2007/3/1, Matthias Schabel <boost@schabel-family.org>:
But wait, there is hope: you can do this:
////////
template<class A> struct ordinal { void * tag() { static int const tag_; return &tag_; } template<class B> bool operator<(ordinal<B> const& other) { return std::less<void*>(tag(), other.tag()); } };
//////
This solution should also be MT safe.
Unfortunately, this doesn't function at compile-time. That is, what I really want to be able to do is:
struct system1 : ordinal<...> { }; struct system2 : ordinal<...> { }; struct system3 : ordinal<...> { };
typedef mpl::list<system2,system3,system1> list_type; typedef mpl::sort<list_type>::type sorted_list_type;
where mpl::less<> can be defined for ordinal<...>
It seems like a hard (maybe impossible) problem...
You could always use the solution we use in boost.typeof. When we emulate typeof support for VC compilers, we use the following trick to associate a unique integer with each type: template<int N> struct encode_counter : encode_counter<N - 1> {}; template<> struct encode_counter<0> {}; //Need to default to a larger value than 4, as due to MSVC's ETI errors. (sizeof(int)==4) char (*encode_index(...))[5]; # define BOOST_TYPEOF_INDEX(T) (sizeof(*boost::type_of::encode_index((boost::type_of::encode_counter<1005>*)0))) # define BOOST_TYPEOF_NEXT_INDEX(next) friend char (*encode_index(encode_counter<next>*))[next]; //Tie it all together template<typename T=int> struct ordinal { //Get the next available compile time constants index BOOST_STATIC_CONSTANT(unsigned,value=BOOST_TYPEOF_INDEX(T)); BOOST_STATIC_CONSTANT(unsigned,next=value+1); //Increment the compile time constant BOOST_TYPEOF_NEXT_INDEX(next); }; It is not exactly a portable solution, though... Peder
Matthias
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