template parameter packs access Nth type and Nth element
C++C++11Variadic TemplatesC++ Problem Overview
The following paper is the first proposal I found for template parameter packs.
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1603.pdf
At page 16, it talks about introducing two new operators [] and <> for accessing parameter pack elements and parameter pack types.
The suggested syntax for such an operator involves two new operators: .[] to access values and .<> to access types. For instance:
template<int N, typename Tuple> struct tuple_element;
template<int N, ... Elements>
struct tuple_element<tuple<Elements...> >
{
typedef Elements.<N> type;
};
template<int N, ... Elements>
Elements.<N>& get(tuple<Elements...>& t)
{ return t.[N]; }
template<int N, ... Elements>
const Elements.<N>& get(const tuple<Elements...>& t)
{ return t.[N]; }
So where are these operators? If there is none, what is their replacement?
C++ Solutions
Solution 1 - C++
Others have already answered that it can be done via std::tuple. If you want to access the Nth type of a parameter pack, you may find the following metafunction handy:
template<int N, typename... Ts> using NthTypeOf =
typename std::tuple_element<N, std::tuple<Ts...>>::type;
Usage:
using ThirdType = NthTypeOf<2, Ts...>;
Solution 2 - C++
C++11 doesn't have corresponding operators which is the reason they are proposed. With C++11 you'll need to either extract the corresponding information yourself or use a class which already does the necessary operation. The easiest approach is probably to just use std::tuple<T...> which already implements the corresponding logic.
If you wonder how std::tuple<T...> currently implements these operations: it is basically an exercise in functional programming using a fairly bad functional programming notation. Once you know how to get the n-th type of the sequence, getting the n-th element using inheritance from base classes parameterized on index and type is fairly trivial. Implementing something like tuple_element<N, T...> could look something like this:
template <int N, typename... T>
struct tuple_element;
template <typename T0, typename... T>
struct tuple_element<0, T0, T...> {
typedef T0 type;
};
template <int N, typename T0, typename... T>
struct tuple_element<N, T0, T...> {
typedef typename tuple_element<N-1, T...>::type type;
};
The actual more challenging bit in implementing something like std::tuple<T...> is conjuring up a list of indices so you got a parallel list of type and integers which can then be expanded, e.g., for a list of base classes using something like (how the internal details look exactly will differ but the basic idea of having a parallel parameters packs for the types and their indices will be somehow there):
template <typename... T, int... I>
class tuple_base<tuple_types<T...>, tuple_indices<I...>>:
public tuple_field<T, I>... {
};
Solution 3 - C++
Access N-th element?
Using std::forward_as_tuple:
template <int I, class... Ts>
decltype(auto) get(Ts&&... ts) {
return std::get<I>(std::forward_as_tuple(ts...));
}
Example usage:
template<class...Ts>
void foo(Ts&&...ts){
auto& first = get<0>(ts...);
auto second = get<1>(ts...);
first = 'H';
second = 'E';
(std::cout << ... << ts);
}
foo('h','e','l','l','o');
// prints "Hello"
This answer is to supplement Emile Cormier's answer which gives only the n-th type.
Solution 4 - C++
To get the Nth element from a pack you can write:
Option 1
Using tuple_element for getting the return type for the Nth element:
template<size_t index, typename T, typename... Ts>
inline constexpr typename enable_if<index==0, T>::type
get(T&& t, Ts&&... ts) {
return t;
}
template<size_t index, typename T, typename... Ts>
inline constexpr typename enable_if<(index > 0) && index <= sizeof...(Ts),
typename tuple_element<index, tuple<T, Ts...>>::type>::type
get(T&& t, Ts&&... ts) {
return get<index-1>(std::forward<Ts>(ts)...);
}
// below is optional - just for getting a more readable compilation error
// in case calling get with a bad index
inline template<long long index, typename... Ts>
constexpr bool index_ok() {
return index >= 0 && index < sizeof...(Ts);
}
template<long long index, typename T, typename... Ts>
inline constexpr
typename enable_if<!index_ok<index, T, Ts...>(), T>::type
get(T&& t, Ts&&... ts) {
static_assert(index_ok<index, T, Ts...>(),
"bad index in call to get, smaller than zero or above pack size");
return t;
}
Option 2
Without using tuple, relying on auto return type and specifically on C++14 decltype(auto) and on using enable_if as a template parameter and not as a return type:
template<size_t index, typename T, typename... Ts,
typename enable_if<index==0>::type* = nullptr>
inline constexpr decltype(auto) get(T&& t, Ts&&... ts) {
return std::forward<T>(t);
}
template<size_t index, typename T, typename... Ts,
typename enable_if<(index > 0 && index <= sizeof...(Ts))>::type* = nullptr>
inline constexpr decltype(auto) get(T&& t, Ts&&... ts) {
return get<index-1>(std::forward<Ts>(ts)...);
}
template<long long index, typename... Ts>
inline constexpr bool index_ok() {
return index >= 0 && index < (long long)sizeof...(Ts);
}
// block (compilation error) the call to get with bad index,
// providing a readable compilation error
template<long long index, typename T, typename... Ts,
typename enable_if<(!index_ok<index, T, Ts...>())>::type* = nullptr>
inline constexpr decltype(auto) get(T&& t, Ts&&... ts) {
static_assert(index_ok<index, T, Ts...>(),
"bad index in call to get, smaller than zero or above pack size");
return std::forward<T>(t); // need to return something...
