Merge 2025-02 CWG Motion 8

P2786R13 Trivial Relocatability for C++26

Author: tkoeppe

source/basic.tex

@@ -5030,6 +5030,14 @@
 Scalar types, trivially copyable class types\iref{class.prop},
 arrays of such types, and cv-qualified versions of these
 types are collectively called \defnadjx{trivially copyable}{types}{type}.
+\label{term.trivially.relocatable.type}%
+Scalar types, trivially relocatable class types\iref{class.prop},
+arrays of such types, and cv-qualified versions of these
+types are collectively called \defnadjx{trivially relocatable}{types}{type}.
+\label{term.replaceable.type}%
+Cv-unqualified scalar types, replaceable class types\iref{class.prop}, and
+arrays of such types are collectively called
+\defnadjx{replaceable}{types}{type}.
 \label{term.standard.layout.type}%
 Scalar types, standard-layout class
 types\iref{class.prop}, arrays of such types, and

source/classes.tex

@@ -36,7 +36,7 @@
 
 \begin{bnf}
 \nontermdef{class-head}\br
-    class-key \opt{attribute-specifier-seq} class-head-name \opt{class-virt-specifier} \opt{base-clause}\br
+    class-key \opt{attribute-specifier-seq} class-head-name \opt{class-property-specifier-seq} \opt{base-clause}\br
     class-key \opt{attribute-specifier-seq} \opt{base-clause}
 \end{bnf}
 
@@ -46,8 +46,15 @@
 \end{bnf}
 
 \begin{bnf}
-\nontermdef{class-virt-specifier}\br
-    \keyword{final}
+\nontermdef{class-property-specifier-seq}\br
+    class-property-specifier \opt{class-property-specifier-seq}
+\end{bnf}
+
+\begin{bnf}
+\nontermdef{class-property-specifier}\br
+    \keyword{final}\br
+    \keyword{trivially_relocatable_if_eligible}\br
+    \keyword{replaceable_if_eligible}
 \end{bnf}
 
 \begin{bnf}
@@ -120,12 +127,13 @@
 \end{note}
 
 \pnum
-If a class is marked with the \grammarterm{class-virt-specifier} \tcode{final} and it appears
-as a \grammarterm{class-or-decltype} in a \grammarterm{base-clause}\iref{class.derived},
-the program is ill-formed. Whenever a
-\grammarterm{class-key} is followed by a \grammarterm{class-head-name}, the
-\grammarterm{identifier} \tcode{final}, and a colon or left brace, \tcode{final} is
-interpreted as a \grammarterm{class-virt-specifier}.
+Each \grammarterm{class-property-specifier} shall appear at most once
+withina single \grammarterm{class-property-specifier-seq}.
+Whenever a \grammarterm{class-key} is followed
+by a \grammarterm{class-head-name},
+the identifier \tcode{final}, \tcode{trivially_relocatable_if_eligible},
+or \tcode{replaceable_if_eligible}, and a colon or left brace,
+the identifier is interpreted as a \grammarterm{class-property-specifier}.
 \begin{example}
 \begin{codeblock}
 struct A;
@@ -134,12 +142,19 @@
 
 struct X {
  struct C { constexpr operator int() { return 5; } };
- struct B final : C{};  // OK, definition of nested class \tcode{B},
-                        // not declaration of a bit-field member \tcode{final}
+ struct B trivially_relocatable_if_eligible : C{};
+                        // OK, definition of nested class \tcode{B},
+                        // not declaration of a bit-field member
+                        // \tcode{trivially_relocatable_if_eligible}
 };
 \end{codeblock}
 \end{example}
 
+\pnum
+If a class is marked with the \grammarterm{class-property-specifier}
+\tcode{final} and that class appears as a \grammarterm{class-or-decltype}
+in a \grammarterm{base-clause}\iref{class.derived}, the program is ill-formed.
+
 \pnum
 \begin{note}
 Complete objects of class type have nonzero size.
@@ -172,6 +187,87 @@
 \item that has a trivial, non-deleted destructor\iref{class.dtor}.
 \end{itemize}
 
