# std::tuple<Types...>::tuple

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< cpp‎ | utility‎ | tuple

Defined in header `<tuple>` |  |
constexpr tuple(); | (1) | (since C++11) (conditionally explicit)
tuple( const Types&... args ); | (2) | (since C++11) (constexpr since C++14) (conditionally explicit)
template< class... UTypes > tuple( UTypes&&... args ); | (3) | (since C++11) (constexpr since C++14) (conditionally explicit)
template< class... UTypes > constexpr tuple( tuple<UTypes...>& other ); | (4) | (since C++23) (conditionally explicit)
template< class... UTypes > tuple( const tuple<UTypes...>& other ); | (5) | (since C++11) (constexpr since C++14) (conditionally explicit)
template< class... UTypes > tuple( tuple<UTypes...>&& other ); | (6) | (since C++11) (constexpr since C++14) (conditionally explicit)
template< class... UTypes > constexpr tuple( const tuple<UTypes...>&& other ); | (7) | (since C++23) (conditionally explicit)
template< class U1, class U2 > constexpr tuple( std::pair<U1, U2>& p ); | (8) | (since C++23) (conditionally explicit)
template< class U1, class U2 > tuple( const std::pair<U1, U2>& p ); | (9) | (since C++11) (constexpr since C++14) (conditionally explicit)
template< class U1, class U2 > tuple( std::pair<U1, U2>&& p ); | (10) | (since C++11) (constexpr since C++14) (conditionally explicit)
template< class U1, class U2 > constexpr tuple( const std::pair<U1, U2>&& p ); | (11) | (since C++23) (conditionally explicit)
template< tuple-like UTuple > constexpr tuple( UTuple&& u ); | (12) | (since C++23) (conditionally explicit)
tuple( const tuple& other ) = default; | (13) | (since C++11)
tuple( tuple&& other ) = default; | (14) | (since C++11)
Allocator-extended constructors |  |
template< class Alloc > tuple( std::allocator_arg_t, const Alloc& a ); | (15) | (since C++11) (constexpr since C++20) (conditionally explicit)
template< class Alloc > tuple( std::allocator_arg_t, const Alloc& a, const Types&... args ); | (16) | (since C++11) (constexpr since C++20) (conditionally explicit)
template< class Alloc, class... UTypes > tuple( std::allocator_arg_t, const Alloc& a, UTypes&&... args ); | (17) | (since C++11) (constexpr since C++20) (conditionally explicit)
template< class Alloc, class... UTypes > constexpr tuple( std::allocator_arg_t, const Alloc& a, tuple<UTypes...>& other ); | (18) | (since C++23) (conditionally explicit)
template< class Alloc, class... UTypes > tuple( std::allocator_arg_t, const Alloc& a, const tuple<UTypes...>& other ); | (19) | (since C++11) (constexpr since C++20) (conditionally explicit)
template< class Alloc, class... UTypes > tuple( std::allocator_arg_t, const Alloc& a, tuple<UTypes...>&& other ); | (20) | (since C++11) (constexpr since C++20) (conditionally explicit)
template< class Alloc, class... UTypes > constexpr tuple( std::allocator_arg_t, const Alloc& a, const tuple<UTypes...>&& other ); | (21) | (since C++23) (conditionally explicit)
template< class Alloc, class U1, class U2 > constexpr tuple( std::allocator_arg_t, const Alloc& a, std::pair<U1, U2>& p ); | (22) | (since C++23) (conditionally explicit)
template< class Alloc, class U1, class U2 > tuple( std::allocator_arg_t, const Alloc& a, const std::pair<U1, U2>& p ); | (23) | (since C++11) (constexpr since C++20) (conditionally explicit)
template< class Alloc, class U1, class U2 > tuple( std::allocator_arg_t, const Alloc& a, std::pair<U1, U2>&& p ); | (24) | (since C++11) (constexpr since C++20) (conditionally explicit)
template< class Alloc, class U1, class U2 > constexpr tuple( std::allocator_arg_t, const Alloc& a, const std::pair<U1, U2>&& p ); | (25) | (since C++23) (conditionally explicit)
template< class Alloc, tuple-like UTuple > constexpr tuple( std::allocator_arg_t, const Alloc& a, UTuple&& u ); | (26) | (since C++23) (conditionally explicit)
template< class Alloc > tuple( std::allocator_arg_t, const Alloc& a, const tuple& other ); | (27) | (since C++11) (constexpr since C++20)
template< class Alloc > tuple( std::allocator_arg_t, const Alloc& a, tuple&& other ); | (28) | (since C++11) (constexpr since C++20)

Constructs a new tuple.

In the descriptions that follow, let

- i be in the range `[`​0​`,` sizeof...(Types)`)` in order,
- `Ti` be the `i`th type in `Types`, and
- `Ui` be the `i`th type in a template parameter pack named `UTypes`,

where indexing is zero-based.

1) Default constructor. Value-initializes all elements, if any. The default constructor is trivial if sizeof...(Types) == 0.

