On Mon, 20 Sep 2010, Larry Evans wrote:
On 09/20/10 10:50, dherring@ll.mit.edu wrote:
See http://www.boost.org/doc/libs/1_44_0/doc/html/variant/design.html
In particular, "Temporary Heap Backup" and "Future Direction".
We have some specialized requirements. We couldn't even use boost::shared_ptr until it added both the custom allocator and deleter. The "never-empty guarantee" conflicts with our needs and is not a concern at this time.
An alternative to boost::variant is the:
http://svn.boost.org/svn/boost/sandbox/variadic_templates /boost/composite_storage/pack/container_one_of_maybe.hpp
which *might* work for you. I say *might* because I don't know exactly what you mean by:
"never-empty guarantee" conflicts with our needs
because those needs are not spelled out.
Rephrase: We don't need the never-empty guarantee; but it conflicts with the always-in-place requirement. Steven's suggestion to add boost::blank looks promising. The documentation should be updated to mention it. This container looks interesting; but other than the sources, I don't see any documentation? For now, we'll stick with what I've cobbled together.
As a side note, we also must expose both the type of the "which" and the discriminant to member mapping (which may be N:1; see the CORBA IDL spec). So I believe boost::variant would still be hard to use even if the heap issue were resolved.
I'm also unsure of what's meant by:
the discriminant to member mapping (which may be N:1).
If you mean types could be duplicated, IOW, allow:
variant<int,int>
then that's what one_of_maybe does.
The one place I found for corba spec (found with google "corba idl spec") was here:
If you're interested, see also Part 1, section 7.11.2.2 of http://www.omg.org/spec/CORBA/3.1/ and the C++ mapping on http://www.omg.org/technology/documents/idl2x_spec_catalog.htm We support a slight variant of this spec.
This layout (discriminant followed by dependent value) is the exact reverse of the container_one_of_maybe is laid out. Originally, the discrimiant was 1st; however, the following post:
That order doesn't bother me too much. The "N:1" notation meant that, just like a "switch statement", there may be N cases that specify one member. It is also possible for multiple members to have the same type. For example, here's an IDL snippet. It shows the discriminant is a "long"; both default and 2 specify member y. union U switch(long) { case 0: long x; default: case 2: char y[10]; case 3: float z; }; Later, Daniel