std::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.

[edit] Template parameters

[edit] Member functions

[edit] Non-member functions

[edit] Helper concepts

[edit] Helper classes

[edit] Helper specializations

This specialization of std::enable_nonlocking_formatter_optimization enables efficient implementation of std::print and std::println for printing a tuple object when each element type enables it.

[edit] Deduction guides (since C++17)

[edit] 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
}

[edit] Example

#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';
}

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

[edit] Defect reports

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

[edit] References

[edit] See also

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