NAME¶

std::tuple::tuple - std::tuple::tuple

Synopsis¶

Defined in header <tuple>
constexpr tuple(); (1) (since C++11)
(conditionally explicit)
(since C++11)
tuple( const Types&... args ); (2) (constexpr since C++14)
(conditionally explicit)
template< class... UTypes > (since C++11)
tuple( UTypes&&... args ); (3) (constexpr since C++14)
(conditionally explicit)
template< class... UTypes > (4) (since C++23)
constexpr tuple( tuple<UTypes...>& other ); (conditionally explicit)
template< class... UTypes > (since C++11)
tuple( const tuple<UTypes...>& other ); (5) (constexpr since C++14)
(conditionally explicit)
template< class... UTypes > (since C++11)
tuple( tuple<UTypes...>&& other ); (6) (constexpr since C++14)
(conditionally explicit)
template< class... UTypes > (7) (since C++23)
constexpr tuple( const tuple<UTypes...>&& other ); (conditionally explicit)
template< class U1, class U2 > (8) (since C++23)
constexpr tuple( std::pair<U1, U2>& p ); (conditionally explicit)
template< class U1, class U2 > (since C++11)
tuple( const std::pair<U1, U2>& p ); (9) (constexpr since C++14)
(conditionally explicit)
template< class U1, class U2 > (since C++11)
tuple( std::pair<U1, U2>&& p ); (10) (constexpr since C++14)
(conditionally explicit)
template< class U1, class U2 > (11) (since C++23)
constexpr tuple( const std::pair<U1, U2>&& p ); (conditionally explicit)
tuple( const tuple& other ) = default; (12) (since C++11)
tuple( tuple&& other ) = default; (13) (since C++11)
template< class Alloc > (since C++11)
tuple( std::allocator_arg_t, const Alloc& a ); (14) (constexpr since C++20)
(conditionally explicit)
template< class Alloc >
(since C++11)
tuple( std::allocator_arg_t, const Alloc& a, (15) (constexpr since C++20)
(conditionally explicit)
const Types&... args );
template< class Alloc, class... UTypes >
(since C++11)
tuple( std::allocator_arg_t, const Alloc& a, (16) (constexpr since C++20)
(conditionally explicit)
UTypes&&... args );
template< class Alloc, class... UTypes >
(since C++23)
constexpr tuple( std::allocator_arg_t, const Alloc& a, (17) (conditionally explicit)

tuple<UTypes...>& other );
template< class Alloc, class... UTypes >
(since C++11)
tuple( std::allocator_arg_t, const Alloc& a, (18) (constexpr since C++20)
(conditionally explicit)
const tuple<UTypes...>& other );
template< class Alloc, class... UTypes >
(since C++11)
tuple( std::allocator_arg_t, const Alloc& a, (19) (constexpr since C++20)
(conditionally explicit)
tuple<UTypes...>&& other );
template< class Alloc, class... UTypes >
(since C++23)
constexpr tuple( std::allocator_arg_t, const Alloc& a, (20) (conditionally explicit)

const tuple<UTypes...>&& other );
template< class Alloc, class U1, class U2 >
(since C++23)
constexpr tuple( std::allocator_arg_t, const Alloc& a, (21) (conditionally explicit)

std::pair<U1, U2>& p );
template< class Alloc, class U1, class U2 >
(since C++11)
tuple( std::allocator_arg_t, const Alloc& a, (22) (constexpr since C++20)
(conditionally explicit)
const std::pair<U1, U2>& p );
template< class Alloc, class U1, class U2 >
(since C++11)
tuple( std::allocator_arg_t, const Alloc& a, (23) (constexpr since C++20)
(conditionally explicit)
std::pair<U1, U2>&& p );
template< class Alloc, class U1, class U2 >
(since C++23)
constexpr tuple( std::allocator_arg_t, const Alloc& a, (24) (conditionally explicit)

const std::pair<U1, U2>&& p );
template< class Alloc >
(since C++11)
tuple( std::allocator_arg_t, const Alloc& a, (25) (constexpr since C++20)

const tuple& other );
template< class Alloc >
(since C++11)
tuple( std::allocator_arg_t, const Alloc& a, (26) (constexpr since C++20)

tuple&& other );

Constructs a new tuple.

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), and
* sizeof...(Types) >= 1, and
* 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 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 (since C++23)
object.

4) Converting copy-constructor. For all i in sizeof...(UTypes), initializes ith
element of the tuple with std::get<i>(other).

* This overload participates in overload resolution only if

* sizeof...(Types) == sizeof...(UTypes) and
* std::is_constructible_v<Ti, Ui&> is true for all i and
* either

* sizeof...(Types) != 1 or
* (when Types... expands to T and UTypes... expands to U)
std::is_convertible_v<tuple<U>&, T>, std::is_constructible_v<T,
tuple<U>&>, and std::is_same_v<T, U> are all false.

* The constructor is explicit if and only if std::is_convertible_v<Ui&, Ti> 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.

5) Converting copy-constructor. For all i in sizeof...(UTypes), initializes ith
element of the tuple with std::get<i>(other).

