How to iterate over a std::tuple in C++ 11 [duplicate]

Answer 1

Here is a similar and more verbose solution than the formerly accepted one given by T.C., which is maybe a little bit easier to understand (-- it's probably the same as thousand others out there in the net):

template<typename TupleType, typename FunctionType>
void for_each(TupleType&&, FunctionType
            , std::integral_constant<size_t, std::tuple_size<typename std::remove_reference<TupleType>::type >::value>) {}

template<std::size_t I, typename TupleType, typename FunctionType
       , typename = typename std::enable_if<I!=std::tuple_size<typename std::remove_reference<TupleType>::type>::value>::type >
void for_each(TupleType&& t, FunctionType f, std::integral_constant<size_t, I>)
{
    f(std::get<I>(std::forward<TupleType>(t)));
    for_each(std::forward<TupleType>(t), f, std::integral_constant<size_t, I + 1>());
}

template<typename TupleType, typename FunctionType>
void for_each(TupleType&& t, FunctionType f)
{
    for_each(std::forward<TupleType>(t), f, std::integral_constant<size_t, 0>());
}

Usage (with std::tuple):

auto some = std::make_tuple("I am good", 255, 2.1);
for_each(some, [](const auto &x) { std::cout << x << std::endl; });

Usage (with std::array):

std::array<std::string,2> some2 = {"Also good", "Hello world"};
for_each(some2, [](const auto &x) { std::cout << x << std::endl; });

DEMO

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General idea: as in the solution of T.C., start with an index I=0 and go up to the size of the tuple. However, here it is done not per variadic expansion but one-at-a-time.

Explanation:

• The first overload of for_each is called if I is equal to the size of the tuple. The function then simply does nothing and such end the recursion.

• The second overload calls the function with the argument std::get<I>(t) and increases the index by one. The class std::integral_constant is needed in order to resolve the value of I at compile time. The std::enable_if SFINAE stuff is used to help the compiler separate this overload from the previous one, and call this overload only if the I is smaller than the tuple size (on Coliru this is needed, whereas in Visual Studio it works without).

• The third starts the recursion with I=0. It is the overload which is usually called from outside.

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EDIT: I've also included the idea mentioned by Yakk to additionally support std::array and std::pair by using a general template parameter TupleType instead of one that is specialized for std::tuple<Ts ...>.

As TupleType type needs to be deduced and is such a "universal reference", this further has the advantage that one gets perfect forwarding for free. The downside is that one has to use another indirection via typename std::remove_reference<TupleType>::type, as TupleType might also be a deduced as a reference type.

Answer 2

template<class F, class...Ts, std::size_t...Is>
void for_each_in_tuple(const std::tuple<Ts...> & tuple, F func, std::index_sequence<Is...>){
    using expander = int[];
    (void)expander { 0, ((void)func(std::get<Is>(tuple)), 0)... };
}

template<class F, class...Ts>
void for_each_in_tuple(const std::tuple<Ts...> & tuple, F func){
    for_each_in_tuple(tuple, func, std::make_index_sequence<sizeof...(Ts)>());
}

Usage:

auto some = std::make_tuple("I am good", 255, 2.1);
for_each_in_tuple(some, [](const auto &x) { std::cout << x << std::endl; });

Demo.

std::index_sequence and family are C++14 features, but they can be easily implemented in C++11 (there are many available on SO). Polymorphic lambdas are also C++14, but can be replaced with a custom-written functor.

Answer 3

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.