Próbuję zadzwonić do funkcji szablonowej takiej jak ta:

typedef std::tuple<int, double, bool> InstrumentTuple;

Cache cache;
InstrumentTuple tuple = cache.get<InstrumentTuple>();

Wiem, że mógłbym „po prostu” przekazać typy krotki. To właśnie wiem, ale jest dość kłopotliwe, ponieważ wykonuję wiele wywołań tej funkcji, a ponieważ krotki są dość długie:

InstrumentTuple tuple = c.get<int, double, bool>(); // syntax I'd like to avoid

Próbowałem więc wielu implementacji metody get, ale bez powodzenia:

Włączanie za pomocą parametru szablonu

#include <tuple>

class Cache
{
private:
    template<int I, typename T, typename = typename std::enable_if<I == std::tuple_size<T>::value>::type>
    std::tuple<> get() // line 6
    {
        return std::tuple<>();
    }

    template<int I, typename T, typename = typename std::enable_if<I != std::tuple_size<T>::value>::type>
    std::tuple<typename std::tuple_element<I,T>::type, decltype(get<I+1, T>())> get() // line 12
    {
        std::tuple<typename std::tuple_element<I,T>::type> value;
        return std::tuple_cat(value, get<I+1, T>());
    }

public:
    template<typename T>
    T get()
    {
        return get<0, T>(); // line 22
    }
};

int main(int argc, char** argv)
{
    Cache cache;
    typedef std::tuple<int, double, bool> InstrumentTuple;
    InstrumentTuple tuple = cache.get<InstrumentTuple>(); // line 30
}

Co daje mi ten błąd:

main.cpp: In instantiation of 'T Cache::get() [with T = std::tuple<int, double, bool>]':
main.cpp:30:56:   required from here
main.cpp:22:26: error: no matching function for call to 'Cache::get()'
main.cpp:22:26: note: candidates are:
main.cpp:6:18: note: template<int I, class T, class> std::tuple<> Cache::get()
main.cpp:6:18: note:   template argument deduction/substitution failed:
main.cpp:5:33: error: no type named 'type' in 'struct std::enable_if<false, void>'
main.cpp:12:81: note: template<int I, class T, class> std::tuple<typename std::tuple_element<I, T>::type, decltype (get<(I + 1), T>())> Cache::get()
// ----- Important part
main.cpp:12:81: note:   template argument deduction/substitution failed:
main.cpp: In substitution of 'template<int I, class T, class> std::tuple<typename std::tuple_element<I, T>::type, decltype (get<(I + 1), T>())> Cache::get() [with int I = 0; T = std::tuple<int, double, bool>; <template-parameter-1-3> = <missing>]':
// -----
main.cpp:22:26:   required from 'T Cache::get() [with T = std::tuple<int, double, bool>]'
main.cpp:30:56:   required from here
main.cpp:12:81: error: no matching function for call to 'Cache::get()'
main.cpp:12:81: note: candidate is:
main.cpp:6:18: note: template<int I, class T, class> std::tuple<> Cache::get()
main.cpp:6:18: note:   template argument deduction/substitution failed:
main.cpp:5:33: error: no type named 'type' in 'struct std::enable_if<false, void>'
main.cpp: In instantiation of 'T Cache::get() [with T = std::tuple<int, double, bool>]':
main.cpp:30:56:   required from here
main.cpp:20:7: note: template<class T> T Cache::get()
main.cpp:20:7: note:   template argument deduction/substitution failed:
main.cpp:22:26: error: wrong number of template arguments (2, should be 1)

Nie rozumiem dlaczego brakuje parametru szablonu.

Próbowałem więc innej implementacji:

Szablon szablonu o nazwie parametr

#include <tuple>

class Cache
{
private:
    template<int>
    std::tuple<> get() // line 7
    {
        return std::tuple<>();
    }

    template<int index, typename type, typename... rest>
    std::tuple<type, rest...> get() // line 13
    {
        return std::tuple_cat(std::tuple<type>(), get<index+1, rest...>());
    }

public:
    template<template<typename... types> class tuple>
    typename std::tuple<(tuple::types)...> get()
    {
        return get<0, (tuple::types)...>();
    }
};  // line 24

int main(int argc, char** argv)
{
    Cache cache;
    typedef std::tuple<int, double, bool> InstrumentTuple;
    InstrumentTuple tuple = cache.get<InstrumentTuple>(); // line 30
}

Ale wtedy dostaję ten błąd:

// ----- Important part
main.cpp:24:1: error: expected identifier before '}' token
main.cpp:24:1: error: expected unqualified-id before '}' token
// -----
main.cpp: In function 'int main(int, char**)':
main.cpp:30:56: error: no matching function for call to 'Cache::get()'
main.cpp:30:56: note: candidates are:
main.cpp:7:18: note: template<int <anonymous> > std::tuple<> Cache::get()
main.cpp:7:18: note:   template argument deduction/substitution failed:
main.cpp:13:31: note: template<int index, class type, class ... rest> std::tuple<_Head, _Tail ...> Cache::get()
main.cpp:13:31: note:   template argument deduction/substitution failed:

Ponownie nie rozumiem błędów z powodu braku identyfikatora.

Zastanawiam się teraz, czy to, co chcę osiągnąć, jest nawet możliwe. Czy można użyć astd::tuple jak chcę? Czy jest jakiś lepszy sposób ?