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std:: get (std::tuple)

From cppreference.net
C++
Utilities library
std::tuple
Defined in header <tuple>
template < std:: size_t I, class ... Types >

typename std:: tuple_element < I, std:: tuple < Types... >> :: type &

get ( std:: tuple < Types... > & t ) noexcept ;
(1) (since C++11)
(constexpr since C++14)
template < std:: size_t I, class ... Types >

typename std:: tuple_element < I, std:: tuple < Types... >> :: type &&

get ( std:: tuple < Types... > && t ) noexcept ;
(2) (since C++11)
(constexpr since C++14)
template < std:: size_t I, class ... Types >

const typename std:: tuple_element < I, std:: tuple < Types... >> :: type &

get ( const std:: tuple < Types... > & t ) noexcept ;
(3) (since C++11)
(constexpr since C++14)
template < std:: size_t I, class ... Types >

const typename std:: tuple_element < I, std:: tuple < Types... >> :: type &&

get ( const std:: tuple < Types... > && t ) noexcept ;
(4) (since C++11)
(constexpr since C++14)
template < class T, class ... Types >
constexpr T & get ( std:: tuple < Types... > & t ) noexcept ;
(5) (since C++14)
template < class T, class ... Types >
constexpr T && get ( std:: tuple < Types... > && t ) noexcept ;
(6) (since C++14)
template < class T, class ... Types >
constexpr const T & get ( const std:: tuple < Types... > & t ) noexcept ;
(7) (since C++14)
template < class T, class ... Types >
constexpr const T && get ( const std:: tuple < Types... > && t ) noexcept ;
(8) (since C++14)
1-4) Extracts the I th element from the tuple. I must be an integer value in [ ​ 0 ​ , sizeof... ( Types ) ) .
5-8) Extracts the element of the tuple t whose type is T . Fails to compile unless the tuple has exactly one element of that type.

Contents

Parameters

t - tuple whose contents to extract

Return value

A reference to the selected element of t .

Notes

Feature-test macro Value Std Feature
__cpp_lib_tuples_by_type 201304L (C++14) Addressing tuples by type ( 5-8 )

Example

Run this code 
#include <cassert>
#include <iostream>
#include <string>
#include <tuple>
int main()
{
    auto x = std::make_tuple(1, "Foo", 3.14);
    // Index-based access
    std::cout << "( " << std::get<0>(x)
              << ", " << std::get<1>(x)
              << ", " << std::get<2>(x)
              << " )\n";
    // Type-based access (since C++14)
    std::cout << "( " << std::get<int>(x)
              << ", " << std::get<const char*>(x)
              << ", " << std::get<double>(x)
              << " )\n";
    const std::tuple<int, const int, double, double> y(1, 2, 6.9, 9.6);
    const int& i1 = std::get<int>(y); // OK: not ambiguous
    assert(i1 == 1);
    const int& i2 = std::get<const int>(y); // OK: not ambiguous
    assert(i2 == 2);
    // const double& d = std::get<double>(y); // Error: ill-formed (ambiguous)
    // Note: std::tie and structured binding can be
    // used to unpack a tuple into individual objects.
}

Output: 

( 1, Foo, 3.14 )
( 1, Foo, 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 2485 C++11 (by index)
C++14 (by type) 		there are no overloads for const tuple && added these overloads ( ( 4 ) and ( 8 ) )

See also

(C++11)
accesses an element of an array
(function template)
(C++11)
accesses an element of a pair
(function template)
(C++17)
reads the value of the variant given the index or the type (if the type is unique), throws on error
(function template)
(C++20)
obtains iterator or sentinel from a std::ranges::subrange
(function template)
(C++26)
obtains a reference to real or imaginary part from a std::complex
(function template)
(C++11)
creates a tuple of lvalue references or unpacks a tuple into individual objects
(function template)
Structured binding (C++17) binds the specified names to sub-objects or tuple elements of the initializer