How can you iterate over the elements of an std::tuple?

Question

How can I iterate over a tuple (using C++11)? I tried the following:

for(int i=0; i<std::tuple_size<T...>::value; ++i) 
  std::get<i>(my_tuple).do_sth();

but this doesn't work:

Error 1: sorry, unimplemented: cannot expand ‘Listener ...’ into a fixed-length argument list.
Error 2: i cannot appear in a constant expression.

So, how do I correctly iterate over the elements of a tuple?

Answer 1

Try to use this:

struct tuple_traits
{
private:
    template<size_t... I, typename T, typename FUNC>
    static constexpr void __handle(T&& tuple, FUNC&& func, std::index_sequence<I...>)
    {
        (func(std::get<I>(tuple)),...);
    }

public:
    template<typename T, typename FUNC>
    static constexpr void for_each(T&& tuple, FUNC&& func)
    {
        using TupleType = std::remove_reference_t<std::remove_cv_t<T>>;
        __handle(
            std::forward<T>(tuple), 
            std::forward<FUNC>(func), 
            std::make_index_sequence<std::tuple_size<TupleType>::value>{}
        );
    }
};

you can use this like this:

using custom_tuple = std::tuple<int, std::string>;

int main(int argc, char* argv[])
{
    custom_tuple _tuple{ 1, "123" };

    tuple_traits::for_each(
        _tuple,
        [](auto&& elem)
        {
            std::cout << elem << std::endl;
        }
    );

    return 0;
}

Answer 2

Here is a solution based on std::interger_sequence.

As I don't know if my_tuple is constructed from std::make_tuple<T>(T &&...) in your code. It's essential for how to construct std::integer_sequence in the solution below.

(1) if your already have a my_tuple outside your function(not using template<typename ...T>), You can use

[](auto my_tuple)
{
    [&my_tuple]<typename N, N... n>(std::integer_sequence<N, n...> int_seq)
    {
        ((std::cout << std::get<n>(my_tuple) << '\n'), ...);
    }(std::make_index_sequence<std::tuple_size_v<decltype(my_tuple)>>{});
}(std::make_tuple());

(2) if your havn't constructed my_tuple in your function and want to handle your T ...arguments

[]<typename ...T>(T... args)
{
    [&args...]<typename N, N... n>(std::integer_sequence<N, n...> int_seq)
    {
        ((std::cout << std::get<n>(std::forward_as_tuple(args...)) << '\n'), ...);
    }(std::index_sequence_for<T...>{});
}();

Answer 3

template <typename F, typename T>
static constexpr size_t
foreach_in_tuple(std::tuple<T> & tuple, F && do_, size_t index_ = 0)
{
    do_(tuple, index_);
    return index_;
}

template <typename F, typename T, typename U, typename... Types>
static constexpr size_t
foreach_in_tuple(std::tuple<T,U,Types...> & tuple, F && do_, size_t index_ = 0)
{
    if(!do_(tuple, index_))
        return index_;
    auto & next_tuple = reinterpret_cast<std::tuple<U,Types...> &>(tuple);
    return foreach_in_tuple(next_tuple, std::forward<F>(do_), index_+1);
}


int main()
{
    using namespace std;
    auto tup = make_tuple(1, 2.3f, 'G', "hello");

    foreach_in_tuple(tup, [](auto & tuple, size_t i)
    {
        auto & value = std::get<0>(tuple);
        std::cout << i << " " << value << std::endl; 
        // if(i >= 2) return false; // break;
        return true; // continue
    });
}

Answer 4

Here's an easy C++17 way of iterating over tuple items with just standard library:

#include <tuple>      // std::tuple
#include <functional> // std::invoke

template <
    size_t Index = 0, // start iteration at 0 index
    typename TTuple,  // the tuple type
    size_t Size =
        std::tuple_size_v<
            std::remove_reference_t<TTuple>>, // tuple size
    typename TCallable, // the callable to be invoked for each tuple item
    typename... TArgs   // other arguments to be passed to the callable 
>
void for_each(TTuple&& tuple, TCallable&& callable, TArgs&&... args)
{
    if constexpr (Index < Size)
    {
        std::invoke(callable, args..., std::get<Index>(tuple));

        if constexpr (Index + 1 < Size)
            for_each<Index + 1>(
                std::forward<TTuple>(tuple),
                std::forward<TCallable>(callable),
                std::forward<TArgs>(args)...);
    }
}