+\pnum
+A class \tcode{C} is \defn{default-movable} if
+
+\begin{itemize}
+\item overload resolution for direct-initializing an object of type \tcode{C}
+from an xvalue of type \tcode{C} selects a constructor that is a direct member
+of \tcode{C} and is neither user-provided nor deleted,
+
+\item overload resolution for assigning to an lvalue of type \tcode{C} from an
+xvalue of type \tcode{C} selects an assignment operator function that is a
+direct member of \tcode{C} and is neither user-provided nor deleted, and
+
+\item \tcode{C} has a destructor that is neither user-provided nor deleted.
+\end{itemize}
+
+\pnum
+A class is \defn{eligible for trivial relocation} unless it
+\begin{itemize}
+\item has any virtual base classes,
+\item has a base class that is not a trivially relocatable class,
+\item has a non-static data member of an object type that is not of a
+trivially relocatable type, or
+
+\item has a deleted destructor,
+\end{itemize}
+except that it is \impldef{whether an otherwise-eligible union having one or
+more subobjects of polymorphic class type is eligible for trivial relocation}
+whether an otherwise-eligible union having one or more subobjects of
+polymorphic class type is eligible for trivial relocation.
+
+\pnum
+A class \tcode{C} is a \defnadj{trivially relocatable}{class}
+if it is eligible for trivial relocation and
+\begin{itemize}
+\item has the \tcode{trivially_relocatable_if_eligible} \grammarterm{class-property-specifier},
+\item is a union with no user-declared special member functions, or
+\item is default-movable.
+\end{itemize}
+
+\pnum
+\begin{note}
+A class with const-qualified or reference non-static data members can be
+trivially relocatable.
+\end{note}
+
+\pnum
+A class \tcode{C} is \defn{eligible for replacement} unless
+\begin{itemize}
+\item it has a base class that is not a replaceable class,
+\item it has a non-static data member that is not of a replaceable type,
+\item overload resolution fails or selects a deleted constructor when
+direct-initializing an object of type \tcode{C} from an xvalue of type
+\tcode{C}\iref{dcl.init.general},
+
+\item overload resolution fails or selects a deleted assignment operator
+function when assigning to an lvalue of type \tcode{C} from an xvalue of type
+\tcode{C} \iref{expr.assign,over.assign}), or
+
+\item it has a deleted destructor.
+\end{itemize}
+
+\pnum
+A class \tcode{C} is a \defnadj{replaceable}{class} if it is
+eligible for replacement and
+\begin{itemize}
+\item has the \tcode{replaceable_if_eligible} \grammarterm{class-property-specifier},
+\item is a union with no user-declared special member functions, or
+\item is default-movable.
+\end{itemize}
+
+\pnum
+\begin{note}
+Accessibility of the special member functions is not considered when
+establishing trivial relocatability or replaceability.
+\end{note}
+
+\pnum
+\begin{note}
+Not all trivially copyable classes are trivially relocatable or replaceable.
+\end{note}
+
 \pnum
 A class \tcode{S} is a \defnadj{standard-layout}{class} if it:
 \begin{itemize}

source/compatibility.tex

@@ -91,6 +91,22 @@
 
 \rSec2[diff.cpp23.dcl.dcl]{\ref{dcl}: declarations}
 
+\diffref{dcl.decl.general}
+\change
+Introduction of \tcode{trivially_relocatable_if_eligible} and
+\tcode{replaceable_if_eligible} as identifiers with special meaning\iref{lex.name}.
+\rationale
+Support declaration of trivially relocatable and replaceable types\iref{class.prop}.
+\effect
+Valid \CppXXIII{} code can become ill-formed.
+\begin{example}
+\begin{codeblock}
+struct C {};
+struct C replaceable_if_eligible {};    // was well-formed (new variable \tcode{replaceable_if_eligible})
+                                        // now ill-formed (redefines \tcode{C})
+\end{codeblock}
+\end{example}
+
 \diffref{dcl.init.list}
 \change
 Pointer comparisons between \tcode{initializer_list} objects' backing arrays
@@ -212,6 +228,17 @@
 Valid \CppXXIII{} code that \tcode{\#include}{s} headers with these names may be
 invalid in this revision of \Cpp{}.
 
