From f7478e4e33252da20e97138c7f1503e7706f300b Mon Sep 17 00:00:00 2001 From: Martin Sebor Date: Thu, 31 Jan 2019 02:33:58 +0000 Subject: [PATCH] PR other/89106 - cast-to-union documentation incorrect w.r.t. lvalueness gcc/ChangeLog: PR other/89106 * doc/extend.texi (cast to a union): Correct and expand. From-SVN: r268411 --- gcc/ChangeLog | 5 +++++ gcc/doc/extend.texi | 37 ++++++++++++++++++++++++++++--------- 2 files changed, 33 insertions(+), 9 deletions(-) diff --git a/gcc/ChangeLog b/gcc/ChangeLog index 895194f65d0..8b632f5f92e 100644 --- a/gcc/ChangeLog +++ b/gcc/ChangeLog @@ -1,3 +1,8 @@ +2019-01-30 Martin Sebor + + PR other/89106 + * doc/extend.texi (cast to a union): Correct and expand. + 2019-01-30 Vladimir Makarov PR rtl-optimization/87246 diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi index f41c7a88a57..b51f4273ade 100644 --- a/gcc/doc/extend.texi +++ b/gcc/doc/extend.texi @@ -2275,27 +2275,46 @@ case 1...5: @cindex cast to a union @cindex union, casting to a -A cast to union type looks similar to other casts, except that the type -specified is a union type. You can specify the type either with the -@code{union} keyword or with a @code{typedef} name that refers to -a union. A cast to a union actually creates a compound literal and -yields an lvalue, not an rvalue like true casts do. +A cast to a union type is a C extension not available in C++. It looks +just like ordinary casts with the constraint that the type specified is +a union type. You can specify the type either with the @code{union} +keyword or with a @code{typedef} name that refers to a union. The result +of a cast to a union is a temporary rvalue of the union type with a member +whose type matches that of the operand initialized to the value of +the operand. The effect of a cast to a union is similar to a compound +literal except that it yields an rvalue like standard casts do. @xref{Compound Literals}. -The types that may be cast to the union type are those of the members -of the union. Thus, given the following union and variables: +Expressions that may be cast to the union type are those whose type matches +at least one of the members of the union. Thus, given the following union +and variables: @smallexample union foo @{ int i; double d; @}; int x; double y; +union foo z; @end smallexample @noindent -both @code{x} and @code{y} can be cast to type @code{union foo}. +both @code{x} and @code{y} can be cast to type @code{union foo} and +the following assignments +@smallexample + z = (union foo) x; + z = (union foo) y; +@end smallexample +are shorthand equivalents of these +@smallexample + z = (union foo) @{ .i = x @}; + z = (union foo) @{ .d = y @}; +@end smallexample + +However, @code{(union foo) FLT_MAX;} is not a valid cast because the union +has no member of type @code{float}. Using the cast as the right-hand side of an assignment to a variable of -union type is equivalent to storing in a member of the union: +union type is equivalent to storing in a member of the union with +the same type @smallexample union foo u; -- 2.30.2