Example:

#include <iostream>

int main()
{
    std::tuple<int, char> items{1, 'a'};
    for_each(items, [](const auto& item) {
        std::cout << item << "\n";
    });
}

Output:

1
a

This can be extended to conditionally break the loop in case the callable returns a value (but still work with callables that do not return a bool assignable value, e.g. void):

#include <tuple>      // std::tuple
#include <functional> // std::invoke

template <
    size_t Index = 0, // start iteration at 0 index
    typename TTuple,  // the tuple type
    size_t Size =
    std::tuple_size_v<
    std::remove_reference_t<TTuple>>, // tuple size
    typename TCallable, // the callable to bo invoked for each tuple item
    typename... TArgs   // other arguments to be passed to the callable 
    >
    void for_each(TTuple&& tuple, TCallable&& callable, TArgs&&... args)
{
    if constexpr (Index < Size)
    {
        if constexpr (std::is_assignable_v<bool&, std::invoke_result_t<TCallable&&, TArgs&&..., decltype(std::get<Index>(tuple))>>)
        {
            if (!std::invoke(callable, args..., std::get<Index>(tuple)))
                return;
        }
        else
        {
            std::invoke(callable, args..., std::get<Index>(tuple));
        }

        if constexpr (Index + 1 < Size)
            for_each<Index + 1>(
                std::forward<TTuple>(tuple),
                std::forward<TCallable>(callable),
                std::forward<TArgs>(args)...);
    }
}

Example:

#include <iostream>

int main()
{
    std::tuple<int, char> items{ 1, 'a' };
    for_each(items, [](const auto& item) {
        std::cout << item << "\n";
    });

    std::cout << "---\n";

    for_each(items, [](const auto& item) {
        std::cout << item << "\n";
        return false;
    });
}

Output:

1
a
---
1

Answer 5

There're many great answers, but for some reason most of them don't consider returning the results of applying f to our tuple... or did I overlook it? Anyway, here's yet another way you can do that:

Doing Foreach with style (debatable)

auto t = std::make_tuple(1, "two", 3.f);
t | foreach([](auto v){ std::cout << v << " "; });

And returning from that:

    auto t = std::make_tuple(1, "two", 3.f);
    auto sizes = t | foreach([](auto v) {
        return sizeof(v);
    });
    sizes | foreach([](auto v) {
        std::cout << v;
    });

Implementation (pretty simple one)

Edit: it gets a little messier.

I won't include some metaprogramming boilerplate here, for it will definitely make things less readable and besides, I believe those have already been answered somewhere on stackoverflow. In case you're feeling lazy, feel free to peek into my github repo for implementation of both

#include <utility>


// Optional includes, if you don't want to implement it by hand or google it
// you can find it in the repo (link below)
#include "typesystem/typelist.hpp"
// used to check if all return types are void, 
// making it a special case 
// (and, alas, not using constexpr-if 
//    for the sake of being compatible with C++14...) 


template <bool Cond, typename T, typename F>
using select = typename std::conditional<Cond, T, F>::type;


template <typename F>
struct elementwise_apply {
    F f;
};

template <typename F>
constexpr auto foreach(F && f) -> elementwise_apply<F> { return {std::forward<F>(f)}; }


template <typename R>
struct tuple_map {
    template <typename F, typename T, size_t... Is>
    static constexpr decltype(auto) impl(std::index_sequence<Is...>, F && f, T&& tuple) {
        return R{ std::forward<F>(f)( std::get<Is>(tuple) )... };
    }
};

template<>
struct tuple_map<void> {
    template <typename F, typename T, size_t... Is>
    static constexpr void impl(std::index_sequence<Is...>, F && f, T&& tuple) {
        [[maybe_unused]] std::initializer_list<int> _ {((void)std::forward<F>(f)( std::get<Is>(tuple) ), 0)... };
    }
};

