Span vs Vector in C++

The main differences between std::span and std::vector are:

Ownership

std::vector owns the elements it contains. It manages the memory allocation and deallocation for those elements.

std::span does not own the elements it references. It simply provides a view into a contiguous sequence of elements stored elsewhere.

Size

The size of a std::vector can change dynamically at runtime. Elements can be added or removed from a vector.

The size of a std::span is fixed at the time it is created. A span cannot change the number of elements it references.

Memory Allocation

std::vector allocates its own memory to store elements. This allocation happens on the heap.

std::span does not allocate any memory. It simply holds a pointer to elements stored elsewhere, either on the stack or the heap.

Cost of Copying

Copying a std::vector is an expensive operation because it involves allocating new memory and copying all the elements.

Copying a std::span is cheap, it only involves copying a pointer and a size.

You should use std::span when you want a lightweight, non-owning view into a sequence of elements, and you don't need to change the size of that sequence. This is often useful for passing arrays to functions:

1#include <span>
2#include <vector>
3
4void Process(std::span<const int> data) {
5  for (int i : data) {
6    // Process each element
7  }
8}
9
10int main() {
11  std::vector<int> v{1, 2, 3, 4, 5};
12  Process(v);
13}

Use std::vector when you need ownership of the elements, or when you need to be able to change the number of elements dynamically.