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doc: type-punning through a union is a GNU extension for C++

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We didn't explicitly say that type-punning through a union is undefined
behavior in C++. Mention that, and that we support it as a GNU extension.

This was reported on LLVM's Discourse (forums) [0].

[0] https://discourse.llvm.org/t/ub-when-type-punning-through-unions/88527/6

gcc/ChangeLog:
	PR c++/117219

	* doc/invoke.texi (-fstrict-aliasing): Explain that type-punning
	through a union in C++ is supported as a GNU extension.
This commit is contained in:
Sam James
2025-10-09 03:40:58 +01:00
parent 24ba6f2f96
commit b52a90e7a0
1 changed files with 5 additions and 4 deletions
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gcc/doc/invoke.texi
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@@ -15049,10 +15049,11 @@ int f() @{
@end smallexample
The practice of reading from a different union member than the one most
recently written to (called ``type-punning'') is common. Even with
@option{-fstrict-aliasing}, type-punning is allowed, provided the memory
is accessed through the union type. So, the code above works as
expected. @xref{Structures unions enumerations and bit-fields
implementation}. However, this code might not:
@option{-fstrict-aliasing}, type-punning is allowed in C, provided the memory
is accessed through the union type. In ISO C++, type-punning through a union
type is undefined behavior, but GCC supports it as an extension. So, the code
above works as expected. @xref{Structures unions enumerations and
bit-fields implementation}. However, this code might not:
@smallexample
int f() @{
union a_union t;