std::get on std::tuple 导致可变参数减少为无，类型错误不完整

Question

以下代码会产生大量编译器错误：

/// Uses template recursion to bind all args
template<std::size_t N, typename... Args> class Binder
{
public:
    Binder(Sqlite3StatementBase &s, std::tuple<Args...> &tup)
    {
        Binder<N - 1, Args...> b(s, tup);
        s.bind(N + 1, std::get<N, Args...>(tup)); // Line 182
    }
};

/// Specialization of Binder to end recursion at 0
template<typename... Args> class Binder<0, Args...>
{
public:
    Binder(Sqlite3StatementBase &s, std::tuple<Args...> &tup)
    {
        s.bind(1, std::get<0, Args...>(tup));
    }
};

第一批错误包括：

In file included from /usr/include/c++/6/bits/unique_ptr.h:37:0,
             from /usr/include/c++/6/condition_variable:43,
             from /home/tony/htpc/Dev/logi/src/db/logi-db.h:22,
             from /home/tony/htpc/Dev/logi/src/db/logi-sqlite.h:26,
             from /home/tony/htpc/Dev/logi/src/db/logi-sqlite.cpp:24:
/usr/include/c++/6/tuple: In instantiation of ‘class std::tuple_element<1ul, std::tuple<unsigned int> >’:
/usr/include/c++/6/tuple:1228:12:   recursively required from ‘class std::tuple_element<2ul, std::tuple<unsigned int, unsigned int> >’
/usr/include/c++/6/tuple:1228:12:   required from ‘class std::tuple_element<3ul, std::tuple<unsigned int, unsigned int, unsigned int> >’
/usr/include/c++/6/utility:106:69:   required by substitution of ‘template<long unsigned int __i, class _Tp> using __tuple_element_t = typename std::tuple_element::type [with long unsigned int __i = 3ul; _Tp = std::tuple<unsigned int, unsigned int, unsigned int>]’
/usr/include/c++/6/tuple:1270:5:   required by substitution of ‘template<long unsigned int __i, class ... _Elements> constexpr std::__tuple_element_t<__i, std::tuple<_Elements ...> >&& std::get(std::tuple<_Elements ...>&&) [with long unsigned int __i = 3ul; _Elements = {unsigned int, unsigned int, unsigned int}]’
/home/tony/htpc/Dev/logi/src/db/logi-sqlite.h:182:47:   required from ‘logi::Sqlite3Database::Binder<N, Args>::Binder(logi::Sqlite3Database::Sqlite3StatementBase&, std::tuple<_Elements ...>&) [with long unsigned int N = 3ul; Args = {unsigned int, unsigned int, unsigned int}]’
/home/tony/htpc/Dev/logi/src/db/logi-sqlite.h:238:71:   required from ‘void logi::Sqlite3Database::Sqlite3Statement<Args>::prepare_row(logi::Sqlite3Database::Sqlite3Statement<Args>::Tup&) [with Args = {unsigned int, unsigned int, unsigned int}; logi::Sqlite3Database::Sqlite3Statement<Args>::Tup = std::tuple<unsigned int, unsigned int, unsigned int>]’
/home/tony/htpc/Dev/logi/src/db/logi-sqlite.h:231:28:   required from ‘void logi::Sqlite3Database::Sqlite3Statement<Args>::execute(typename logi::Sqlite3Database::Sqlite3Statement<Args>::Parent::ArgsVector&) [with Args = {unsigned int, unsigned int, unsigned int}; typename logi::Sqlite3Database::Sqlite3Statement<Args>::Parent::ArgsVector = std::vector<std::tuple<unsigned int, unsigned int, unsigned int>, std::allocator<std::tuple<unsigned int, unsigned int, unsigned int> > >]’
/home/tony/htpc/Dev/logi/src/db/logi-sqlite.cpp:354:1:   required from here
/usr/include/c++/6/tuple:1228:12: error: invalid use of incomplete type ‘class std::tuple_element<0ul, std::tuple<> >’

所以似乎 std::get 调用导致系统库的其他部分（与 std::get 或 std::tuple 没有直接关系）递归减少 Args... 到什么都没有，这是独立于我对数字模板参数 N 的递归而发生的。是否可以在不从根本上改变我的方法的情况下在我的代码中修复某些东西，或者只是不可能在上下文中使用 std::get元组的模板参数是可变的？

Answer 1

鉴于 an implementation 的 C++14 make_index_sequence 函数（及其关联类型），您可以轻松地做您想做的事：

template<size_t ...indices, typename ...Args>
void binder_helper(Sqlite3StatementBase &s, std::integer_sequence<size_t, indices...>, std::tuple<Args...> &tup)
{
    auto dump = {(s.bind(indices, std::get<indices>(tup)), 0)...};
}

template<typename ...Args>
void binder(Sqlite3StatementBase &s, std::tuple<Args...> &tup)
{
    binder_helper(s, std::make_index_sequence<sizeof...(Args)>(), tup);
}

使用 C++17 的折叠表达式，binder_helper 中的古怪东西可以简化为更合理的东西：

template<size_t ...indices, typename ...Args>
void binder_helper(Sqlite3StatementBase &s, std::integer_sequence<size_t, indices...>, std::tuple<Args...> &tup)
{
    (s.bind(indices, std::get<indices>(tup)), ...);
}

请注意，在后一种情况下，标准确实不保证对s.bind的调用将按顺序执行。在前一种情况下，因为表达式包含在花括号初始化列表中，所以可以保证按顺序求值。

std::get on std::tuple 导致可变参数减少为无，类型错误不完整

std::get on std::tuple results in variadic args reducing to nothing with incomplete type error

gcc

variadic-templates

c++11

stdtuple