- This overload participates in overload resolution only if std::is_default_constructible<Ti>::value is true for all i
- The constructor is explicit if and only if `Ti` is not copy-list-initializable from {} for at least one i.

2) Direct constructor. Initializes each element of the tuple with the corresponding parameter.

- This overload participates in overload resolution only if sizeof...(Types) >= 1 and std::is_copy_constructible<Ti>::value is true for all i.
- This constructor is explicit if and only if std::is_convertible<const Ti&, Ti>::value is false for at least one i.

3) Converting constructor. Initializes each element of the tuple with the corresponding value in std::forward<UTypes>(args).

- This overload participates in overload resolution only if
  - sizeof...(Types) == sizeof...(UTypes),
  - sizeof...(Types) >= 1,
  - std::is_constructible<Ti, Ui>::value is true for all i, and
  - let `D` be std::decay<U0>::type (until C++20)std::remove_cvref_t<U0> (since C++20),
    - if sizeof...(Types) == 1, then `D` is not `std::tuple`, otherwise,
    - if sizeof...(Types) == 2 or sizeof...(Types) == 3, then either `D` is not std::allocator_arg_t, or `T0` is std::allocator_arg_t.
- The constructor is explicit if and only if std::is_convertible<Ui, Ti>::value is false for at least one i.

This constructor is defined as deleted if the initialization of any element that is a reference would bind it to a temporary object. | (since C++23)

4-7) Converting constructor. Initializes each element of the tuple with the corresponding element of other.

Formally, let FWD(other) be std::forward<decltype(other)>(other), for all i, initializes `i`th element of the tuple with std::get<i>(FWD(other)).

- This overload participates in overload resolution only if
  - sizeof...(Types) == sizeof...(UTypes),
  - std::is_constructible_v<Ti, decltype(std::get<i>(FWD(other)))> is true for all i, and
  - either
    - sizeof...(Types) is not 1, or
    - (when `Types...` expands to `T` and `UTypes...` expands to `U`) std::is_convertible_v<decltype(other), T>, std::is_constructible_v<T, decltype(other)>, and std::is_same_v<T, U> are all false.
- These constructors are explicit if and only if std::is_convertible_v<decltype(std::get<i>(FWD(other))), Ti> is false for at least one i.

These constructors are defined as deleted if the initialization of any element that is a reference would bind it to a temporary object. | (since C++23)

8-11) Pair constructor. Constructs a 2-element tuple with each element constructed from the corresponding element of p.

Formally, let FWD(p) be std::forward<decltype(p)>(p), initializes the first element with std::get<0>(FWD(p)) and the second element with std::get<1>(FWD(p)).

- This overload participates in overload resolution only if
  - sizeof...(Types) == 2,
  - std::is_constructible_v<T0, decltype(std::get<0>(FWD(p)))> is true, and
  - std::is_constructible_v<T1, decltype(std::get<1>(FWD(p)))> is true
- The constructor is explicit if and only if std::is_convertible_v<decltype(std::get<0>(FWD(p))), T0> or std::is_convertible_v<decltype(std::get<1>(FWD(p))), T1> is false.

These constructors are defined as deleted if the initialization of any element that is a reference would bind it to a temporary object. | (since C++23)

12) `tuple-like` constructor. Constructs a tuple with each element constructed from the corresponding element of u.

Formally, for all i, initializes `i`th element of the tuple with std::get<i>(std::forward<UTuple>(u)).

- This overload participates in overload resolution only if
  - std::same_as<std::remove_cvref_t<UTuple>, std::tuple> is false,
  - std::remove_cvref_t<UTuple> is not a specialization of std::ranges::subrange,
  - sizeof...(Types) equals std::tuple_size_v<std::remove_cvref_t<UTuple>>,
  - std::is_constructible_v<Ti, decltype(std::get<i>(std::forward<UTuple>(u)))> is true for all i, and
  - either
    - sizeof...(Types) is not 1, or
    - (when `Types...` expands to `T`) std::is_convertible_v<UTuple, T> and std::is_constructible_v<T, UTuple> are both false.

13) Implicitly-defined copy constructor. Initializes each element of the tuple with the corresponding element of other.

- This constructor is constexpr if every operation it performs is constexpr. For the empty tuple std::tuple<>, it is constexpr.
- std::is_copy_constructible<Ti>::value must be true for all i, otherwise the behavior is undefined (until C++20)the program is ill-formed (since C++20).

14) Implicitly-defined move constructor. For all i, initializes the `i`th element of the tuple with std::forward<Ui>(std::get<i>(other)).

- This constructor is constexpr if every operation it performs is constexpr. For the empty tuple std::tuple<>, it is constexpr.
- std::is_move_constructible<Ti>::value must be true for all i, otherwise the behavior is undefined (until C++20)this overload does not participate in overload resolution (since C++20).

15-28) Identical to (1-14) except each element is created by uses-allocator construction, that is, the Allocator object a is passed as an additional argument to the constructor of each element for which std::uses_allocator<Ui, Alloc>::value is true.

