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cpp17_1.cpp
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cpp17_1.cpp
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#include <utility>
#include <iostream>
template<typename T1, typename T2>
struct plus;
template<typename T1, typename T2>
struct times;
template<typename T, typename U>
constexpr auto operator+(T l, U r){ return plus(l,r);}
template<typename T, typename U>
constexpr auto operator*(T l, U r){ return times(l,r);}
template<typename T1, typename T2>
struct plus{
constexpr plus(T1 t1_, T2 t2_):t1{t1_},t2{t2_}{}
T1 t1;
T2 t2;
template<typename T>
constexpr auto operator()(T&& t) const {return t1(std::forward<T>(t))+t2(std::forward<T>(t));}
};
template<typename T>
struct is_plus : std::false_type{};
template<typename T1, typename T2>
struct is_plus<plus<T1, T2>> : std::true_type{};
template<typename T1, typename T2>
struct times{
constexpr times(T1 t1_, T2 t2_):t1{t1_},t2{t2_}{}
T1 t1;
T2 t2;
template<typename T>
constexpr auto operator()(T&& t) const {return t1(std::forward<T>(t))*t2(std::forward<T>(t));}
};
template<typename T>
struct is_times : std::false_type{};
template<typename T1, typename T2>
struct is_times<times<T1, T2>> : std::true_type{};
template <typename T, typename U>
using same = std::is_same<typename std::decay<T>::type,typename std::decay<U>::type>;
int main(){
constexpr auto x = [](auto t){ return t ;};
constexpr auto c = [](auto t){ return [t](auto){return t;};};
constexpr auto zero = [](auto t){ return 0 ;};
constexpr auto one = [](auto t){ return 1 ;};
constexpr auto recursion = [one, zero, x](auto t, auto self){
using t_t = decltype(t);
using x_t = decltype(x);
if constexpr (same<t_t,x_t>::value)
return one;
else if constexpr( is_plus<typename std::decay<decltype(t)>::type>::value)
return plus{self(t.t1, self), self(t.t2, self)};
else if constexpr( is_times<typename std::decay<decltype(t)>::type>::value)
return plus{times{self(t.t1, self), t.t2}, times{t.t1, self(t.t2, self)}};
else
return zero;
};
constexpr auto D = [recursion](auto t){return recursion(t, recursion);};
auto constexpr ex = D(x*x*c(4.));
auto constexpr val=ex(5.);
std::cout<<val<<"\n";
}