C++ array std::get

The std::get function retrieves the element at the specified index from a std::array. It is a template function and provides a type-safe way to access elements by their index. The index must be a compile-time constant, and bounds checking is not performed.

Syntax of std::get

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template<std::size_t I, class T, std::size_t N>
constexpr typename std::array<T, N>::reference get(std::array<T, N>& arr) noexcept;

template<std::size_t I, class T, std::size_t N>
constexpr typename std::array<T, N>::const_reference get(const std::array<T, N>& arr) noexcept;

Parameters

ParameterDescription
IA compile-time constant representing the index of the element to access.
arrThe std::array object from which the element is retrieved.

Return Value

Returns a reference to the element at the specified index I. If the array is const, the returned reference is also const.

Exceptions

The std::get function does not throw exceptions. However, accessing an out-of-bounds index results in a compilation error.

Examples for std::get

Example 1: Accessing Elements Using std::get

This example demonstrates how to access elements of a std::array using std::get:

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#include <iostream>
#include <array>

int main() {
    std::array<int, 5> arr = {10, 20, 30, 40, 50};

    std::cout << "Element at index 0: " << std::get<0>(arr) << std::endl;
    std::cout << "Element at index 2: " << std::get<2>(arr) << std::endl;

    std::get<4>(arr) = 100; // Modify the element at index 4
    std::cout << "Modified element at index 4: " << std::get<4>(arr) << std::endl;

    return 0;
}

Explanation:

  1. A std::array named arr is initialized with the elements {10, 20, 30, 40, 50}.
  2. The std::get<I>(arr) function is used to retrieve the element at index I, where I is a compile-time constant.
  3. The element at index 4 is modified to 100 using std::get.
  4. The modified value is printed to confirm the change.

Output:

Element at index 0: 10
Element at index 2: 30
Modified element at index 4: 100

Example 2: Compile-Time Error for Invalid Index

This example demonstrates the compile-time error when trying to access an invalid index:

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#include <iostream>
#include <array>

int main() {
    std::array<int, 3> arr = {1, 2, 3};

    std::cout << std::get<3>(arr) << std::endl; // Index 3 is out of bounds

    return 0;
}

Explanation:

Attempting to access index 3 in a std::array of size 3 results in a compile-time error, as std::get requires the index to be within bounds.

Output:

In file included from main.cpp:2:
/usr/include/c++/11/array: In instantiation of ‘constexpr _Tp& std::get(std::array<_Tp, _Nm>&) [with long unsigned int _Int = 3; _Tp = int; long unsigned int _Nm = 3]’:
main.cpp:7:29:   required from here
/usr/include/c++/11/array:363:26: error: static assertion failed: array index is within bounds
  363 |       static_assert(_Int < _Nm, "array index is within bounds");
      |                     ~~~~~^~~~~