Where should I put [[gnu :: transparent_union]]?
Clang supports the use of the C ++ 11 generic attribute syntax for vendor-specific attributes . This means that in theory I can use any attribute supported by Clang and mark it with syntax [[attribute]]
.
However, I am having problems using the attribute transparent_union
. I remember that C ++ allows you to put attributes basically anywhere, so I tried all of the following, to no avail:
[[gnu::transparent_union]] union x { int foo; };
// error: an attribute list cannot appear here
union [[gnu::transparent_union]] x { int foo; };
// warning: transparent_union attribute can only be applied to a union definition;
// attribute ignored [-Wignored-attributes]
union x [[gnu::transparent_union]] { int foo; };
// error: an attribute list cannot appear here
// warning: transparent_union attribute can only be applied to a union definition;
// attribute ignored [-Wignored-attributes]
union x { int foo; } [[gnu::transparent_union]];
// error: an attribute list cannot appear here
// warning: transparent_union attribute can only be applied to a union definition;
// attribute ignored [-Wignored-attributes]
The correct place to use the syntax __attribute__
seems to be at the end of the union definition:
union x { int foo; } __attribute__((transparent_union));
// OK
Where is the right place, if any, to put [[gnu::transparent_union]]
with Clang?
source to share
In the C ++ 11 standard, it shows in the grammar that after the class key comes the seq-specifier, which is union
.
class-head:
class-key attribute-specifier-seqopt class-head-name class-virt-specifieropt base-clauseopt
class-key attribute-specifier-seqopt base-clauseopt
[..]
class-key:
class
struct
union
This is confirmed as:
union [[gnu::transparent_union]] x { int foo; };
seems to be the only syntax that doesn't trigger diagnostics. This is probably a Clang bug.
Keep in mind that this __attribute__
is a GNU extension and [[attribute]]
is part of C ++.