构造std :: tuple时定义元素初始化顺序 [英] Defining element initialization order when constructing std::tuple
问题描述
我想将元素的初始化值存储在单独的元组内的元组中，以便可以将相同的值用作相应类型的其他元组的预设。
I would like to store the initialization values for elements in a tuple inside a separate tuple, so that I can use the same values as a preset for other tuples of the respective type.
对我的程序而言，元组元素的构造函数以从左到右的顺序调用是非常重要的（否则，它充其量是非常令人困惑的）。
It is very important to my program that the constructors of the tuple elements are called in left-to-right order (otherwise it would at best turn out very confusing).
这是我程序的简化版本：
Here is a simplified version of my program:
#include <tuple>
// Elements are the end points of a Widget hierarchy
struct Element
{
using initer_t = int;
Element( const initer_t pIniter )
:data{ pIniter }
{
printf("Creating %i\n", data);
}
const initer_t data;
};
// A Widget class stores any number of Elements and/or other Widget instances
template<typename... Elems>
struct Widget
{
using initer_t = std::tuple<typename Elems::initer_t...>;
Widget( const initer_t pIniter )
:elements{ pIniter }
{}
const std::tuple<Elems...> elements;
};
int main()
{
using Button = Widget<Element, Element>;
using ButtonList = Widget<Button, Button, Element>;
Button::initer_t basic_button_initer{ 0, 1 }; // presets for Buttons
Button::initer_t other_button_initer{ 2, 3 };
ButtonList::initer_t buttonlist_initer{ basic_button_initer, other_button_initer, 4 }; //a preset for a ButtonList
ButtonList buttonlist{ buttonlist_initer };
return 0;
}
所以我正在初始化 std :: tuple<元素...> 成员和 Widget< Elems ...> 的元素 std :: tuple< typename Elems :: initer_t。 ..> 在 Widget< Elems ...> 的构造函数初始化列表中。
应该用 initer_t 定义的类型的相应初始化值来初始化 elements 的每个元素。使用pIniter中的值。
initer_t 类型是Widget层次结构中每个成员的类型（例如 Widget< typename ...> 或 Element ），这是初始化层次结构成员的类型。
但是这种情况发生的顺序是从右到左，而我需要从左到右。
So I am initializing the std::tuple<Elems...> elements member of Widget<Elems...> with a std::tuple<typename Elems::initer_t...> in Widget<Elems...>'s constructor initialization list.
This is supposed to initialize each element of elements with its corresponding initialization value of the type defined by initer_t using the values in pIniter.
The initer_t type is a type for each member of a Widget hierarchy(for example a Widget<typename...> or an Element), which is the type that the hierarchy member should be initialized with.
But the order in which this happens is right-to-left, while I need it in left-to-right.
程序的输出是
Creating 4
Creating 3
Creating 2
Creating 1
Creating 0
但是我想撤销此顺序。
在这种情况下，我该怎么办？
How can I do this in this case?
推荐答案
对于对解决方案感兴趣的人，我想出了一种方法控制初始化顺序并保留 elements 的常数：
For anyone interested in a solution, I came up with a way to control the initialization order and retain the constness of elements:
#include <tuple>
template<typename... Elems>
struct construct
{
template<size_t... Ns, typename Head, typename... Rest>
static constexpr const std::tuple<Rest...>
drop_head_impl( const std::index_sequence<Ns...> ns,
const std::tuple<Head, Rest...> tup )
{
return std::tuple<Rest...>( std::get<Ns + 1u>( tup )... );
}
template<typename Head, typename... Rest>
static constexpr const std::tuple<Rest...>
drop_head( const std::tuple<Head, Rest...> tup )
{
return drop_head_impl( std::make_index_sequence<sizeof...(Rest)>(), tup );
}
template<typename Head>
static constexpr const std::tuple<Head>
func_impl( const std::tuple<typename Head::initer_t> initer )
{
return std::tuple<Head>( { std::get<0>( initer ) } );
}
template<typename Head, typename Next, typename... Rest>
static constexpr const std::tuple<Head, Next, Rest...>
func_impl( const std::tuple<typename Head::initer_t, typename Next::initer_t, typename Rest::initer_t...> initer )
{
std::tuple<Head> head( { std::get<0>( initer ) } );
return std::tuple_cat( head, func_impl<Next, Rest...>( drop_head(initer) ) );
}
static constexpr const std::tuple<Elems...>
func( const std::tuple<typename Elems::initer_t...> initer )
{
return func_impl<Elems...>( initer );
}
};
// Elements are the end points of a Widget hierarchy
struct Element
{
using initer_t = int;
Element( const initer_t pIniter )
:data{ pIniter }
{
printf( "Creating %i\n", data );
}
const initer_t data;
};
// A Widget class stores any number of Elements and/or other Widget instances
template<typename... Elems>
struct Widget
{
using initer_t = std::tuple<typename Elems::initer_t...>;
Widget( const initer_t pIniter )
:elements( construct<Elems...>::func( pIniter ) )
{}
const std::tuple<Elems...> elements;
};
int main()
{
using Button = Widget<Element, Element>;
using ButtonList = Widget<Button, Button, Element>;
Button::initer_t basic_button_initer{ 0, 1 }; // presets for Buttons
Button::initer_t other_button_initer{ 2, 3 };
ButtonList::initer_t buttonlist_initer{ basic_button_initer, other_button_initer, 4 }; //a preset for a ButtonList
ButtonList buttonlist{ buttonlist_initer };
return 0;
}
构造结构接受 initer_t s（初始值）的元组，构造一个包含 Elems ... 的第一个元素的元组。 initer的第一个元素，然后删除initer的第一个元素，并将剩余的元组传递给自身，这将导致使用下一个 Elems ... 元素的元组initer中的下一个元素。对于具有一个元素的元组， func_impl 的重载停止了此递归，该元组只是从 initer_t 中构造该元素一个元组并返回它。该单元素元组被连接到具有前一个元素的元组，结果被返回到更高的级别，并被连接到那里的单元素元组，依此类推。
The construct structure takes the tuple of initer_ts (initer), constructs a tuple containing the first element of Elems... using the first element of initer, then drops the first element of initer and passes the remaining tuple to itself, which causes a tuple with the next element of Elems... to be constructed using the next element in initer. This recursion is stopped by an overload of func_impl for a tuple with one element which simply constructs that element from its initer_t in a tuple and returns it. This single-element tuple gets concatenated to the tuple with the previous element, the result gets returned to the higher level and is concatenated to the single-element tuple there and so on.
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