[英]Variadic template declaration from single integer argument?
Is it possible to statically declare N same-type template arguments from single integer type template argument?是否可以从单个整数类型模板参数静态声明N 个相同类型的模板参数? Potentially something similar to this:
可能与此类似的内容:
template < int N >
class MyTemplatedType {
// base type, known
typedef float base_t;
// instance of some other templated class with *N* template arguments
SomeOtherClass< base_t, base_t, base_t, base_t, ..., base_t > foo;
/* ... */
}
I understand I can use a variadic template and use it directly to the member instance declaration, but I was wondering if there would be some kind of SFINAE implementation that would resolve a single integer template parameter, as the syntax would be much cleaner and intuitive.我知道我可以使用可变参数模板并将其直接用于成员实例声明,但我想知道是否会有某种 SFINAE 实现可以解析单个整数模板参数,因为语法会更清晰和直观。
One possible solution, using tuples and helper templates to assemble the desired class.一种可能的解决方案,使用元组和辅助模板来组装所需的类。 A little additional sugar can make the resulting syntax "cleaner" Tested with gcc 10.2:
一点额外的糖可以使生成的语法“更干净” 用 gcc 10.2 测试:
#include <tuple>
#include <type_traits>
// Create std::tuple<T ...>, with T repeated N times.
template<int N, typename T>
struct tuple_list : tuple_list<N-1, T> {
typedef decltype(std::tuple_cat(std::declval<typename tuple_list<N-1, T>
::tuple_t>(),
std::declval<std::tuple<T> &&>())
) tuple_t;
};
template<typename T>
struct tuple_list<0, T> {
typedef std::tuple<> tuple_t;
};
template<typename ...> struct SomeOtherClass {};
// And now, replace `std::tuple` with SomeOtherClass
template<typename tuple_t> struct make_some_other_class;
template<typename ...Args>
struct make_some_other_class<std::tuple<Args...>> {
typedef SomeOtherClass<Args...> type_t;
};
template < int N >
class MyTemplatedType {
typedef float base_t;
public:
// The payload is not exactly "clean", but one more helper
// template can make the syntax here a little bit nicer...
typename make_some_other_class< typename tuple_list<N, base_t>
::tuple_t >::type_t foo;
};
void foobar(MyTemplatedType<3> &bar)
{
SomeOtherClass<float, float, float> &this_works=bar.foo;
}
You can write a meta-function that accepts N
, base_t
, and SomeOtherClass
, and recursively calls itself with a smaller N
, each time tacking on base_t
to the end of a growing parameter pack:您可以编写一个接受
N
、 base_t
和SomeOtherClass
的元函数,并使用较小的N
递归调用自身,每次将base_t
到不断增长的参数包的末尾:
template <int N, typename T, template<typename...> typename C, typename ...Ts>
struct expand {
using type = typename expand<N-1, T, C, T, Ts...>::type;
};
For the base case, when N
goes down to 0, the meta-function yields SomeOtherClass
instantiated with the parameter pack of N
base_t
types:对于基本情况,当
N
下降到 0 时,元函数产生SomeOtherClass
用N
base_t
类型的参数包实例化:
template <typename T, template<typename...> typename C, typename ...Ts>
struct expand<0, T, C, Ts...> {
using type = C<Ts...>;
};
Also, convenience alias is nice to avoid having to say typename
at the call site:此外,便利别名可以很好地避免在调用站点上说
typename
:
template <int N, typename T, template<typename...> typename C>
using expand_t = typename expand<N, T, C>::type;
Now at the call site you can write:现在在呼叫站点你可以写:
expand_t<N, base_t, SomeOtherClass> foo;
// equivalent to
// SomeOtherClass< base_t, base_t, ..., base_t > foo;
// ^^^ N times ^^^
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