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PR other/89106 - cast-to-union documentation incorrect w.r.t. lvalueness

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gcc/ChangeLog:

	PR other/89106
	* doc/extend.texi (cast to a union): Correct and expand.

From-SVN: r268411
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Martin Sebor 2019-01-31 02:33:58 +00:00 • committed by Martin Sebor
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@ -1,3 +1,8 @@
2019-01-30 Martin Sebor <msebor@redhat.com>
PR other/89106
* doc/extend.texi (cast to a union): Correct and expand.
2019-01-30 Vladimir Makarov <vmakarov@redhat.com>
PR rtl-optimization/87246

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gcc/doc/extend.texi
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@ -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;