Difference between std::ranges::subrange and std::span

std::ranges::subrange and std::span both provide views over a sequence of elements without owning the elements themselves, but they serve different purposes and have distinct characteristics.

std::ranges::subrange has the following characteristics:

• Part of Ranges Library: std::ranges::subrange is part of the Ranges library, introduced in C++20. It provides a way to create views of a range defined by iterators.
• Flexible Iterators: It can work with any type of iterator, not just pointers. This means you can create a subrange from various types of containers and iterators.
• Slicing: You can create a subrange from a subset of a container, not necessarily starting from the beginning.
• Non-Contiguous: Subranges do not require the underlying data to be contiguous in memory.

The following program uses std::ranges::subrange to view part of an underlying std::vector:

1#include <iostream>
2#include <ranges>
3#include <vector>
4
5int main() {
6  std::vector<int> Nums{1, 2, 3, 4, 5};
7  std::ranges::subrange View{
8    Nums.begin() + 1, Nums.end() - 1};
9
10  for (int n : View) {
11    std::cout << n << ", ";
12  }
13}

12, 3, 4,

std::span has these characteristics:

• Part of Utilities Library: std::span is part of the Utilities library and was introduced in C++20. It provides a view over a contiguous sequence of elements, typically from an array or a std::vector.
• Pointer-Based: It is essentially a pair of a pointer and a size, meaning it only works with contiguous memory blocks.
• Fixed Size: std::span views are fixed at the time of creation and cannot be resized.
• More Limited Use: It is more limited in scope compared to std::ranges::subrange, focusing on contiguous memory.

The following example program uses std::span to create a view of an underlying std::vector:

1#include <vector>
2#include <span>
3#include <iostream>
4
5int main() {
6  std::vector<int> Nums{1, 2, 3, 4, 5};
7  std::span<int> View{Nums.data(), Nums.size()};
8
9  for (int n : View) {
10    std::cout << n << ", ";
11  }
12}

11, 2, 3, 4, 5,

Key Differences

• Flexibility: std::ranges::subrange is more flexible, working with various iterators and non-contiguous memory, while std::span is limited to contiguous memory.
• Purpose: std::ranges::subrange is useful for range-based operations and slicing, whereas std::span is more suited for working with contiguous arrays and interfacing with legacy APIs.
• Usage Context: Use std::ranges::subrange for general range operations and std::span when you need a simple, contiguous view of data.

By understanding these differences, you can choose the appropriate type for your specific needs in C++.