span-of-incomplete-element-type.md

Support span of elements of incomplete type

Document Number: D????R0
Date: 2021-12-08
Author: Andrey Davydov (andrey.a.davydov@gmail.com)
Audience: Library Evolution Incubator (LEWGI)

Introduction

This paper proposes allowing span to be instantiated with incomplete element type.

Motivation and Scope

Currently span requires element type to be a complete object type ([span.overview]). This requirement contradicts to guideline prefer span over the common idiom of a “pointer plus length” pair of parameters. For instance, C++ Core Guidelines contains the following recommendation:

Example The safety Profiles recommend replacing

void f(int* some_ints, int some_ints_length);  // BAD: C style, unsafe

with

void f(gsl::span<int> some_ints);              // GOOD: safe, bounds-checked

Let's consider the following quite natural and innocent code:

class Element;

class Storage {
  ...
  span<Element> get_elements();
};

extern void process(span<Element>);

void do_something(Storage& storage) {
  process(storage.get_elements());
}

Right now (C++20) it is not valid due to the requirement that class Element must be complete, but replacement of span<Element> with pair<Element*, size_t> "fixes" the code, because nor declaration of the function Storage::get_elements() nor declaration of the function process require complete type Element. The goal of this proposal is to make the example above valid as is.

Impact on the Standard

This proposal is a pure library extension.

Proposed Wording

Remove 22.7.3.1 p4 [span.overview].

TODO

Modify 22.7.3.*: add requirement "ElementType must be a complete object type that is not an abstract class type" to some members of span.