* This overload participates in overload resolution only if

* sizeof...(Types) == sizeof...(UTypes) and
* std::is_constructible<Ti, const Ui&>::value is true for all i and
* either

* sizeof...(Types) != 1 or
* (when Types... expands to T and UTypes... expands to U)
std::is_convertible<const tuple<U>&, T>::value,
std::is_constructible<T, const tuple<U>&>::value, and std::is_same<T,
U>::value are all false.

* The constructor is explicit if and only if std::is_convertible<const 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 (since C++23)
object.

6) Converting move-constructor. For all i in sizeof...(UTypes), initializes ith
element of the tuple with std::forward<Ui>(std::get<i>(other)).

* This overload participates in overload resolution only if

* sizeof...(Types) == sizeof...(UTypes) and
* std::is_constructible<Ti, Ui>::value is true for all i and
* either

* sizeof...(Types) != 1 or
* (when Types... expands to T and UTypes... expands to U)
std::is_convertible<tuple<U>, T>::value, std::is_constructible<T,
tuple<U>>::value, and std::is_same<T, U>::value are all false.

* 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 (since C++23)
object.

7) Converting move-constructor. For all i in sizeof...(UTypes), initializes ith
element of the tuple with std::forward<const Ui>(std::get<i>(other)).

* This overload participates in overload resolution only if

* sizeof...(Types) == sizeof...(UTypes) and
* std::is_constructible_v<Ti, const Ui> is true for all i and
* either

* sizeof...(Types) != 1 or
* (when Types... expands to T and UTypes... expands to U)
std::is_convertible_v<const tuple<U>, T>, std::is_constructible_v<T,
const tuple<U>>, and std::is_same_v<T, U> are all false.

* The constructor is explicit if and only if std::is_convertible_v<const Ui, Ti>
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.

8) Pair copy constructor. Constructs a 2-element tuple with the first element
constructed from p.first and the second element from p.second

* This overload participates in overload resolution only if sizeof...(Types) == 2
and std::is_constructible_v<T0, U1&> and std::is_constructible_v<T1, U2&> are
both true
* The constructor is explicit if and only if std::is_convertible_v<U1&, T0> or
std::is_convertible_v<U2&, T1> is false.
* This constructor is defined as deleted if the initialization of any element that
is a reference would bind it to a temporary object.

9) Pair copy constructor. Constructs a 2-element tuple with the first element
constructed from p.first and the second element from p.second

* This overload participates in overload resolution only if sizeof...(Types) == 2
and std::is_constructible<T0,const U1&>::value and std::is_constructible<T1,
const U2&>::value are both true
* The constructor is explicit if and only if std::is_convertible<const U1&,
T0>::value or std::is_convertible<const U2&, T1>::value is false.

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

10) Pair move constructor. Constructs a 2-element tuple with the first element
constructed from std::forward<U1>(p.first) and the second element from
std::forward<U2>(p.second)

* This overload participates in overload resolution only if sizeof...(Types) == 2
and std::is_constructible<T0, U1>::value and std::is_constructible<T1,
U2>::value are both true
* The constructor is explicit if and only if std::is_convertible<U1, T0>::value or
std::is_convertible<U2, T1>::value is false.

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

11) Pair move constructor. Constructs a 2-element tuple with the first element
constructed from std::forward<const U1>(p.first) and the second element from
std::forward<const U2>(p.second)

* This overload participates in overload resolution only if sizeof...(Types) == 2
and std::is_constructible_v<T0, const U1> and std::is_constructible_v<T1, const
U2> are both true
* The constructor is explicit if and only if std::is_convertible_v<U1, T0> or
std::is_convertible_v<U2, T1> is false.
* This constructor is defined as deleted if the initialization of any element that
is a reference would bind it to a temporary object.

12) 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).

13) Implicitly-defined move constructor. Initializes each 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).

14-26) Identical to (1-13) 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 - a tuple of values used to initialize each element of the tuple
p - pair of values used to initialize both elements of this 2-tuple
a - 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 <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(const std::tuple<Args...>& t)
{
std::cout << "(";
TuplePrinter<decltype(t), sizeof...(Args)>::print(t);
std::cout << ")\n";
}
// end helper function

int main()
{
std::tuple<int, std::string, double> t1;
std::cout << "Value-initialized, t1: "; print(t1);
std::tuple<int, std::string, double> t2{42, "Test", -3.14};
std::cout << "Initialized with values, t2: "; print(t2);
std::tuple<char, std::string, int> t3{t2};
std::cout << "Implicitly converted, t3: "; print(t3);
std::tuple<int, double> t4{std::make_pair(42, 3.14)};
std::cout << "Constructed from a pair, t4: "; print(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};
std::cout << "Constructed with allocator, t5: "; print(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
N4387 C++11 some constructors were explicit, most constructors made
preventing useful behavior conditionally-explicit
LWG 2510 C++11 default constructor was implicit made conditionally-explicit
constructor of 1-tuple might
LWG 3121 C++11 recursively check the constraints; furtherly constrained the
allocator_arg_t argument brought constructor
ambiguity
the uses-allocator constructor
LWG 3158 C++11 corresponding to made conditionally-explicit
default constructor was implicit
LWG 3211 C++11 whether the default constructor of require to be trivial
tuple<> is trivial was unspecified

See also¶

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