Slices vs Arrays in Carbon language

Question

What are the differences between Array and Slice in Carbon? I find a document from the official repo. However, it is uncompleted currently.

The following are example codes from the Carbon official repo.

// Carbon:
package Geometry api;
import Math;

class Circle {
  var r: f32;
}

fn PrintTotalArea(circles: Slice(Circle)) {
  var area: f32 = 0;
  for (c: Circle in circles) {
    area += Math.Pi * c.r * c.r;
  }
  Print("Total area: {0}", area);
}

fn Main() -> i32 {
  // A dynamically sized array, like `std::vector`.
  var circles: Array(Circle) = ({.r = 1.0}, {.r = 2.0});
  // Implicitly constructs `Slice` from `Array`.
  PrintTotalArea(circles);
  return 0;
}

I guess that the Slice is something like std::span, since the C++ version of the codes above uses std::span when the Carbon version uses Slice. Am I correct?

// C++:
#include <math.h>
#include <iostream>
#include <span>
#include <vector>

struct Circle {
  float r;
};

void PrintTotalArea(std::span<Circle> circles) {
  float area = 0;
  for (const Circle& c : circles) {
    area += M_PI * c.r * c.r;
  }
  std::cout << "Total area: " << area << "\n";
}

auto main(int argc, char** argv) -> int {
  std::vector<Circle> circles = {{1.0}, {2.0}};
  // Implicitly constructors `span` from `vector`.
  PrintTotalArea(circles);
  return 0;
}

Answer 1

Currently, a first version Arrays exist, while continuing to being worked on, and the Slice type is basically not defined, so it is a bit early to give you a real answer.

As you guessed, it seems like slices might eventually have similarities with std::span, based on some design discussions, but nothing has been approved yet.

You can check the open design proposals on GitHub directly.