23]{\Cpp{} and ISO \CppXXIII{}}
+
+\rSec2[diff.cpp23.general]{General}
+
+\pnum
+\indextext{summary!compatibility with ISO \CppXXIII{}}%
+Subclause \ref{diff.cpp23} lists the differences between \Cpp{} and
+ISO \CppXXIII{} (ISO/IEC 14882:2023, \doccite{Programming Languages --- \Cpp{}}),
+by the chapters of this document.
+
+\rSec2[diff.cpp23.expr]{\ref{expr}: expressions}
+
+\diffref{dcl.init.list}
+\change
+Pointer comparisons between \tcode{initializer_list} objects' backing arrays
+are unspecified.
+\rationale
+Permit the implementation to store backing arrays in static read-only memory.
+\effect
+Valid \CppXXIII{} code
+that relies on the result of pointer comparison between backing arrays
+may change behavior.
+For example:
+\begin{example}
+bool ne(std::initializer_list<int> a, std::initializer_list<int> b) {
+  return a.begin() != b.begin() + 1;
+}
+bool b = ne({2,3}, {1,2,3}); // unspecified result; previously \tcode{false}
+\end{example}
+
 \rSec1[diff.cpp20]{\Cpp{} and ISO \CppXX{}}
 
 \rSec2[diff.cpp20.general]{General}

source/declarations.tex

@@ -5991,36 +5991,64 @@
 an initializer list as if
 the implementation generated and materialized\iref{conv.rval}
 a prvalue of type ``array of $N$ \tcode{const E}'',
-where $N$ is the number of elements in the
-initializer list. Each element of that array is copy-initialized with the
+where $N$ is the number of elements in the initializer list;
+this is called the initializer list's \defnadj{backing}{array}.
+Each element of the backing array is copy-initialized with the
 corresponding element of the initializer list, and the
 \tcode{std::initializer_list<E>} object is constructed to refer to that array.
 \begin{note}
 A constructor or conversion function selected for the copy is required to be
 accessible\iref{class.access} in the context of the initializer list.
 \end{note}
-If a narrowing conversion is required to initialize any of the elements, the program is ill-formed.
+If a narrowing conversion is required to initialize any of the elements,
+the program is ill-formed.
+Whether all backing arrays are distinct
+(that is, are stored in non-overlapping objects) is unspecified.
+
+\pnum
+The backing array has the same lifetime as any other temporary
+object\iref{class.temporary}, except that initializing an
+\tcode{initializer_list} object from the array extends the lifetime of
+the array exactly like binding a reference to a temporary.
 \begin{example}
 \begin{codeblock}
-struct X {
-  X(std::initializer_list<double> v);
+void f(std::initializer_list<double> il);
+void g(float x) {
+  f({1, x, 3});
+}
+void h() {
+  f({1, 2, 3});
+}
+
+struct A {
+  mutable int i;
 };
-X x{ 1,2,3 };
+void q(std::initializer_list<A>);
+void r() {
+  q({A{1}, A{2}, A{3}});
+}
 \end{codeblock}
 
 The initialization will be implemented in a way roughly equivalent to this:
 \begin{codeblock}
-const double __a[3] = {double{1}, double{2}, double{3}};
-X x(std::initializer_list<double>(__a, __a+3));
+void g(float x) {
+  const double __a[3] = {double{1}, double{x}, double{3}};              // backing array
+  f(std::initializer_list<double>(__a, __a+3));
+}
+void h() {
+  static constexpr double __b[3] = {double{1}, double{2}, double{3}};   // backing array
+  f(std::initializer_list<double>(__b, __b+3));
+}
+void r() {
+  const A __c[3] = {A{1}, A{2}, A{3}};                                  // backing array
+  q(std::initializer_list<A>(__c, __c+3));
+}
 \end{codeblock}
-assuming that the implementation can construct an \tcode{initializer_list} object with a pair of pointers.
+assuming that the implementation
+can construct an \tcode{initializer_list} object with a pair of pointers, and
+with the understanding that \tcode{__b} does not outlive the call to \tcode{f}.
 \end{example}
 
-\pnum
-The array has the same lifetime as any other temporary
-object\iref{class.temporary}, except that initializing an
-\tcode{initiali\-zer_list} object from the array extends the lifetime of
-the array exactly like binding a reference to a temporary.
 \begin{example}
 \begin{codeblock}
 typedef std::complex<double> cmplx;
@@ -6047,9 +6075,6 @@
 a temporary array to a reference member, so the program is
 ill-formed\iref{class.base.init}.
 \end{example}
-\begin{note}
-The implementation is free to allocate the array in read-only memory if an explicit array with the same initializer can be so allocated.
-\end{note}
 
 \pnum
 A \defnadj{narrowing}{conversion} is an implicit conversion

source/macros.tex

@@ -274,6 +274,7 @@
 \newcommand{\CppXIV}{\Cpp{} 2014}
 \newcommand{\CppXVII}{\Cpp{} 2017}
 \newcommand{\CppXX}{\Cpp{} 2020}
+\newcommand{\CppXXIII}{\Cpp{} 2023}
 \newcommand{\opt}[1]{#1\ensuremath{_\mathit{\color{black}opt}}}
 \newcommand{\bigoh}[1]{\ensuremath{\mathscr{O}(#1)}}