Apply stateful lambda to integer sequence values - c++

Question

I am playing around with trying to implement the numeric literal operator template.
#include <string_view>
#include <cstdint>
#include <cmath>
#include <iostream>
#include <boost/mp11/integer_sequence.hpp>
#include <boost/mp11/algorithm.hpp>
using namespace boost::mp11;
template <char... Cs>
[[nodiscard]] constexpr auto operator""_c(){
int weight =std::pow(10, sizeof... (Cs));
// unused, would like to transform it using lambda that mutably captures
// weight
using ints = index_sequence<sizeof... (Cs)>;
// ugly fold way
auto val = ((weight/=10,(int)(Cs-'0')*weight) + ...);
return val;
}
int main(){
std::cout << 0_c << std::endl;
std::cout << 00_c << std::endl;
std::cout << 01_c << std::endl;
std::cout << 123_c << std::endl;
}
This code works for simple cases(correctness is not important, e.g. negative numbers), it is just an example, but code looks ugly and clang emits a warning for modifying weight multiple times, so I guess code is buggy(undefined or unspecified behavior) although it seems to work...
Now I wonder is there a way for me to transform the ints I use(it is from boost::mp11, but same thing exists in std::) with a stateful lambda (that modifies weight).
So I would like to transfer ints, that are <0,1,2> into something like <100,10,1>
I presume this has been asked before but this is very hard to search for.
To be clear: operator "" is just a toy problem, my real question is about mapping the values of integer sequence with a stateful lambda.
Also if not clear from question: I am perfectly happy to use boost mp11, but could not find anything in the docs.

Answer 1

So I would like to transfer ints, that are <0,1,2> into something like
<100,10,1>
First, you can convert std::index_sequence to std::array, then perform your operations on it as you normally do, and finally, convert std::array to std::index_sequence again.
In order for the stateful lambda to work at compile-time, we can accept a function that can return the stateful lambda then get it internally:
template<std::size_t... Is>
constexpr auto transform_seq(std::index_sequence<Is...>, auto get_op) {
// index_sequence -> array
constexpr auto arr = [op = get_op()]() mutable {
std::array<std::size_t, sizeof...(Is)> arr{Is...};
for (auto& value : arr)
value = op(value);
return arr;
}();
// array -> index_sequence
constexpr auto seq = [&]<std::size_t... Js>(std::index_sequence<Js...>) {
return std::index_sequence<std::get<Js>(arr)...>{};
}(std::make_index_sequence<arr.size()>{});
return seq;
};
Then you can perform the index_sequence conversion according to op you pass in:
using input1 = std::index_sequence<0,1,2>;
auto gen_op1 = [] {
return [w = 1000](auto x) mutable { w /= 10; return w; };
};
using res1 = decltype(transform_seq(input1{}, gen_op1));
static_assert(std::same_as<res1, std::index_sequence<100, 10, 1>>);
using input2 = std::index_sequence<0,1,2,3>;
auto gen_op2 = [] {
return [b = true] (auto x) mutable { b = !b; return b * 10 + x; };
};
using res2 = decltype(transform_seq(input2{}, gen_op2));
static_assert(std::same_as<res2, std::index_sequence<0,11,2,13>>);
Demo.

Answer 2

I think you want:
template <typename F, std::size_t ... Is>
constexpr auto apply(F f, std::index_sequence<Is...>)
-> std::index_sequence<f(Is)...>
{
return {};
}
template <char... Cs>
[[nodiscard]] constexpr auto operator""_c(){
return []<std::size_t ... Pows>(std::index_sequence<Pows...>){
return ((Pows * (Cs - '0')) + ...);
}(apply([](std::size_t n){ return ipow(10, sizeof...(Cs) - n - 1);},
std::make_index_sequence<sizeof...(Cs)>()));
}
Demo
But doing computation directly seems even simpler:
template <char... Cs>
[[nodiscard]] constexpr auto operator""_c(){
constexpr auto res =
[]<std::size_t ... Is>(std::index_sequence<Is...>){
return ((ipow(10, sizeof...(Cs) - Is - 1) * (Cs - '0')) + ...);
}(std::make_index_sequence<sizeof...(Cs)>());
return res;
}