I have function at
designed to access std::tuple element by index specified in runtime
template<std::size_t _Index = 0, typename _Tuple, typename _Function>
inline typename std::enable_if<_Index == std::tuple_size<_Tuple>::value, void>::type
for_each(_Tuple &, _Function)
{}
template<std::size_t _Index = 0, typename _Tuple, typename _Function>
inline typename std::enable_if < _Index < std::tuple_size<_Tuple>::value, void>::type
for_each(_Tuple &t, _Function f)
{
f(std::get<_Index>(t));
for_each<_Index + 1, _Tuple, _Function>(t, f);
}
namespace detail { namespace at {
template < typename _Function >
struct helper
{
inline helper(size_t index_, _Function f_) : index(index_), f(f_), count(0) {}
template < typename _Arg >
void operator()(_Arg &arg_) const
{
if(index == count++)
f(arg_);
}
const size_t index;
mutable size_t count;
_Function f;
};
}} // end of namespace detail
template < typename _Tuple, typename _Function >
void at(_Tuple &t, size_t index_, _Function f)
{
if(std::tuple_size<_Tuple> ::value <= index_)
throw std::out_of_range("");
for_each(t, detail::at::helper<_Function>(index_, f));
}
It has linear complexity. How can i achive O(1) complexity?
Assuming you pass something similar to a generic lambda, i.e. a function object with an overloaded function call operator:
#include <iostream>
struct Func
{
template<class T>
void operator()(T p)
{
std::cout << __PRETTY_FUNCTION__ << " : " << p << "\n";
}
};
The you can build an array of function pointers:
#include <tuple>
template<int... Is> struct seq {};
template<int N, int... Is> struct gen_seq : gen_seq<N-1, N-1, Is...> {};
template<int... Is> struct gen_seq<0, Is...> : seq<Is...> {};
template<int N, class T, class F>
void apply_one(T& p, F func)
{
func( std::get<N>(p) );
}
template<class T, class F, int... Is>
void apply(T& p, int index, F func, seq<Is...>)
{
using FT = void(T&, F);
static constexpr FT* arr[] = { &apply_one<Is, T, F>... };
arr[index](p, func);
}
template<class T, class F>
void apply(T& p, int index, F func)
{
apply(p, index, func, gen_seq<std::tuple_size<T>::value>{});
}
Usage example:
int main()
{
std::tuple<int, double, char, double> t{1, 2.3, 4, 5.6};
for(int i = 0; i < 4; ++i) apply(t, i, Func{});
}
clang++ also accepts an expansion applied to a pattern that contains a lambda expression:
static FT* arr[] = { [](T& p, F func){ func(std::get<Is>(p)); }... };
(although I've to admit that looks really weird)
g++4.8.1 rejects this.
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