tuple_element Class

Artikkeli
12/07/2021

Wraps a tuple element. Specializations wrap array elements and pair elements.

Syntax

// CLASS tuple_element (find element by index)
template <size_t Index, class Tuple>
   struct tuple_element;

// tuple_element for const
template <size_t Index, class Tuple>
   struct tuple_element<Index, const Tuple>;

// tuple_element for volatile
template <size_t Index, class Tuple>
   struct tuple_element<Index, volatile Tuple>;

// tuple_element for const volatile
template <size_t Index, class Tuple>
   struct tuple_element<Index, const volatile Tuple>;

// Helper typedef
template <size_t Index, class Tuple>
   using tuple_element_t = typename tuple_element<Index, Tuple>::type;

// Specialization for arrays
template <size_t Index, class Elem, size_t Size>
   struct tuple_element<Index, array<Elem, Size>>;

// Specializations for pairs
// struct to determine type of element 0 in pair
template <class T1, class T2>
   struct tuple_element<0, pair<T1, T2>>;

// struct to determine type of element 1 in pair
template <class T1, class T2>
   struct tuple_element<1, pair<T1, T2>>;

Parameters

Index
The index of the designated element.

Tuple
The type of the tuple.

Elem
The type of an array element.

Size
The size of the array.

T1
The type of the first element in a pair.

T2
The type of the second element in a pair.

Remarks

The class template tuple_element has a nested typedef type that is a synonym for the type at index Index of the tuple type Tuple.

The typedef tuple_element_t is a convenient alias for tuple_element<Index, Tuple>::type.

The class template specialization for arrays provides an interface to an array as a tuple of Size elements, each of which has the same type. Each specialization has a nested typedef type that is a synonym for the type of the Index element of the array, with any const-volatile qualifications preserved.

The template specializations for pair types each provide a single member typedef, type, which is a synonym for the type of the element at the specified position in the pair, with any const and/or volatile qualifications preserved. The typedef tuple_element_t is a convenient alias for tuple_element<N, pair<T1, T2>>::type.

Use the get function to return the element at a specified position, or of a specified type.

Example: Get an element from a tuple

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

using namespace std;
typedef tuple<int, double, string> MyTuple;

int main() {
    MyTuple c0{ 0, 1.5, "Tail" };

    tuple_element_t<0, MyTuple> val = get<0>(c0); //get by index
    tuple_element_t<1, MyTuple> val2 = get<1>(c0);
    tuple_element_t<2, MyTuple> val3 = get<string>(c0); // get by type

    cout << val << " " << val2 << " " << val3 << endl;
}

0 1.5 Tail

Example: Get an element from an array

#include <array>
#include <iostream>

using namespace std;
typedef array<int, 4> MyArray;

int main()
{
    MyArray c0 { 0, 1, 2, 3 };

    for (const auto& e : c0)
    {
        cout << e << " ";
    }
    cout << endl;

    // display first element "0"
    tuple_element<0, MyArray>::type val = c0.front();
    cout << val << endl;
}

0 1 2 3
0

Example: Get an element from a pair

#include <utility>
#include <iostream>

using namespace std;

typedef pair<int, double> MyPair;
int main() {
    MyPair c0(0, 1.333);

    // display contents "0 1"
    cout << get<0>(c0) << " ";
    cout << get<1>(c0) << endl;

    // display first element "0 " by index
    tuple_element<0, MyPair>::type val = get<0>(c0);
    cout << val << " ";

    // display second element by type
    tuple_element<1, MyPair>::type val2 = get<double>(c0);
    cout << val2 << endl;
}

0 1.333
0 1.333

Requirements

Header: <tuple>

Header: <array> (for array specialization)

Header: <utility> (for pair specializations)

Namespace: std

Jaa