NAME¶

std::tuple - std::tuple

Synopsis¶

Defined in header <tuple>
template< class... Types > (since C++11)
class tuple;

Class template std::tuple is a fixed-size collection of heterogeneous values. It is
a generalization of std::pair.

If std::is_trivially_destructible<Ti>::value is true for every Ti in Types, the
destructor of std::tuple is trivial.

If a program declares an explicit or partial specialization of std::tuple, the
program is ill-formed, no diagnostic required.

Template parameters¶

Types... - the types of the elements that the tuple stores. Empty list is supported.

Member functions¶

constructor constructs a new tuple
(public member function)
operator= assigns the contents of one tuple to another
(public member function)
swap swaps the contents of two tuples
(public member function)

Non-member functions¶

make_tuple creates a tuple object of the type defined by the argument
(C++11) types
(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)
tuple_cat creates a tuple by concatenating any number of tuples
(C++11) (function template)
get(std::tuple) tuple accesses specified element
(C++11) (function template)
operator==
operator!=
operator<
operator<=
operator>
operator>= lexicographically compares the values in the tuple
operator<=> (function template)
(removed in C++20)
(removed in C++20)
(removed in C++20)
(removed in C++20)
(removed in C++20)
(C++20)
std::swap(std::tuple) specializes the std::swap algorithm
(C++11) (function template)

Helper concepts

tuple-like specifies that a type implemented the tuple protocol
pair-like (std::get, std::tuple_element, std::tuple_size)
(C++23) (exposition-only concept*)

Helper classes¶

std::tuple_size<std::tuple> obtains the size of tuple at compile time
(C++11) (class template specialization)
std::tuple_element<std::tuple> obtains the type of the specified element
(C++11) (class template specialization)
std::uses_allocator<std::tuple> specializes the std::uses_allocator type
(C++11) trait
(class template specialization)
std::basic_common_reference<tuple-like> determines the common reference type of a
(C++23) tuple and a tuple-like type
(class template specialization)
std::common_type<tuple-like> determines the common type of a tuple and a
(C++23) tuple-like type
(class template specialization)
std::formatter<std::tuple> formatting support for tuple
(C++23) (class template specialization)
ignore placeholder to skip an element when
(C++11) unpacking a tuple using tie
(constant)

Deduction guides (since C++17)

Notes¶

Since the "shape" of a tuple – its size, the types of its elements, and the ordering
of those types – are part of its type signature, they must all be available at
compile time and can only depend on other compile-time information. This means that
many conditional operations on tuples – in particular, conditional prepend/append
and filter – are only possible if the conditions can be evaluated at compile time.
For example, given a std::tuple<int, double, int>, it is possible to filter on types
– e.g. returning a std::tuple<int, int> – but not to filter on whether or not each
element is positive (which would have a different type signature depending on
runtime values of the tuple), unless all the elements were themselves constexpr.

As a workaround, one can work with tuples of std::optional, but there is still no
way to adjust the size based on runtime information.

Until N4387 (applied as a defect report for C++11), a function could not return a
tuple using copy-list-initialization:

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

Example¶

// Run this code

#include <iostream>
#include <stdexcept>
#include <string>
#include <tuple>

std::tuple<double, char, std::string> get_student(int id)
{
switch (id)
{
case 0: return {3.8, 'A', "Lisa Simpson"};
case 1: return {2.9, 'C', "Milhouse Van Houten"};
case 2: return {1.7, 'D', "Ralph Wiggum"};
case 3: return {0.6, 'F', "Bart Simpson"};
}

throw std::invalid_argument("id");
}

int main()
{
const auto student0 = get_student(0);
std::cout << "ID: 0, "
<< "GPA: " << std::get<0>(student0) << ", "
<< "grade: " << std::get<1>(student0) << ", "
<< "name: " << std::get<2>(student0) << '\n';

const auto student1 = get_student(1);
std::cout << "ID: 1, "
<< "GPA: " << std::get<double>(student1) << ", "
<< "grade: " << std::get<char>(student1) << ", "
<< "name: " << std::get<std::string>(student1) << '\n';

double gpa2;
char grade2;
std::string name2;
std::tie(gpa2, grade2, name2) = get_student(2);
std::cout << "ID: 2, "
<< "GPA: " << gpa2 << ", "
<< "grade: " << grade2 << ", "
<< "name: " << name2 << '\n';

// C++17 structured binding:
const auto [gpa3, grade3, name3] = get_student(3);
std::cout << "ID: 3, "
<< "GPA: " << gpa3 << ", "
<< "grade: " << grade3 << ", "
<< "name: " << name3 << '\n';
}

Output:¶

ID: 0, GPA: 3.8, grade: A, name: Lisa Simpson
ID: 1, GPA: 2.9, grade: C, name: Milhouse Van Houten
ID: 2, GPA: 1.7, grade: D, name: Ralph Wiggum
ID: 3, GPA: 0.6, grade: F, name: Bart Simpson

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 2796 C++11 triviality of the destructor of specified
std::tuple was unspecified
the program is ill-formed
LWG 3990 C++11 a program could declare an explicit or in this
partial specialization of std::tuple case (no diagnostic
required)

References¶

* C++23 standard (ISO/IEC 14882:2023):

* 22.4 Tuples [tuple]
* C++20 standard (ISO/IEC 14882:2020):

* 20.5 Tuples [tuple]
* C++17 standard (ISO/IEC 14882:2017):

* 23.5 Tuples [tuple]
* C++14 standard (ISO/IEC 14882:2014):

* 20.4 Tuples [tuple]
* C++11 standard (ISO/IEC 14882:2011):

* 20.4 Tuples [tuple]

See also¶

pair implements binary tuple, i.e. a pair of values
(class template)