"Dave Dribin" <dave-ml@dribin.org> wrote in message news:F773AA4D-E17B-4FB2-B872-116B17E7715B@dribin.org...
Hello,
I started a thread on Boost-users about the proposed endian library, but it's probably more suitable on the dev list. Here's a link to the thread in the archives:
<http://lists.boost.org/boost-users/2006/07/20887.php>
I see two issues with the library as of 0.5. First, the endian classes cannot be used in unions.
Yep, that's a problem. But it goes beyond union requirements - the class needs to be changed be POD's. The primary use is I/O, and that means they need to be memcpyable. And that means they must be POD's. I know some have argued that both POD and non-POD versions should be supplied, but I don't want to depend on undefined behavior, which is what will happen with a non-POD endian class. Constructors can be added later if the committee decides to relax the POD requirements.
And, second the classes cannot be used in variable argument lists. I'm attaching a patch to help resolve both of these issues.
In order to be used in a union, a class must not have any constructors. However, removing constructors breaks the following code:
big4_t x = 42;
My solution was to provide a static init() method to help in these situations:
big4_t x = big_t::init(42);
Interesting. Of course you could have also written it like this: big4_t x; x=45;
The second problem shows up using varargs. For example, this will not compile:
printf("x = %d\n", x);
The solution I came up with is to use operator() to get the native value:
printf("x = %d\n", x());
My old implementation also did that, but over time I came to feel it was too cryptic. I'd prefer to call the functon value() I think.
In my patch, I've also added a check_unions() function to endian_test.cpp that tests the sizes of unions.
Thanks for spotting these problems. I fell stupid for no thinking the POD issue throught before this. My old interface was a POD - I guess I got seduced by the elegance of Darin's interface and wishful thinking about changing C++ to allow POD's to have constructors (except copy constructors). --Beman