diff --git a/source/basic.tex b/source/basic.tex index a3020c43c3..bbae15075c 100644 --- a/source/basic.tex +++ b/source/basic.tex @@ -3453,6 +3453,100 @@ \end{note} \indextext{object model|)} +\rSec2[basic.align]{Alignment} + +\pnum +Object types have \defnx{alignment requirements}{alignment requirement!implementation-defined}\iref{basic.fundamental,basic.compound} +which place restrictions on the addresses at which an object of that type +may be allocated. An \defn{alignment} is an \impldef{alignment} +integer value representing the number of bytes between successive addresses +at which a given object can be allocated. An object type imposes an alignment +requirement on every object of that type; stricter alignment can be requested +using the alignment specifier\iref{dcl.align}. +Attempting to create an object\iref{intro.object} in storage that +does not meet the alignment requirements of the object's type +is undefined behavior. + +\pnum +A \defnadj{fundamental}{alignment} is represented by an alignment +less than or equal to the greatest alignment supported by the implementation in +all contexts, which is equal to +\tcode{\keyword{alignof}(std::max_align_t)}\iref{support.types}. +The alignment required for a type may be different when it is used as the type +of a complete object and when it is used as the type of a subobject. +\begin{example} +\begin{codeblock} +struct B { long double d; }; +struct D : virtual B { char c; }; +\end{codeblock} + +When \tcode{D} is the type of a complete object, it will have a subobject of +type \tcode{B}, so it must be aligned appropriately for a \tcode{\keyword{long} \keyword{double}}. +If \tcode{D} appears as a subobject of another object that also has \tcode{B} +as a virtual base class, the \tcode{B} subobject might be part of a different +subobject, reducing the alignment requirements on the \tcode{D} subobject. +\end{example} +The result of the \keyword{alignof} operator reflects the alignment +requirement of the type in the complete-object case. + +\pnum +An \defnadj{extended}{alignment} is represented by an alignment +greater than \tcode{\keyword{alignof}(std::max_align_t)}. It is \impldef{support for extended alignments} +whether any extended alignments are supported and the contexts in which they are +supported\iref{dcl.align}. A type having an extended alignment +requirement is an \defnadj{over-aligned}{type}. +\begin{note} +Every over-aligned type is or contains a class type +to which extended alignment applies (possibly through a non-static data member). +\end{note} +A \defnadj{new-extended}{alignment} is represented by +an alignment greater than \mname{STDCPP_DEFAULT_NEW_ALIGNMENT}\iref{cpp.predefined}. + +\pnum +Alignments are represented as values of the type \tcode{std::size_t}. +Valid alignments include only those values returned by an \keyword{alignof} +expression for the fundamental types plus an additional \impldef{alignment additional +values} +set of values, which may be empty. +Every alignment value shall be a non-negative integral power of two. + +\pnum +Alignments have an order from \defnx{weaker}{alignment!weaker} to +\defnx{stronger}{alignment!stronger} or \defnx{stricter}{alignment!stricter} alignments. Stricter +alignments have larger alignment values. An address that satisfies an alignment +requirement also satisfies any weaker valid alignment requirement. + +\pnum +The alignment requirement of a complete type can be queried using an +\keyword{alignof} expression\iref{expr.alignof}. Furthermore, +the narrow character types\iref{basic.fundamental} shall have the weakest +alignment requirement. +\begin{note} +This enables the ordinary character types to be used as the +underlying type for an aligned memory area\iref{dcl.align}. +\end{note} + +\pnum +Comparing alignments is meaningful and provides the obvious results: + +\begin{itemize} +\item Two alignments are equal when their numeric values are equal. +\item Two alignments are different when their numeric values are not equal. +\item When an alignment is larger than another it represents a stricter alignment. +\end{itemize} + +\pnum +\begin{note} +The runtime pointer alignment function\iref{ptr.align} +can be used to obtain an aligned pointer within a buffer; +an \grammarterm{alignment-specifier}\iref{dcl.align} +can be used to align storage explicitly. +\end{note} + +\pnum +If a request for a specific extended alignment in a specific context is not +supported by an implementation, the program is ill-formed. + \rSec2[basic.life]{Lifetime} \pnum @@ -4249,100 +4343,6 @@ deallocation function shall deallocate the storage referenced by the pointer, ending the duration of the region of storage. -\rSec2[basic.align]{Alignment} - -\pnum -Object types have \defnx{alignment requirements}{alignment requirement!implementation-defined}\iref{basic.fundamental,basic.compound} -which place restrictions on the addresses at which an object of that type -may be allocated. An \defn{alignment} is an \impldef{alignment} -integer value representing the number of bytes between successive addresses -at which a given object can be allocated. An object type imposes an alignment -requirement on every object of that type; stricter alignment can be requested -using the alignment specifier\iref{dcl.align}. -Attempting to create an object\iref{intro.object} in storage that -does not meet the alignment requirements of the object's type -is undefined behavior. - -\pnum -A \defnadj{fundamental}{alignment} is represented by an alignment -less than or equal to the greatest alignment supported by the implementation in -all contexts, which is equal to -\tcode{\keyword{alignof}(std::max_align_t)}\iref{support.types}. -The alignment required for a type may be different when it is used as the type -of a complete object and when it is used as the type of a subobject. -\begin{example} -\begin{codeblock} -struct B { long double d; }; -struct D : virtual B { char c; }; -\end{codeblock} - -When \tcode{D} is the type of a complete object, it will have a subobject of -type \tcode{B}, so it must be aligned appropriately for a \tcode{\keyword{long} \keyword{double}}. -If \tcode{D} appears as a subobject of another object that also has \tcode{B} -as a virtual base class, the \tcode{B} subobject might be part of a different -subobject, reducing the alignment requirements on the \tcode{D} subobject. -\end{example} -The result of the \keyword{alignof} operator reflects the alignment -requirement of the type in the complete-object case. - -\pnum -An \defnadj{extended}{alignment} is represented by an alignment -greater than \tcode{\keyword{alignof}(std::max_align_t)}. It is \impldef{support for extended alignments} -whether any extended alignments are supported and the contexts in which they are -supported\iref{dcl.align}. A type having an extended alignment -requirement is an \defnadj{over-aligned}{type}. -\begin{note} -Every over-aligned type is or contains a class type -to which extended alignment applies (possibly through a non-static data member). -\end{note} -A \defnadj{new-extended}{alignment} is represented by -an alignment greater than \mname{STDCPP_DEFAULT_NEW_ALIGNMENT}\iref{cpp.predefined}. - -\pnum -Alignments are represented as values of the type \tcode{std::size_t}. -Valid alignments include only those values returned by an \keyword{alignof} -expression for the fundamental types plus an additional \impldef{alignment additional -values} -set of values, which may be empty. -Every alignment value shall be a non-negative integral power of two. - -\pnum -Alignments have an order from \defnx{weaker}{alignment!weaker} to -\defnx{stronger}{alignment!stronger} or \defnx{stricter}{alignment!stricter} alignments. Stricter -alignments have larger alignment values. An address that satisfies an alignment -requirement also satisfies any weaker valid alignment requirement. - -\pnum -The alignment requirement of a complete type can be queried using an -\keyword{alignof} expression\iref{expr.alignof}. Furthermore, -the narrow character types\iref{basic.fundamental} shall have the weakest -alignment requirement. -\begin{note} -This enables the ordinary character types to be used as the -underlying type for an aligned memory area\iref{dcl.align}. -\end{note} - -\pnum -Comparing alignments is meaningful and provides the obvious results: - -\begin{itemize} -\item Two alignments are equal when their numeric values are equal. -\item Two alignments are different when their numeric values are not equal. -\item When an alignment is larger than another it represents a stricter alignment. -\end{itemize} - -\pnum -\begin{note} -The runtime pointer alignment function\iref{ptr.align} -can be used to obtain an aligned pointer within a buffer; -an \grammarterm{alignment-specifier}\iref{dcl.align} -can be used to align storage explicitly. -\end{note} - -\pnum -If a request for a specific extended alignment in a specific context is not -supported by an implementation, the program is ill-formed. - \rSec2[class.temporary]{Temporary objects} \pnum