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

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std::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 ith type in Types, and
  • Ui be the ith 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
  • The constructor is explicit if and only if std::is_convertible<Ui, Ti>::value is false for at least one i.
(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 ith element of the tuple with std::get<i>(FWD(other)).

  • 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)).

  • 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 ith element of the tuple with std::get<i>(std::forward<UTuple>(u)).

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 ith 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

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