C++ Lambdas Using lambdas for inline parameter pack unpacking


Example

C++14

Parameter pack unpacking traditionally requires writing a helper function for each time you want to do it.

In this toy example:

template<std::size_t...Is>
void print_indexes( std::index_sequence<Is...> ) {
  using discard=int[];
  (void)discard{0,((void)(
    std::cout << Is << '\n' // here Is is a compile-time constant.
  ),0)...};
}
template<std::size_t I>
void print_indexes_upto() {
  return print_indexes( std::make_index_sequence<I>{} );
}

The print_indexes_upto wants to create and unpack a parameter pack of indexes. In order to do so, it must call a helper function. Every time you want to unpack a parameter pack you created, you end up having to create a custom helper function to do it.

This can be avoided with lambdas.

You can unpack parameter packs into a set of invocations of a lambda, like this:

template<std::size_t I>
using index_t = std::integral_constant<std::size_t, I>;
template<std::size_t I>
constexpr index_t<I> index{};

template<class=void, std::size_t...Is>
auto index_over( std::index_sequence<Is...> ) {
  return [](auto&& f){
    using discard=int[];
    (void)discard{0,(void(
      f( index<Is> )
    ),0)...};
  };
}

template<std::size_t N>
auto index_over(index_t<N> = {}) {
  return index_over( std::make_index_sequence<N>{} );
}
C++17

With fold expressions, index_over() can be simplified to:

template<class=void, std::size_t...Is>
auto index_over( std::index_sequence<Is...> ) {
  return [](auto&& f){
    ((void)(f(index<Is>)), ...);
  };
}

Once you have done that, you can use this to replace having to manually unpack parameter packs with a second overload in other code, letting you unpack parameter packs "inline":

template<class Tup, class F>
void for_each_tuple_element(Tup&& tup, F&& f) {
  using T = std::remove_reference_t<Tup>;
  using std::tuple_size;
  auto from_zero_to_N = index_over< tuple_size<T>{} >();

  from_zero_to_N(
    [&](auto i){
      using std::get;
      f( get<i>( std::forward<Tup>(tup) ) );
    }
  );
}

The auto i passed to the lambda by the index_over is a std::integral_constant<std::size_t, ???>. This has a constexpr conversion to std::size_t that does not depend on the state of this, so we can use it as a compile-time constant, such as when we pass it to std::get<i> above.

To go back to the toy example at the top, rewrite it as:

template<std::size_t I>
void print_indexes_upto() {
  index_over(index<I>)([](auto i){
    std::cout << i << '\n'; // here i is a compile-time constant
  });
}

which is much shorter, and keeps logic in the code that uses it.

Live example to play with.