template <typename F, typename... Ts>
constexpr decltype(auto) operator| (std::tuple<Ts...> & t, fmap<F> && op) {
    constexpr bool all_void = core::Types<decltype( std::move(op).f(std::declval<Ts&>()) )...>.all( core::is_void );
    using R = meta::select<all_void, void, std::tuple<decltype(std::move(op).f(std::declval<Ts&>()))...>>;
    return tuple_map<R>::impl(std::make_index_sequence<sizeof...(Ts)>{}, std::move(op).f, t);
}

template <typename F, typename... Ts>
constexpr decltype(auto) operator| (std::tuple<Ts...> const& t, fmap<F> && op) {
    constexpr bool all_void = check if all "decltype( std::move(op).f(std::declval<Ts>()) )..." types are void, since then it's a special case
    // e.g. core::Types<decltype( std::move(op).f(std::declval<Ts>()) )...>.all( core::is_void );
    using R = meta::select<all_void, void, std::tuple<decltype(std::move(op).f(std::declval<Ts const&>()))...>>;
    return tuple_map<R>::impl(std::make_index_sequence<sizeof...(Ts)>{}, std::move(op).f, t);
}

template <typename F, typename... Ts>
constexpr decltype(auto) operator| (std::tuple<Ts...> && t, fmap<F> && op) {
    constexpr bool all_void = core::Types<decltype( std::move(op).f(std::declval<Ts&&>()) )...>.all( core::is_void );
    using R = meta::select<all_void, void, std::tuple<decltype(std::move(op).f(std::declval<Ts&&>()))...>>;
    return tuple_map<R>::impl(std::make_index_sequence<sizeof...(Ts)>{}, std::move(op).f, std::move(t));
}

Yeah, that would be much nicer if we were to use C++17

This is also an example of std::moving object's members, for which I'll better refer to this nice brief article

P.S. If you're stuck checking if all "decltype( std::move(op).f(std::declval()) )..." types are void you can find some metaprogramming library, or, if those libraries seem too hard to grasp (which some of them may be due to some crazy metaprogramming tricks), you know where to look

Answer 6

Expanding on @Stypox answer, we can make their solution more generic (C++17 onward). By adding a callable function argument:

template<size_t I = 0, typename... Tp, typename F>
void for_each_apply(std::tuple<Tp...>& t, F &&f) {
    f(std::get<I>(t));
    if constexpr(I+1 != sizeof...(Tp)) {
        for_each_apply<I+1>(t, std::forward<F>(f));
    }
}

Then, we need a strategy to visit each type.

Let start with some helpers (first two taken from cppreference):

template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
template<class ... Ts> struct variant_ref { using type = std::variant<std::reference_wrapper<Ts>...>; };

variant_ref is used to allow tuples' state to be modified.

Usage:

std::tuple<Foo, Bar, Foo> tuples;

for_each_apply(tuples,
               [](variant_ref<Foo, Bar>::type &&v) {
                   std::visit(overloaded {
                       [](Foo &arg) { arg.foo(); },
                       [](Bar const &arg) { arg.bar(); },
                   }, v);
               });

Result:

Foo0
Bar
Foo0
Foo1
Bar
Foo1

For completeness, here are my Bar & Foo:

struct Foo {
    void foo() {std::cout << "Foo" << i++ << std::endl;}
    int i = 0;
};
struct Bar {
    void bar() const {std::cout << "Bar" << std::endl;}
};

Answer 7

Another option would be to implement iterators for tuples. This has the advantage that you can use a variety of algorithms provided by the standard library and range-based for loops. An elegant approach to this is explained here https://foonathan.net/2017/03/tuple-iterator/. The basic idea is to turn tuples into a range with begin() and end() methods to provide iterators. The iterator itself returns a std::variant<...> which can then be visited using std::visit.