### Parameters

args | - | values used to initialize each element of the tuple
other | - | the tuple of values used to initialize each element of the tuple
p | - | the pair of values used to initialize both elements of the 2-tuple
u | - | the `tuple-like` object of values used to initialize each element of the tuple
a | - | the allocator to use in uses-allocator construction

### Notes

Conditionally-explicit constructors make it possible to construct a tuple in copy-initialization context using list-initialization syntax:

    std::tuple<int, int> foo_tuple()
    {
        // return {1, -1};             // Error before N4387
        return std::make_tuple(1, -1); // Always works
    }

Note that if some element of the list is not implicitly convertible to the corresponding element of the target tuple, the constructors become explicit:

    using namespace std::chrono;
    void launch_rocket_at(std::tuple<hours, minutes, seconds>);

    launch_rocket_at({hours(1), minutes(2), seconds(3)}); // OK
    launch_rocket_at({1, 2, 3}); // Error: int is not implicitly convertible to duration
    launch_rocket_at(std::tuple<hours, minutes, seconds>{1, 2, 3}); // OK

### Example

Run this code

    #include <iomanip>
    #include <iostream>
    #include <memory>
    #include <string>
    #include <string_view>
    #include <tuple>
    #include <type_traits>
    #include <vector>

    // helper function to print a vector to a stream
    template<class Os, class T>
    Os& operator<< (Os& os, std::vector<T> const& v)
    {
        os << '{';
        for (auto i{v.size()}; const T& e : v)
            os << e << (--i ? "," : "");
        return os << '}';
    }

    template<class T>
    void print_single(T const& v)
    {
        if constexpr (std::is_same_v<T, std::decay_t<std::string>>)
            std::cout << std::quoted(v);
        else if constexpr (std::is_same_v<std::decay_t<T>, char>)
            std::cout << "'" << v << "'";
        else
            std::cout << v;
    }

    // helper function to print a tuple of any size
    template<class Tuple, std::size_t N>
    struct TuplePrinter
    {
        static void print(const Tuple& t)
        {
            TuplePrinter<Tuple, N - 1>::print(t);
            std::cout << ", ";
            print_single(std::get<N - 1>(t));
        }
    };

    template<class Tuple>
    struct TuplePrinter<Tuple, 1>
    {
        static void print(const Tuple& t)
        {
            print_single(std::get<0>(t));
        }
    };

    template<class... Args>
    void print(std::string_view message, const std::tuple<Args...>& t)
    {
        std::cout << message << " (";
        TuplePrinter<decltype(t), sizeof...(Args)>::print(t);
        std::cout << ")\n";
    }
    // end helper function

    int main()
    {
        std::tuple<int, std::string, double> t1;
        print("Value-initialized, t1:", t1);

        std::tuple<int, std::string, double> t2{42, "Test", -3.14};
        print("Initialized with values, t2:", t2);

        std::tuple<char, std::string, int> t3{t2};
        print("Implicitly converted, t3:", t3);

        std::tuple<int, double> t4{std::make_pair(42, 3.14)};
        print("Constructed from a pair, t4:", t4);

        // given Allocator my_alloc with a single-argument constructor
        // my_alloc(int); use my_alloc(1) to allocate 5 ints in a vector
        using my_alloc = std::allocator<int>;
        std::vector<int, my_alloc> v{5, 1, my_alloc{/* 1 */}};

        // use my_alloc(2) to allocate 5 ints in a vector in a tuple
        std::tuple<int, std::vector<int, my_alloc>, double> t5
            {std::allocator_arg, my_alloc{/* 2 */}, 42, v, -3.14};
        print("Constructed with allocator, t5:", t5);
    }

Possible output:

    Value-initialized, t1: (0, "", 0)
    Initialized with values, t2: (42, "Test", -3.14)
    Implicitly converted, t3: ('*', "Test", -3)
    Constructed from a pair, t4: (42, 3.14)
    Constructed with allocator, t5: (42, {1,1,1,1,1}, -3.14)

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2510 | C++11 | default constructor was implicit | made conditionally-explicit
LWG 3121 | C++11 | constructor of 1-tuple might recursively check the constraints; `allocator_arg_t` argument brought ambiguity | furtherly constrained the constructor
LWG 3158 | C++11 | the uses-allocator constructor corresponding to default constructor was implicit | made conditionally-explicit
LWG 3211 | C++11 | whether the default constructor of `tuple<>` is trivial was unspecified | require to be trivial
N4387 | C++11 | some constructors were explicit, preventing useful behavior | most constructors made conditionally-explicit

### See also

operator= (C++11) | assigns the contents of one `tuple` to another (public member function)
make_tuple (C++11) | creates a `tuple` object of the type defined by the argument types (function template)
tie (C++11) | creates a tuple of lvalue references or unpacks a tuple into individual objects (function template)
forward_as_tuple (C++11) | creates a `tuple` of forwarding references (function template)
(constructor) | constructs new pair (public member function of `std::pair<T1,T2>`)

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