In short: I want to extract various options from variadic template parameters, but not only by tag but by index for those parameters, that have no known tag. I like the approach in boost (e.g. heap or lockfree policies), but want to make it compatible with STL containers - the allocator parameter.
Preface
I am currently writing template for queue/buffer of variable-size records/objects with this signature:
// current:
template <typename R = byte, class Alloc = std::allocator<byte>,
class... Opts> class rqueue;
// what I want:
template <class... Opts> class rqueue;
I have few other possible options which I described like this:
namespace tag {
struct allocator {}; ///< specify allocator for data storage
struct virtual_offset {}; ///< manage offset in virtual memory
struct fill_empty {}; ///< fill empty space (security or reconstruction)
struct reconstructible {}; ///< fill empty and leave one slot between wp&rp
} namespace opt {
/// allocator for data storage
template <class Alloc> struct allocator: tag::allocator {
typedef Alloc type; };
/// type for offset in virtual memory
template <class Off> struct virtual_offset: tag::virtual_offset {
typedef Off type; };
/// type and value to fill empty space
template <class T, T V> struct fill_empty: tag::fill_empty {
typedef T type; static constexpr T value = V; };
/// make state pointers reconstructible by leaving one slot between wp&rp
template <class T, T V> struct reconstructible
: tag::reconstructible, fill_empty<T, V> {};
}
Usage
// basic queue for custom record class
rqueue<record>;
// advanced record storage that can be written to a file and reconstructed back
rqueue<opt::virtual_offset<unsigned>, opt::reconstructible<byte,0xFF>>;
// specialization for strings with custom allocator
rqueue<string, myalloc>;
// alternative to above
rqueue<const char*, opt::allocator<myalloc>>;
Option Pack Helper
namespace opt {
template<class... Opts> struct bind {
template<class Tag> static constexpr bool has() {
return false; }
template<class Tag, class Default = void>
using get = Default; };
template<class First, class... More> struct bind<First, More...> {
private:
template<class Tag> static constexpr bool first() {
return std::is_same<Tag, First>::value
|| std::is_base_of<Tag, First>::value; }
template<class Tag, class Default, bool> struct get_ {
typedef typename bind<More...>::template get<Tag, Default> type; };
template<class Tag, class Default> struct get_<Tag, Default, true> {
typedef First type; };
public:
template<class Tag> static constexpr bool has() {
return first<Tag>() || bind<More...>::template has<Tag>(); }
template<class Tag, class Default = void>
using get = typename get_<Tag, Default, first<Tag>()>::type; };
}
It is not that advanced as The Boost Parameter Library, but gets the job done... so far, with list of untagged required params and tagged optional params.
Test Code
cout << boolalpha;
typedef opt::bind<
opt::virtual_offset<unsigned>,
opt::reconstructible<char,0>
> opts;
cout << opts::has<tag::virtual_offset>() << endl;
cout << opts::has<tag::fill_empty>() << endl;
cout << opts::has<tag::reconstructible>() << endl;
cout << typeid(opts::get<tag::virtual_offset>::type).name() << endl;
cout << typeid(opts::get<tag::fill_empty>::type).name() << endl;
cout << (int)opts::get<tag::fill_empty>::value << endl;
typedef opt::bind<> no;
cout << no::has<tag::virtual_offset>() << endl;
cout << no::has<tag::fill_empty>() << endl;
cout << no::has<tag::reconstructible>() << endl;
cout << typeid(no::get<tag::virtual_offset>).name() << endl;
typedef opt::bind<opt::fill_empty<char,0>> one;
cout << one::has<tag::virtual_offset>() << endl;
cout << one::has<tag::fill_empty>() << endl;
cout << one::has<tag::reconstructible>() << endl;
cout << typeid(one::get<tag::virtual_offset>).name() << endl;
Questions
I was searching the complex boost parameter / metaprogramming library and ended in metafunctions which can do the same work as my small helper (and of course much more), but didn't find a solution for extracting untagged options by index.
Maybe I should just forget about it and stick with those tags, or write some complex helper to filter out all tagged options and acces those left by index... But before I surrender or go the long way, here is nice place to ask :)
Note: If you refer to some library, leave a description how to use it, please. Thank you.