+\diffref{res.on.macro.definitions}
+\change
+Additional restrictions on macro names.
+\rationale
+Avoid hard to diagnose or non-portable constructs.
+\effect
+Names of special identifiers may not be used as macro names.
+Valid \CppXXIII{} code that defines \tcode{replaceable_if_eligible} or
+\tcode{trivially_relocatable_if_eligible} as macros is invalid
+in this revision of \Cpp{}.
+
 \rSec2[diff.cpp23.strings]{\ref{strings}: strings library}
 
 \diffref{string.conversions}

source/expressions.tex

@@ -2157,9 +2157,9 @@
 other than by changing:
 \begin{itemize}
 \item the size and/or alignment of the closure type,
-
-\item whether the closure type is trivially copyable\iref{class.prop}, or
-
+\item whether the closure type is trivially copyable\iref{class.prop},
+\item whether the closure type is trivially relocatable\iref{class.prop},
+\item whether the closure type is replaceable\iref{class.prop}, or
 \item whether the closure type is a standard-layout class\iref{class.prop}.
 \end{itemize}
 

source/lex.tex

@@ -907,6 +907,8 @@
 \indextext{\idxcode{final}}%
 \indextext{\idxcode{module}}%
 \indextext{\idxcode{override}}%
+\indextext{\idxcode{replaceable_if_eligible}}%
+\indextext{\idxcode{trivially_relocatable_if_eligible}}%
 The identifiers in \tref{lex.name.special} have a special meaning when
 appearing in a certain context. When referred to in the grammar, these identifiers
 are used explicitly rather than using the \grammarterm{identifier} grammar production.
@@ -915,18 +917,15 @@
 token as a regular \grammarterm{identifier}.
 
 \begin{multicolfloattable}{Identifiers with special meaning}{lex.name.special}
-{llllll}
-\keyword{final}           \\
-\columnbreak
-\keyword{import}          \\
-\columnbreak
-\keyword{module}          \\
-\columnbreak
-\keyword{override}        \\
-\columnbreak
-\keyword{post}            \\
-\columnbreak
-\keyword{pre}             \\
+{llll}
+\keyword{final}                                 \\
+\keyword{override}                              \\\columnbreak
+\keyword{import}                                \\
+\keyword{module}                                \\\columnbreak
+\keyword{post}                                  \\
+\keyword{pre}                                   \\\columnbreak
+\keyword{replaceable_if_eligible}               \\
+\keyword{trivially_relocatable_if_eligible}     \\
 \end{multicolfloattable}
 
 \pnum

source/lib-intro.tex

@@ -3785,6 +3785,28 @@
 side effects.
 \end{note}
 
+\rSec3[library.class.props]{Properties of library classes}
+
+\pnum
+Unless explicitly stated otherwise, it is unspecified whether any class
+described in \ref{\firstlibchapter} through \ref{\lastlibchapter} and
+\ref{depr} is a trivially copyable class, a standard-layout class, or an
+implicit-lifetime class\iref{class.prop}.
+
+\pnum
+Unless explicitly stated otherwise, it is unspecified whether any class for
+which trivial relocation (i.e., the effects of
+\tcode{trivially_relocate}\iref{obj.lifetime}) would be semantically equivalent
+to move-construction of the destination object followed by destruction of the
+source object is a trivially relocatable class\iref{class.prop}.
+
+\pnum
+Unless explicitly stated otherwise, it is unspecified whether a class \tcode{C}
+is a replaceable class\iref{class.prop} if assigning an xvalue \tcode{a} of
+type \tcode{C} to an object \tcode{b} of type \tcode{C} is semantically
+equivalent to destroying \tcode{b} and then constructing from \tcode{a} in
+\tcode{b}'s place.
+
 \rSec3[protection.within.classes]{Protection within classes}
 
 \pnum