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Is there anything better in c++17 (maybe C++2a) than the classic C++14 way to unpack variadic tuple with std::index_sequence?
Anything better than this:
template <typename ...I>
class MultiIterator
{
public:
MultiIterator(I const& ...i)
: i(i...)
{}
MultiIterator& operator ++()
{
increment(std::index_sequence_for<I...>{});
return *this;
}
private:
template <std::size_t ...C>
void increment(std::index_sequence<C...>)
{
std::ignore = std::make_tuple(++std::get<C>(i)...);
}
std::tuple<I...> i;
};
Like fold expression, structured-bindings? Any hint? I can accept answer why I cannot use these mentioned C++17 features here - but I prefer "solution.
656
std::tuple_cat () concatenates two tuples into one. But I recently needed a tuple_cdr () to remove the first item from the tuple, leaving me the rest. A tuple_car () would just do std::get (tuple) to get the first time, so nobody bothers to write one.
Unpacking a fixed size container in C++ can be tedious, and require you to fiddle around with std::get or std::tie. But not anymore, thanks to the new structured bindings introduced in C++17.
Tuples in C++. 1. get() :- get() is used to access the tuple values and modify them, it accepts the index and tuple name as arguments to access a particular tuple element. 2. make_tuple() :- make_tuple() is used to assign tuple with values. The values passed should be in order with the values declared in tuple.
But now you have a tuple instead of a parameter pack. Trick with std::index_sequence<> and a helper method lets you call a normal method (that takes normal parameters), passing a tuple that holds those parameter values: template <class... T_values>
Since C++14 we have generic lambdas, and since C++17 we have fold expressions and std::apply effectively hiding the usual unpack logic:
std::apply( [](auto&... i){ ((void)++i,...); }, some_tuple );
note: for your information, the (void) thing is just to avoid any custom comma operator to kick in... you never know :)
62
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