CWG2609 Padding in class types

tkoeppe · Apr 12, 2026 · Apr 4, 2026 · Apr 4, 2026 · Apr 4, 2026 · Apr 4, 2026
commit 68767eaadec53b0b4c8c6a6238c251667e0fe76f
diff --git a/source/expressions.tex b/source/expressions.tex
@@ -5675,6 +5675,8 @@
 When applied to a class, the result is the number of bytes in an object
 of that class including any padding required for placing objects of that
 type in an array.
+The amount and placement of padding in a class type
+is a property of the implementation.
 The result of applying \keyword{sizeof} to a
 potentially-overlapping subobject is
 the size of the type, not the size of the subobject.

diff --git a/source/intro.tex b/source/intro.tex
@@ -462,6 +462,11 @@
 \end{codeblock}
 \end{example}
 
+\indexdefn{property!of the implementation}%
+\definition{property of the implementation}{defns.impl.prop}
+behavior, for a well-formed program\iref{defns.well.formed}
+construct and correct data, that depends on the implementation
+
 \definition{referenceable type}{defns.referenceable}
 \indexdefn{type!referenceable}%
 type that is either an
@@ -920,10 +925,14 @@
 
 \pnum
 \indextext{behavior!implementation-defined}%
-Certain aspects and operations of the abstract machine are described in this
+Certain aspects and operations of the abstract machine
+constitute the parameters of the abstract machine and
+are described in this
 document as implementation-defined behavior (for example,
-\tcode{sizeof(int)}). These constitute the parameters of the abstract machine.
-Each implementation shall include documentation describing its characteristics
+\tcode{sizeof(int)})
+or as properties of the implementation (for example, padding in class types).
+For implementation-defined behavior,
+each implementation shall include documentation describing its characteristics
 and behavior in these respects.
 \begin{footnote}
 This documentation also includes