我正在尝试将不同类型的值的集合映射到连续的空间中。为了做到这一点(有效地),我需要知道所有元素的总大小和每个元素的偏移量。如果将集合作为元组给出,则获取总大小很容易。它还允许预先计算偏移量。我很难获得类型的偏移量。为了简化示例,我假设Tuple中的类型将是唯一的(实际上Tuple将具有成对的唯一标记+非唯一值)。这是我当前未编译的尝试:
#include <cstddef>
#include <iostream>
#include <tuple>
struct A
{
std::size_t size()
{
return 3;
}
};
struct B
{
std::size_t size()
{
return 2;
}
};
struct C
{
std::size_t size()
{
return 4;
}
};
template <typename Tuple>
struct Foo
{
const Tuple& tuple_;
std::array<int, std::tuple_size<Tuple>::value> array_;
Foo(const Tuple& tuple) : tuple_(tuple)
{
std::cout << init() << '\n';
}
////////////////////////////////////////////////////////////////////////////////////////////////////
template <std::size_t INDEX = 0>
typename std::enable_if<std::tuple_size<Tuple>::value == INDEX, std::size_t>::type
init()
{
return 0;
}
////////////////////////////////////////////////////////////////////////////////////////////////////
template <std::size_t INDEX = 0>
typename std::enable_if<std::tuple_size<Tuple>::value != INDEX, std::size_t>::type
init()
{
auto offset = init<INDEX + 1>();
std::cout << "index: "<< INDEX << "; offset: " << offset << '\n';
array_[INDEX] = offset;
return offset + std::get<INDEX>(tuple_).size();
}
////////////////////////////////////////////////////////////////////////////////////////////////////
template <std::size_t INDEX = 0, typename T>
typename std::enable_if<std::tuple_size<Tuple>::value == INDEX, std::size_t>::type
offset(const T&)
{
return 0;
}
////////////////////////////////////////////////////////////////////////////////////////////////////
template <std::size_t INDEX = 0, typename T>
typename std::enable_if<(std::tuple_size<Tuple>::value != INDEX) && !std::is_same<typename std::tuple_element<INDEX, Tuple>::type, T>::value, std::size_t>::type
offset(const T& t)
{
return offset<INDEX + 1>(t);
}
////////////////////////////////////////////////////////////////////////////////////////////////////
template <std::size_t INDEX = 0, typename T>
typename std::enable_if<(std::tuple_size<Tuple>::value != INDEX) && std::is_same<typename std::tuple_element<INDEX, Tuple>::type, T>::value, std::size_t>::type
offset(const T&)
{
return array_[INDEX];
}
};
int main()
{
A a;
B b;
C c;
auto t = std::tie(a, b, c);
using T = decltype(t);
Foo<T> foo(t);
std::cout << foo.offset(a) << '\n';
//std::cout << foo.offset(b) << '\n';
//std::cout << foo.offset(c) << '\n';
}
编译器错误:
In file included from prog.cpp:3:0:
/usr/include/c++/4.8/tuple: In instantiation of ‘struct std::tuple_element<1u, std::tuple<C&> >’:
/usr/include/c++/4.8/tuple:680:12: recursively required from ‘struct std::tuple_element<2u, std::tuple<B&, C&> >’
/usr/include/c++/4.8/tuple:680:12: required from ‘struct std::tuple_element<3u, std::tuple<A&, B&, C&> >’
prog.cpp:79:22: recursively required from ‘typename std::enable_if<((std::tuple_size<_Tp>::value != INDEX) && (! std::is_same<typename std::tuple_element<INDEX, Tuple>::type, T>::value)), unsigned int>::type Foo<Tuple>::offset(const T&) [with unsigned int INDEX = 1u; T = A; Tuple = std::tuple<A&, B&, C&>; typename std::enable_if<((std::tuple_size<_Tp>::value != INDEX) && (! std::is_same<typename std::tuple_element<INDEX, Tuple>::type, T>::value)), unsigned int>::type = unsigned int]’
prog.cpp:79:22: required from ‘typename std::enable_if<((std::tuple_size<_Tp>::value != INDEX) && (! std::is_same<typename std::tuple_element<INDEX, Tuple>::type, T>::value)), unsigned int>::type Foo<Tuple>::offset(const T&) [with unsigned int INDEX = 0u; T = A; Tuple = std::tuple<A&, B&, C&>; typename std::enable_if<((std::tuple_size<_Tp>::value != INDEX) && (! std::is_same<typename std::tuple_element<INDEX, Tuple>::type, T>::value)), unsigned int>::type = unsigned int]’
prog.cpp:101:27: required from here
/usr/include/c++/4.8/tuple:680:12: error: invalid use of incomplete type ‘struct std::tuple_element<0u, std::tuple<> >’
struct tuple_element<__i, tuple<_Head, _Tail...> >
^
In file included from /usr/include/c++/4.8/tuple:38:0,
from prog.cpp:3:
/usr/include/c++/4.8/utility:84:11: error: declaration of ‘struct std::tuple_element<0u, std::tuple<> >’
class tuple_element;
^
您的代码是正确的,除了一个问题。
问题是您使用std::tuple
辅助函数构造了一个std::tie()
,这导致引用的元组-std::tuple<A&, B&, C&>
。
另一方面,您具有offset(const T& t)
,其中推导类型T
可以只是A
,B
或C
(即,引用不是推导类型的一部分,仅是参数的类型)。也就是说,您的条件is_same<tuple_element<...>::type, T>
始终会失败,因为一种类型是引用,而另一种类型不是,例如。 std::is_same<A&, A>
。为了解决这个问题,您应该先decay类型(或至少remove a reference),然后再将其与另一个类型进行相等性比较。
话虽如此,您的每种情况应如下所示:
std::is_same<typename std::decay<typename std::tuple_element<INDEX, Tuple>::type>::type, T>::value
// ~~~~~~~~~~~~~~~~~~~^ ~~~~~~^
或在c++14中:
std::is_same<std::decay_t<typename std::tuple_element<INDEX, Tuple>::type>::type>, T>::value
// ~~~~~~~~~~~~^ ^
似乎您想要类似的东西:
template <typename T, typename Tuple> struct get_index;
template <typename T, typename... Ts>
struct get_index<T, std::tuple<T, Ts...>> : std::integral_constant<std::size_t, 0> {};
template <typename T, typename Tail, typename... Ts>
struct get_index<T, std::tuple<Tail, Ts...>> :
std::integral_constant<std::size_t, 1 + get_index<T, std::tuple<Ts...>>::value> {};
然后将所有偏移方法替换为
template <typename T>
std::size_t
offset(const T&)
{
return array_[get_index<T&, Tuple>::value];
}
注意,在您的示例中,Tuple
是std::tuple<A&, B&, C&>
,并且您测试了与A
的相等性...