3.2.2 Compile-time Integer Sequences

What was the problem

PIC <utility>

If you start playing with parameter packs [] and tuples [2.5.8], you will soon encounter a case where you will need to so something “for each item in the tuple”.

A typical use case consists in calling a function with its arguments taken from a tuple, something we already tried in the previous pages [], in an argument-bindings example. Go check, and see how much code we wrote to store an sequence of integers in a type.

How the Problem is Solved

Starting with C++14, this code can be replaced by std::integer_sequence<typename T, T... Ints> and std::make_integer_sequence<typename T, T N>. Writing a function that runs a function with its arguments taken from a tuple is approximately as simple as this:

// The call_helper will help us to get the elements of a tuple, because we 
// cannot get them by type due to possibly duplicate types. 
template<typename IntegerSequence> 
struct call_helper; 
template<unsigned... I> 
struct call_helper<std::integer_sequence<std::size_t, I...>> 
  template<typename F, typename... Args> 
  static void call(F&& function, std::tuple<Args...>&& arguments) 
    // Here we unpack I... to get the arguments from the tuple. 
    // See [2.3.3]. 
    // Note that it does not work with member functions, as they require another 
    // call syntax. This is still left as an exercise for the reader. 
template<typename F, typename... Args> 
decltype(auto) apply(F&& function, std::tuple<Args...>&& arguments) 
  return call_helper 
      std::make_integer_sequence<std::size_t, sizeof...(Args)> 
    >::call(std::forward<F>(function), std::move(arguments)); 

Isn’t it clean? Hold on, we’ll soon be able to remove this code too [??]4!

4And it will be for the best, as this example code misses many features, like calling member functions, accepting a const tuple as argument, and probably more.