gsl::copy is not in the Core Guidelines

[dmitrykobets-msft]

Microsoft's GSL currently has a definition for the gsl::copy function. However, gsl::copy isn't properly mentioned anywhere in the Core Guidelines, including the GSL: Guidelines support library section.

Should this function be added to the CG with a specification and corresponding rule?

History of the current implementation

gsl::copy was first brought up in std::copy doesn't work on 2 span · Issue #248 · microsoft/GSL (github.com), in reference to an example code snippet mentioned in I.13: Do not pass an array as a single pointer:

Alternative Consider using explicit spans:

void copy(span<const T> r, span<T> r2); // copy r to r2

It was then implemented in PR Add a copy function for span as mentioned in issue #248 by MikeGitb · Pull Request #344 · microsoft/GSL (github.com), eventually leading to the current implementation in Microsoft's GSL.

Note: there is also another instance of this idea in I.24: Avoid adjacent parameters that can be invoked by the same arguments in either order with different meaning, except with the name name copy_n:

Alternative Don’t pass arrays as pointers, pass an object representing a range (e.g., a span):

void copy_n(span<const T> p, span<T> q);  // copy from p to q

[hsutter]

Editors call: Yes, we should add a note to the GSL specification that copy(span<const T>, span<T>) be provided in GSL.

[adah1972]

It may also be worth mentioning that using std::copy on gsl::span iterators causes pathetic performance on major compilers except for MSVC, which has a special workaround. Using std::copy, there are boundary checks in each copy step. Using gsl::copy, the boundary checks are done only at the beginning.