###### tags: `C++`
# Meta Programming to unroll a function

> The code is copied from the [Lightning Talk: C++20 - A New Way of Meta-Programming? - Kris Jusiak - CppCon 2022](https://www.youtube.com/watch?v=zRYlQGMdISI&t=271s)

## What I want:

void print() {
    std::cout<<"hello world\n";
}
int main() {
    unroll<5>(print);
}

/* expected output

hello world
hello world
hello world
hello world
hello world

*/

## template design:

### The method in the video before c++20

template<auto N>
constexpr auto unroll = [](auto expr) {
    [expr]<auto ...Is>(std::index_sequence<Is...>) {
        ((expr(), void(Is)), ...);
    }(std::make_index_sequence<N>{});
};

### The method should be after c++20

template<auto N>
constexpr auto unroll = [](auto expr) {
    auto [... Is] = std::make_index_sequence<N>{};
    ((expr(), void(Is)), ...);
}

### The method without lambda

template <typename Callable, auto... Is>
constexpr void deep_unroll(Callable expr, std::index_sequence<Is...>) {
    ((expr(), void(Is)), ...);
}
template <auto N, typename Callable>
constexpr void unroll2(Callable expr) {
    deep_unroll(expr, std::make_index_sequence<N>{});
}

### demo code
[code](https://godbolt.org/z/oMhrKc1Md)