views.md

std::span

std::span is a feature in C++20 that serves as a lightweight, non-owning reference to a subsequence (a contiguous range) of an array or a container like std::vector, std::array, or even a C-style array. The primary use case for std::span is to provide a way to pass around the information of a "view" of a data structure without actually copying the underlying data. This can be particularly useful for improving performance and reducing memory overhead in various algorithms and data manipulations.

Here's a simple example using std::span:

#include <iostream>
#include <span>

void print_elements(std::span<int> arr) {
    for (int element : arr) {
        std::cout << element << " ";
    }
    std::cout << std::endl;
}

int main() {
    int my_array[5] = {1, 2, 3, 4, 5};
    std::span<int> my_span(my_array);

    print_elements(my_span);

    return 0;
}

In this example, print_elements takes a std::span parameter. This allows it to operate on the array without needing to know its size at compile time or owning a copy of the data.

Key Properties

1. Non-owning: std::span does not own the data it points to; it's just a view.
2. Contiguous: Points to a contiguous block of memory (like arrays or std::vector).
3. Type-safe: Offers compile-time checking.
4. Dynamic Extent: The size can be determined at runtime.
5. Static Extent: The size is known at compile-time.

Member Functions

Here are some key member functions that you'll likely use:

• size(): Returns the number of elements in the span.
• empty(): Checks if the span is empty.
• front(), back(): Returns the first and the last element, respectively.
• operator[]: Allows indexed access to elements.
• begin(), end(): Provide iterators to the first and past-the-last elements.

Construction

int a[10];
std::vector<int> v;
std::array<int, 10> arr;

std::span<int> span1(a, 10);        // from pointer and length
std::span<int> span2(a, a + 10);    // from two pointers
std::span<int> span3(v);            // from std::vector
std::span<int> span4(arr);          // from std::array

Subviews

You can easily create subviews:

std::span<int> span1(a, 10);
std::span<int> subspan = span1.subspan(0, 5);  // First 5 elements