Here some examples:

auto t = std::tuple{ 1, 2.f, 3.0 };
auto r = to_range(t);

for(auto v : r)
{
    std::visit(unwrap([](auto& x)
        {
            x = 1;
        }), v);
}

std::for_each(begin(r), end(r), [](auto v)
    {
        std::visit(unwrap([](auto& x)
            {
                x = 0;
            }), v);
    });

std::accumulate(begin(r), end(r), 0.0, [](auto acc, auto v)
    {
        return acc + std::visit(unwrap([](auto& x)
        {
            return static_cast<double>(x);
        }), v);
    });

std::for_each(begin(r), end(r), [](auto v)
{
    std::visit(unwrap([](const auto& x)
        {
            std::cout << x << std::endl;
        }), v);
});

std::for_each(begin(r), end(r), [](auto v)
{
    std::visit(overload(
        [](int x) { std::cout << "int" << std::endl; },
        [](float x) { std::cout << "float" << std::endl; },
        [](double x) { std::cout << "double" << std::endl; }), v);
});

My implementation (which is heavily based on the explanations in the link above):

#ifndef TUPLE_RANGE_H
#define TUPLE_RANGE_H

#include <utility>
#include <functional>
#include <variant>
#include <type_traits>

template<typename Accessor>
class tuple_iterator
{
public:
    tuple_iterator(Accessor acc, const int idx)
        : acc_(acc), index_(idx)
    {

    }

    tuple_iterator operator++()
    {
        ++index_;
        return *this;
    }

    template<typename T>
    bool operator ==(tuple_iterator<T> other)
    {
        return index_ == other.index();
    }

    template<typename T>
    bool operator !=(tuple_iterator<T> other)
    {
        return index_ != other.index();
    }

    auto operator*() { return std::invoke(acc_, index_); }

    [[nodiscard]] int index() const { return index_; }

private:
    const Accessor acc_;
    int index_;
};

template<bool IsConst, typename...Ts>
struct tuple_access
{
    using tuple_type = std::tuple<Ts...>;
    using tuple_ref = std::conditional_t<IsConst, const tuple_type&, tuple_type&>;

    template<typename T>
    using element_ref = std::conditional_t<IsConst,
        std::reference_wrapper<const T>,
        std::reference_wrapper<T>>;

    using variant_type = std::variant<element_ref<Ts>...>;
    using function_type = variant_type(*)(tuple_ref);
    using table_type = std::array<function_type, sizeof...(Ts)>;

private:
    template<size_t Index>
    static constexpr function_type create_accessor()
    {
        return { [](tuple_ref t) -> variant_type
        {
            if constexpr (IsConst)
                return std::cref(std::get<Index>(t));
            else
                return std::ref(std::get<Index>(t));
        } };
    }

    template<size_t...Is>
    static constexpr table_type create_table(std::index_sequence<Is...>)
    {
        return { create_accessor<Is>()... };
    }

public:
    static constexpr auto table = create_table(std::make_index_sequence<sizeof...(Ts)>{}); 
};

template<bool IsConst, typename...Ts>
class tuple_range
{
public:
    using tuple_access_type = tuple_access<IsConst, Ts...>;
    using tuple_ref = typename tuple_access_type::tuple_ref;

    static constexpr auto tuple_size = sizeof...(Ts);

    explicit tuple_range(tuple_ref tuple)
        : tuple_(tuple)
    {
    }

    [[nodiscard]] auto begin() const 
    { 
        return tuple_iterator{ create_accessor(), 0 };
    }

    [[nodiscard]] auto end() const 
    { 
        return tuple_iterator{ create_accessor(), tuple_size };
    }

private:
    tuple_ref tuple_;

    auto create_accessor() const
    { 
        return [this](int idx)
        {
            return std::invoke(tuple_access_type::table[idx], tuple_);
        };
    }
};

template<bool IsConst, typename...Ts>
auto begin(const tuple_range<IsConst, Ts...>& r)
{
    return r.begin();
}

template<bool IsConst, typename...Ts>
auto end(const tuple_range<IsConst, Ts...>& r)
{
    return r.end();
}

template <class ... Fs>
struct overload : Fs... {
    explicit overload(Fs&&... fs) : Fs{ fs }... {}
    using Fs::operator()...;

    template<class T>
    auto operator()(std::reference_wrapper<T> ref)
    {
        return (*this)(ref.get());
    }

    template<class T>
    auto operator()(std::reference_wrapper<const T> ref)
    {
        return (*this)(ref.get());
    }
};

template <class F>
struct unwrap : overload<F>
{
    explicit unwrap(F&& f) : overload<F>{ std::forward<F>(f) } {}
    using overload<F>::operator();
};

template<typename...Ts>
auto to_range(std::tuple<Ts...>& t)
{
    return tuple_range<false, Ts...>{t};
}

template<typename...Ts>
auto to_range(const std::tuple<Ts...>& t)
{
    return tuple_range<true, Ts...>{t};
}


