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 ifsizeof...(Types) == 0
. std::is_default_constructible<Ti>::value
is true
for all i
Ti
is not copy-list-initializable from {}
for at least one i
.sizeof...(Types) >= 1
and std::is_copy_constructible<Ti>::value
is true
for all i
. std::is_convertible<const Ti&, Ti>::value
is false
for at least one i
.std::forward<UTypes>(args)
. sizeof...(Types) == sizeof...(UTypes)
, sizeof...(Types) >= 1
, std::is_constructible<Ti, Ui>::value
is true
for all i
, and D
be std::decay<U0>::type
(until C++20)std::remove_cvref_t<U0>
(since C++20), sizeof...(Types) == 1
, then D
is not std::tuple
, otherwise, sizeof...(Types) == 2
or sizeof...(Types) == 3
, then either D
is not std::allocator_arg_t
, or T0
is std::allocator_arg_t
. std::is_convertible<Ui, Ti>::value
is false
for at least one i
.
| (since C++23) |
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))
.
sizeof...(Types) == sizeof...(UTypes)
, std::is_constructible_v<Ti, decltype(std::get<i>(FWD(other)))>
is true
for all i
, and sizeof...(Types)
is not 1
, or 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
. std::is_convertible_v<decltype(std::get<i>(FWD(other))), Ti>
is false
for at least one i
.
| (since C++23) |
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))
.
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
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
.
| (since C++23) |
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))
.
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 sizeof...(Types)
is not 1
, or Types...
expands to T
) std::is_convertible_v<UTuple, T>
and std::is_constructible_v<T, UTuple>
are both false
.other
. 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).i
, initializes the i
th element of the tuple with std::forward<Ui>(std::get<i>(other))
. 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).a
is passed as an additional argument to the constructor of each element for which std::uses_allocator<Ui, Alloc>::value
is true
.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 |
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
#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)
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 oftuple<> is trivial was unspecified | require to be trivial |
N4387 | C++11 | some constructors were explicit, preventing useful behavior | most constructors made conditionally-explicit |
(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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