Neil Mayhew wrote:
On 28/05/08 04:25 PM Beman Dawes wrote:
Neil Mayhew wrote:
... have a base class that contains everything except the constructors, and a derived class that adds the constructors There are some issues with the base class approach. For it to be a POD, it can't have constructors, base classes, or private members.
Strictly speaking, yes, although we are only concerned about memory layout here, so the class doesn't have to be a true POD. The other concern is for the class to be able to live inside a union, and I think the constructors are the the only thing stopping that. (I #if'd the constructors and I was then able to use endian in a union.)
I was wrong above - you are correct that just the constructs have to be removed. Private members and base classes are OK.
It might be better to just have two separate types, endian and old_endian. (old_endian isn't very clear as a name, but calling it pod_endian becomes invalid the moment C++0x ships.)
The typedefs to big32_t etc. would then be #if'd to correspond to the POD or non-POD classes as desired. I think this is better than putting the #if's around the constructors themselves. Hummm... Seems too obscure. Better to have two sets of typedefs, with the C++03 set prefixed by old_ (or whatever better name anyone can come up with.)
I don't like "old_", at least not if it has to appear in my code! :-)
I would like my client code to remain essentially the same when I upgrade to a C++0x compiler. This means having the same type names, which is why I suggested conditional typedefs. I thought this was cleaner than putting the conditionals inside the endian class itself. However, it doesn't make a lot of difference.
Perhaps the simplest and best solution is therefore:
class endian< ... > : cover_operators< ... > { public: #if defined(CXX_0X) || !defined(ENDIANS_IN_UNIONS) endian() {} endian(T val) { ... } #endif
That would work. The macro names need tweaks. Maybe !defined(BOOST_NO_RELAXED_PODS) || !defined(BOOST_ENDIANS_IN_UNIONS) or !(defined(BOOST_NO_RELAXED_PODS) && defined(BOOST_ENDIANS_IN_UNIONS)) It's been a long day so that'll need checking in the morning:-) Thanks for the ideas, --Beman