#endif

Read-only access is also supported by passing a const std::tuple<>& to to_range().

Answer 8

A more simple, intuitive and compiler-friendly way of doing this in C++17, using if constexpr:

// prints every element of a tuple
template<size_t I = 0, typename... Tp>
void print(std::tuple<Tp...>& t) {
    std::cout << std::get<I>(t) << " ";
    // do things
    if constexpr(I+1 != sizeof...(Tp))
        print<I+1>(t);
}

This is compile-time recursion, similar to the one presented by @emsr. But this doesn't use SFINAE so (I think) it is more compiler-friendly.

Answer 9

Using constexpr and if constexpr(C++17) this is fairly simple and straight forward:

template <std::size_t I = 0, typename ... Ts>
void print(std::tuple<Ts...> tup) {
  if constexpr (I == sizeof...(Ts)) {
    return;
  } else {
    std::cout << std::get<I>(tup) << ' ';
    print<I+1>(tup);
  }
}

Answer 10

C++ is introducing expansion statements for this purpose. They were originally on track for C++20 but narrowly missed the cut due to a lack of time for language wording review (see here and here).

The currently agreed syntax (see the links above) is:

{
    auto tup = std::make_tuple(0, 'a', 3.14);
    template for (auto elem : tup)
        std::cout << elem << std::endl;
}

Answer 11

In C++17, you can use std::apply with fold expression:

std::apply([](auto&&... args) {((/* args.dosomething() */), ...);}, the_tuple);

A complete example for printing a tuple:

#include <tuple>
#include <iostream>

int main()
{
    std::tuple t{42, 'a', 4.2}; // Another C++17 feature: class template argument deduction
    std::apply([](auto&&... args) {((std::cout << args << '\n'), ...);}, t);
}

[Online Example on Coliru]

This solution solves the issue of evaluation order in M. Alaggan's answer.

Answer 12

Others have mentioned some well-designed third-party libraries that you may turn to. However, if you are using C++ without those third-party libraries, the following code may help.

namespace detail {

template <class Tuple, std::size_t I, class = void>
struct for_each_in_tuple_helper {
  template <class UnaryFunction>
  static void apply(Tuple&& tp, UnaryFunction& f) {
    f(std::get<I>(std::forward<Tuple>(tp)));
    for_each_in_tuple_helper<Tuple, I + 1u>::apply(std::forward<Tuple>(tp), f);
  }
};

template <class Tuple, std::size_t I>
struct for_each_in_tuple_helper<Tuple, I, typename std::enable_if<
    I == std::tuple_size<typename std::decay<Tuple>::type>::value>::type> {
  template <class UnaryFunction>
  static void apply(Tuple&&, UnaryFunction&) {}
};

}  // namespace detail

template <class Tuple, class UnaryFunction>
UnaryFunction for_each_in_tuple(Tuple&& tp, UnaryFunction f) {
  detail::for_each_in_tuple_helper<Tuple, 0u>
      ::apply(std::forward<Tuple>(tp), f);
  return std::move(f);
}

Note: The code compiles with any compiler supporing C++11, and it keeps consistency with design of the standard library:

1. The tuple need not be std::tuple, and instead may be anything that supports std::get and std::tuple_size; in particular, std::array and std::pair may be used;

2. The tuple may be a reference type or cv-qualified;

3. It has similar behavior as std::for_each, and returns the input UnaryFunction;

4. For C++14 (or laster version) users, typename std::enable_if<T>::type and typename std::decay<T>::type could be replaced with their simplified version, std::enable_if_t<T> and std::decay_t<T>;

5. For C++17 (or laster version) users, std::tuple_size<T>::value could be replaced with its simplified version, std::tuple_size_v<T>.

