如何在 C++ 11 中迭代 std::tuple

I want to know how should I iterate over it? There is tupl_size(), but reading the documentation, I didn't get how to utilize it. Also I have search SO, but questions seem to be around Boost::tuple .

auto some = make_tuple("I am good", 255, 2.1);

Here is an attempt to break down iterating over a tuple into component parts.

First, a function that represents doing a sequence of operations in order. Note that many compilers find this hard to understand, despite it being legal C++11 as far as I can tell:

template<class... Fs>
void do_in_order( Fs&&... fs ) {
  int unused[] = { 0, ( (void)std::forward<Fs>(fs)(), 0 )... }
  (void)unused; // blocks warnings
}

Next, a function that takes a std::tuple, and extracts the indexes required to access each element. By doing so, we can perfect forward later on.

As a side benefit, my code supports std::pair and std::array iteration:

template<class T>
constexpr std::make_index_sequence<std::tuple_size<T>::value>
get_indexes( T const& )
{ return {}; }

The meat and potatoes:

template<size_t... Is, class Tuple, class F>
void for_each( std::index_sequence<Is...>, Tuple&& tup, F&& f ) {
  using std::get;
  do_in_order( [&]{ f( get<Is>(std::forward<Tuple>(tup)) ); }... );
}

and the public-facing interface:

template<class Tuple, class F>
void for_each( Tuple&& tup, F&& f ) {
  auto indexes = get_indexes(tup);
  for_each(indexes, std::forward<Tuple>(tup), std::forward<F>(f) );
}

while it states Tuple it works on std::arrays and std::pairs. It also forward the r/l value category of said object down to the function object it invokes. Also note that if you have a free function get<N> on your custom type, and you override get_indexes, the above for_each will work on your custom type.

As noted, do_in_order while neat isn't supported by many compilers, as they don't like the lambda with unexpanded parameter packs being expanded into parameter packs.

We can inline do_in_order in that case

template<size_t... Is, class Tuple, class F>
void for_each( std::index_sequence<Is...>, Tuple&& tup, F&& f ) {
  using std::get;
  int unused[] = { 0, ( (void)f(get<Is>(std::forward<Tuple>(tup)), 0 )... }
  (void)unused; // blocks warnings
}

this doesn't cost much verbosity, but I personally find it less clear. The shadow magic of how do_in_order works is obscured by doing it inline in my opinion.

index_sequence (and supporting templates) is a C++14 feature that can be written in C++11. Finding such an implementation on stack overflow is easy. A current top google hit is a decent O(lg(n)) depth implementation, which if I read the comments correctly may be the basis for at least one iteration of the actual gcc make_integer_sequence (the comments also point out some further compile-time improvements surrounding eliminating sizeof... calls).

Alternatively we can write:

template<class F, class...Args>
void for_each_arg(F&&f,Args&&...args){
  using discard=int[];
  (void)discard{0,((void)(
    f(std::forward<Args>(args))
  ),0)...};
}

And then:

template<size_t... Is, class Tuple, class F>
void for_each( std::index_sequence<Is...>, Tuple&& tup, F&& f ) {
  using std::get;
  for_each_arg(
    std::forward<F>(f),
    get<Is>(std::forward<Tuple>(tup))...
  );
}

Which avoids the manual expand yet compiles on more compilers. We pass the Is via the auto&&i parameter.

In C++1z we can also use std::apply with a for_each_arg function object to do away with the index fiddling.