Исключить повторяющиеся записи из аргументов шаблона C ++ 11

Моя стратегия для решения этой проблемы заключается в использовании нескольких уровней косвенности.

• молния < Args …> отправляет обработку своих аргументов функции process_zipper_arguments путем наследования:

Пример:

template < typename... Args >
struct zipper : zipper < typename process_zipper_arguments < Args... >::type > {};

• использовать template < typename... Args > struct typelist {} отслеживать типы объектов, от которых вы хотите наследовать.
• специализироваться struct zipper < typelist < Args... > >: public virtual Args... сделать фактическое наследование

Чтобы избавиться от дубликатов родительских типов, используются две вспомогательные функции в process_zipper_arguments:

• is_in < CandidateType, typelist< Args... > >::type либо true_type или же false_type и может быть определен рекурсивно
• add_unique < CandidateType, typelist< Args... > >::type это typelist <...> К этому добавлен либо CandidateType, либо нет. Это вызывает is_in определить это.

Вот полный код, который компилируется хотя бы с g ++ (GCC) 4.6.3 с —std = c ++ 0x. Критика на это приветствуется.

// Forward declarations
template < typename... Args >
struct zipper;

// Two types meaning true and false
struct true_type {};
struct false_type {};

// The only purpose of this struct is to be associated with Types...
template < typename... Types >
struct typelist {};// ===================================================
// is_in < type, typelist<...> >::type
//     is true_type if type is in typelist
//     is false_type if type is not in typelist

// Assume TElement is not in the list unless proven otherwise
template < typename TElement, typename TList >
struct is_in {
typedef false_type type;
};

// If it matches the first type, it is definitely in the list
template < typename TElement, typename... TTail >
struct is_in < TElement, typelist < TElement, TTail... > >
{
typedef true_type type;
};

// If it is not the first element, check the remaining list
template < typename TElement, typename THead, typename... TTail >
struct is_in < TElement, typelist < THead, TTail... > >
{
typedef typename is_in < TElement, typelist < TTail... > >::type type;
};

// ===================================================
// add_unique < TNew, typelist<...> >::type
//     is typelist < TNew, ... > if TNew is not already in the list
//     is typelist <...> otherwise

// Append a type to a type_list unless it already exists
template < typename TNew, typename TList,
typename Tis_duplicate = typename is_in < TNew, TList >::type
>
struct add_unique;

// If TNew is in the list, return the list unmodified
template < typename TNew, typename... TList >
struct add_unique < TNew, typelist < TList... >, true_type >
{
typedef typelist < TList... > type;
};

// If TNew is not in the list, append it
template < typename TNew, typename... TList >
struct add_unique < TNew, typelist < TList... >, false_type >
{
typedef typelist < TNew, TList... > type;
};

// ===================================================
// process_zipper_arguments < Args... >::type
//     returns a typelist of types to be inherited from.
//
// It performs the following actions:
// a) Unpack zipper<...> and typelist <...> arguments
// b) Ignore values that are already in the list

template < typename... Args >
struct process_zipper_arguments;

// Unpack a zipper in the first argument
template < typename... ZipperArgs, typename... Args >
struct process_zipper_arguments < zipper < ZipperArgs... >, Args... >
{
typedef typename process_zipper_arguments < ZipperArgs..., Args... >::type type;
};

// Unpack a typelist in the first argument
template < typename... TypeListArgs, typename... Args >
struct process_zipper_arguments < typelist < TypeListArgs... >, Args... >
{
typedef typename process_zipper_arguments < TypeListArgs..., Args... >::type type;
};

// End the recursion if the list is empty
template < >
struct process_zipper_arguments < >
{
typedef typelist < > type;
};

// Construct the list of unique types by appending them one by one
template < typename THead, typename... TTail >
struct process_zipper_arguments < THead, TTail... >
{
typedef typename
add_unique < THead,
typename process_zipper_arguments < TTail... >::type
>::type type;
};// ===================================================
// The zipper class that you might want// If the list of types is not yet known, process it.
// The inheritance is ugly, but there is a workaround
template < typename... Args >
struct zipper : zipper < typename process_zipper_arguments < Args... >::type >
{
// // Instead of inheriting, you can use zipper as a factory.
// // So this:
// typedef zipper < meas2, zipper < meas1, meas > > mymeas;
// // Turns to:
// typedef typename zipper < meas2, zipper < meas1, meas > >::type mymeas;
typedef zipper < typename process_zipper_arguments < Args... >::type > type;
};

// If the list of types is known, inherit from each type
template < typename... Args >
struct zipper < typelist < Args... > >
: public virtual Args...
{};

// ===================================================
// Short usage demo, replace with your own code

struct meas {
int i;
};

struct meas2 {
int j;
};

struct meas3 {
int k;
};typedef zipper < meas, meas, meas3 > meas_type;
typedef zipper < meas2, meas_type, meas2 > meas_type2;

typedef typename zipper < meas_type2 >::type nicer_meas_type2;int main ( int, char** )
{
meas * m = new meas_type2;
meas_type2 n;
nicer_meas_type2 o;

return 0;
}

Отладка дает следующий результат (точка останова на return 0; линия):

(gdb) print *m
$1 = {i = 0}
(gdb) print n
$2 = {<zipper<typelist<meas, meas3, meas2> >> = {<meas> = {i = 4196320}, <meas3> = {k = 0}, <meas2> = {j = 0},
_vptr.zipper = 0x400928}, <No data fields>}
(gdb) print o
$3 = {<meas> = {i = 4195719}, <meas3> = {k = 0}, <meas2> = {j = 1}, _vptr.zipper = 0x4009a8 <VTT for zipper<typelist<meas, meas3, meas2> >>}