6. For C++20 (or laster version) users, the SFINAE feature could be implemented with the Concepts.

Answer 13

I have stumbled on the same problem for iterating over a tuple of function objects, so here is one more solution:

#include <tuple> 
#include <iostream>

// Function objects
class A 
{
    public: 
        inline void operator()() const { std::cout << "A\n"; };
};

class B 
{
    public: 
        inline void operator()() const { std::cout << "B\n"; };
};

class C 
{
    public:
        inline void operator()() const { std::cout << "C\n"; };
};

class D 
{
    public:
        inline void operator()() const { std::cout << "D\n"; };
};


// Call iterator using recursion.
template<typename Fobjects, int N = 0> 
struct call_functors 
{
    static void apply(Fobjects const& funcs)
    {
        std::get<N>(funcs)(); 

        // Choose either the stopper or descend further,  
        // depending if N + 1 < size of the tuple. 
        using caller = std::conditional_t
        <
            N + 1 < std::tuple_size_v<Fobjects>,
            call_functors<Fobjects, N + 1>, 
            call_functors<Fobjects, -1>
        >;

        caller::apply(funcs); 
    }
};

// Stopper.
template<typename Fobjects> 
struct call_functors<Fobjects, -1>
{
    static void apply(Fobjects const& funcs)
    {
    }
};

// Call dispatch function.
template<typename Fobjects>
void call(Fobjects const& funcs)
{
    call_functors<Fobjects>::apply(funcs);
};


using namespace std; 

int main()
{
    using Tuple = tuple<A,B,C,D>; 

    Tuple functors = {A{}, B{}, C{}, D{}}; 

    call(functors); 

    return 0; 
}

Output:

A 
B 
C 
D

Answer 14

Of all the answers I've seen here, here and here, I liked @sigidagi's way of iterating best. Unfortunately, his answer is very verbose which in my opinion obscures the inherent clarity.

This is my version of his solution which is more concise and works with std::tuple, std::pair and std::array.

template<typename UnaryFunction>
void invoke_with_arg(UnaryFunction)
{}

/**
 * Invoke the unary function with each of the arguments in turn.
 */
template<typename UnaryFunction, typename Arg0, typename... Args>
void invoke_with_arg(UnaryFunction f, Arg0&& a0, Args&&... as)
{
    f(std::forward<Arg0>(a0));
    invoke_with_arg(std::move(f), std::forward<Args>(as)...);
}

template<typename Tuple, typename UnaryFunction, std::size_t... Indices>
void for_each_helper(Tuple&& t, UnaryFunction f, std::index_sequence<Indices...>)
{
    using std::get;
    invoke_with_arg(std::move(f), get<Indices>(std::forward<Tuple>(t))...);
}

/**
 * Invoke the unary function for each of the elements of the tuple.
 */
template<typename Tuple, typename UnaryFunction>
void for_each(Tuple&& t, UnaryFunction f)
{
    using size = std::tuple_size<typename std::remove_reference<Tuple>::type>;
    for_each_helper(
        std::forward<Tuple>(t),
        std::move(f),
        std::make_index_sequence<size::value>()
    );
}

Demo: coliru

C++14's std::make_index_sequence can be implemented for C++11.

Answer 15

In MSVC STL there's a _For_each_tuple_element function (not documented):

#include <tuple>

// ...

std::tuple<int, char, float> values{};
std::_For_each_tuple_element(values, [](auto&& value)
{
    // process 'value'
});

Answer 16

Use Boost.Hana and generic lambdas:

#include <tuple>
#include <iostream>
#include <boost/hana.hpp>
#include <boost/hana/ext/std/tuple.hpp>

struct Foo1 {
    int foo() const { return 42; }
};

struct Foo2 {
    int bar = 0;
    int foo() { bar = 24; return bar; }
};

int main() {
    using namespace std;
    using boost::hana::for_each;

    Foo1 foo1;
    Foo2 foo2;

    for_each(tie(foo1, foo2), [](auto &foo) {
        cout << foo.foo() << endl;
    });

    cout << "foo2.bar after mutation: " << foo2.bar << endl;
}

http://coliru.stacked-crooked.com/a/27b3691f55caf271

Answer 17

In C++17 you can do this:

std::apply([](auto ...x){std::make_tuple(x.do_something()...);} , the_tuple);

This already works in Clang++ 3.9, using std::experimental::apply.