// we hope to fail on the static_assert above
}
Usage example:
template<size_t index, typename... Ts>
void resetElementN(Ts&&... ts) {
get<index>(std::forward<Ts>(ts)...) = {}; // assuming element N has an empty ctor
}
int main() {
int i = 0;
string s = "hello";
get<0>(i,2,"hello","hello"s, 'a') += get<0>(2);
get<1>(1,i,"hello",4) += get<1>(1, 2);
get<3>(1,2,"hello",i) += get<2>(0, 1, 2);
get<2>(1,2,s,4) = get<2>(0, 1, "hi");
cout << i << ' ' << s << endl;
resetElementN<1>(0, i, 2);
resetElementN<0>(s, 1, 2);
cout << i << ' ' << s << endl;
// not ok - and do not compile
// get<0>(1,i,"hello","hello"s) = 5;
// get<1>(1,i*2,"hello") = 5;
// get<2>(1,i*2,"hello")[4] = '!';
// resetElementN<1>(s, 1, 2);
// ok
const int j = 2;
cout << get<0>(j,i,3,4) << endl;
// not ok - and do not compile
// get<0>(j,i,3,4) = 5;
// not ok - and do not compile
// with a readable compilation error
// cout << get<-1>("one", 2, '3') << endl;
// cout << get<3>("one", 2, '3') << endl;
}
Code
Option 1: http://coliru.stacked-crooked.com/a/60ad3d860aa94453<br/>
Option 2: http://coliru.stacked-crooked.com/a/09f6e8e155612f8b<br/>
Solution 5 - C++
We can implement a simple function to get nth parameter directly without any recursive calls but many pure type operations in compile-time. Let's look at the key code firstly:
template<class...Ts>
struct GetImp {
template<class T, class...Us>
static decltype(auto) impl(Ts&&..., T&& obj, Us&&...) {
return std::forward<T>(obj);
}
};
template<size_t n, class...Ts>
decltype(auto) get(Ts&&...args) {
static_assert(n<sizeof...(args), "index over range");
return Transform<GetImp, Before_s<n, Seq<Ts...>> >
::impl(std::forward<Ts>(args)...);
}
What does Transform means?
For example, if we have a type T that is std::tuple<int,double,float>,
then Transform<GetImp,T> would be GetImp<int,double,float>.
note that I define another empty struct "Seq" instead of std::tuple to do
the same thing with less compile time.(In fact both of them could be compiled very quickly,but I guess an empty struct would be more effectively)
So Before_s<n,Seq<Ts...>> generate a Seq<?> and then we transform it into GetImp, so that we can know what type of [0]~[n-1] parameters are,
and then drop them off to index the nth parameter directly.
For example, Before_s<3,Seq<T0,T1,T2,T3,T4...>> is Seq<T0,T1,T2>,
Before_s<2,Seq<T0,T1,T2,T3,T4...>> is Seq<T0,T1> etc.
We use Before_s to deal with our Seq type to reduce compile time when we
use one meta function to implement another meta function for less compile
time.
Implementation
#define OMIT_T(...) typename __VA_ARGS__::type
template<class...Args>
struct Seq { };
template< template<class...> class Dst >
struct TransformImp{
template< template<class...>class Src, class...Args >
static Dst<Args...> from(Src<Args...>&&);
};
template< template<class...> class Dst, class T>
using Transform = decltype(TransformImp<Dst>::from(std::declval<T>()));
template<class T>
using Seqfy = Transform<Seq, T>;
template<class...>struct MergeImp;
template<class...Ts, class...Others>
struct MergeImp<Seq<Ts...>, Seq<Others...>>
{
using type = Seq<Ts..., Others...>;
};
template<class first, class second>
using Merge = OMIT_T(MergeImp<Seqfy<first>, Seqfy<second> >);
template<class T, class U>
using Merge_s = OMIT_T(MergeImp<T, U>);
template<size_t, class...>struct BeforeImp;
template<size_t n, class T, class...Ts>
struct BeforeImp<n, Seq<T, Ts...>> {
static_assert(n <= sizeof...(Ts)+1, "index over range");
using type = Merge_s<Seq<T>, OMIT_T(BeforeImp<n - 1, Seq<Ts...>>)>;
};
template<class T, class...Ts>
struct BeforeImp<1, Seq<T, Ts...>> {
using type = Seq<T>;
};
template<class T, class...Ts>
struct BeforeImp<0, Seq<T, Ts...>> {
using type = Seq<>;
};
template<size_t n>
struct BeforeImp<n, Seq<>> {
using type = Seq<>;
};
template<size_t n, class T>
using Before = OMIT_T(BeforeImp<n, Seqfy<T>>);
template<size_t n, class T>
using Before_s = OMIT_T(BeforeImp<n, T>);
Edited: Advanced Implementation
We needn't use Before_s to calculate n-1 types before nth type,instead, we can ignore them:
struct EatParam{
constexpr EatParam(...)noexcept{}
};
template<size_t n>
struct GenSeqImp {
using type = Merge_s<OMIT_T(GenSeqImp<n / 2>), OMIT_T(GenSeqImp<n - n / 2>)>;
};
template<>
struct GenSeqImp<0> {
using type = Seq<>;
};
template<>
struct GenSeqImp<1> {
using type = Seq<EatParam>;
};
template<size_t n>
using GenSeq = OMIT_T(GenSeqImp<n>);
template<class...Ts>
struct GetImp {
template<class T>
static constexpr decltype(auto) impl(Ts&&..., T&& obj, ...)noexcept {
return std::forward<T>(obj);
}
};
template<size_t n, class...Ts>
constexpr decltype(auto) get(Ts&&...args)noexcept {
static_assert(n<sizeof...(args), "index over range.");
//return Transform<GetImp, Before_s<n, Seq<Ts...>> >
return Transform<GetImp, GenSeq<n>>
::impl(std::forward<Ts>(args)...);
}
In addition , there is a very interesting article about implementation of getting nth type:
Thanks for their work, I didn't know we could use (...) to do the hack before.