Answer 18

I might have missed this train, but this will be here for future reference.
Here's my construct based on this answer and on this gist:

#include <tuple>
#include <utility>

template<std::size_t N>
struct tuple_functor
{
    template<typename T, typename F>
    static void run(std::size_t i, T&& t, F&& f)
    {
        const std::size_t I = (N - 1);
        switch(i)
        {
        case I:
            std::forward<F>(f)(std::get<I>(std::forward<T>(t)));
            break;

        default:
            tuple_functor<I>::run(i, std::forward<T>(t), std::forward<F>(f));
        }
    }
};

template<>
struct tuple_functor<0>
{
    template<typename T, typename F>
    static void run(std::size_t, T, F){}
};

You then use it as follow:

template<typename... T>
void logger(std::string format, T... args) //behaves like C#'s String.Format()
{
    auto tp = std::forward_as_tuple(args...);
    auto fc = [](const auto& t){std::cout << t;};

    /* ... */

    std::size_t some_index = ...
    tuple_functor<sizeof...(T)>::run(some_index, tp, fc);

    /* ... */
}

There could be room for improvements.

---

As per OP's code, it would become this:

const std::size_t num = sizeof...(T);
auto my_tuple = std::forward_as_tuple(t...);
auto do_sth = [](const auto& elem){/* ... */};
for(int i = 0; i < num; ++i)
    tuple_functor<num>::run(i, my_tuple, do_sth);

Answer 19

I have an answer based on Iterating over a Tuple:

#include <tuple>
#include <utility> 
#include <iostream>

template<std::size_t I = 0, typename... Tp>
inline typename std::enable_if<I == sizeof...(Tp), void>::type
  print(std::tuple<Tp...>& t)
  { }

template<std::size_t I = 0, typename... Tp>
inline typename std::enable_if<I < sizeof...(Tp), void>::type
  print(std::tuple<Tp...>& t)
  {
    std::cout << std::get<I>(t) << std::endl;
    print<I + 1, Tp...>(t);
  }

int
main()
{
  typedef std::tuple<int, float, double> T;
  T t = std::make_tuple(2, 3.14159F, 2345.678);

  print(t);
}

The usual idea is to use compile time recursion. In fact, this idea is used to make a printf that is type safe as noted in the original tuple papers.

This can be easily generalized into a for_each for tuples:

#include <tuple>
#include <utility> 

template<std::size_t I = 0, typename FuncT, typename... Tp>
inline typename std::enable_if<I == sizeof...(Tp), void>::type
  for_each(std::tuple<Tp...> &, FuncT) // Unused arguments are given no names.
  { }

template<std::size_t I = 0, typename FuncT, typename... Tp>
inline typename std::enable_if<I < sizeof...(Tp), void>::type
  for_each(std::tuple<Tp...>& t, FuncT f)
  {
    f(std::get<I>(t));
    for_each<I + 1, FuncT, Tp...>(t, f);
  }

Though this then requires some effort to have FuncT represent something with the appropriate overloads for every type the tuple might contain. This works best if you know all the tuple elements will share a common base class or something similar.

Answer 20

Boost.Fusion is a possibility:

Untested example:

struct DoSomething
{
    template<typename T>
    void operator()(T& t) const
    {
        t.do_sth();
    }
};

tuple<....> t = ...;
boost::fusion::for_each(t, DoSomething());

Answer 21

boost's tuple provides helper functions get_head() and get_tail() so your helper functions may look like this:

inline void call_do_sth(const null_type&) {};

template <class H, class T>
inline void call_do_sth(cons<H, T>& x) { x.get_head().do_sth(); call_do_sth(x.get_tail()); }

as described in here http://www.boost.org/doc/libs/1_34_0/libs/tuple/doc/tuple_advanced_interface.html

with std::tuple it should be similar.

Actually, unfortunately std::tuple does not seem to provide such interface, so methods suggested before should work, or you would need to switch to boost::tuple which has other benefits (like io operators already provided). Though there is downside of boost::tuple with gcc - it does not accept variadic templates yet, but that may be already fixed as I do not have latest version of boost installed on my machine.

Answer 22

If you want to use std::tuple and you have C++ compiler which supports variadic templates, try code bellow (tested with g++4.5). This should be the answer to your question.

#include <tuple>

// ------------- UTILITY---------------
template<int...> struct index_tuple{}; 

template<int I, typename IndexTuple, typename... Types> 
struct make_indexes_impl; 

template<int I, int... Indexes, typename T, typename ... Types> 
struct make_indexes_impl<I, index_tuple<Indexes...>, T, Types...> 
{ 
    typedef typename make_indexes_impl<I + 1, index_tuple<Indexes..., I>, Types...>::type type; 
}; 

template<int I, int... Indexes> 
struct make_indexes_impl<I, index_tuple<Indexes...> > 
{ 
    typedef index_tuple<Indexes...> type; 
}; 

template<typename ... Types> 
struct make_indexes : make_indexes_impl<0, index_tuple<>, Types...> 
{}; 

// ----------- FOR EACH -----------------
template<typename Func, typename Last>
void for_each_impl(Func&& f, Last&& last)
{
    f(last);
}

template<typename Func, typename First, typename ... Rest>
void for_each_impl(Func&& f, First&& first, Rest&&...rest) 
{
    f(first);
    for_each_impl( std::forward<Func>(f), rest...);
}

template<typename Func, int ... Indexes, typename ... Args>
void for_each_helper( Func&& f, index_tuple<Indexes...>, std::tuple<Args...>&& tup)
{
    for_each_impl( std::forward<Func>(f), std::forward<Args>(std::get<Indexes>(tup))...);
}

template<typename Func, typename ... Args>
void for_each( std::tuple<Args...>& tup, Func&& f)
{
   for_each_helper(std::forward<Func>(f), 
                   typename make_indexes<Args...>::type(), 
                   std::forward<std::tuple<Args...>>(tup) );
}

template<typename Func, typename ... Args>
void for_each( std::tuple<Args...>&& tup, Func&& f)
{
   for_each_helper(std::forward<Func>(f), 
                   typename make_indexes<Args...>::type(), 
                   std::forward<std::tuple<Args...>>(tup) );
}

boost::fusion is another option, but it requires its own tuple type: boost::fusion::tuple. Lets better stick to the standard! Here is a test:

#include <iostream>

// ---------- FUNCTOR ----------
struct Functor 
{
    template<typename T>
    void operator()(T& t) const { std::cout << t << std::endl; }
};

int main()
{
    for_each( std::make_tuple(2, 0.6, 'c'), Functor() );
    return 0;
}

the power of variadic templates!

Answer 23

You need to use template metaprogramming, here shown with Boost.Tuple:

#include <boost/tuple/tuple.hpp>
#include <iostream>

template <typename T_Tuple, size_t size>
struct print_tuple_helper {
    static std::ostream & print( std::ostream & s, const T_Tuple & t ) {
        return print_tuple_helper<T_Tuple,size-1>::print( s, t ) << boost::get<size-1>( t );
    }
};

template <typename T_Tuple>
struct print_tuple_helper<T_Tuple,0> {
    static std::ostream & print( std::ostream & s, const T_Tuple & ) {
        return s;
    }
};

template <typename T_Tuple>
std::ostream & print_tuple( std::ostream & s, const T_Tuple & t ) {
    return print_tuple_helper<T_Tuple,boost::tuples::length<T_Tuple>::value>::print( s, t );
}

int main() {

    const boost::tuple<int,char,float,char,double> t( 0, ' ', 2.5f, '\n', 3.1416 );
    print_tuple( std::cout, t );

    return 0;
}

In C++0x, you can write print_tuple() as a variadic template function instead.