Dear Tooling/EWG/LEWG,
On Mon, Mar 9, 2020 at 3:36 PM Steve Downey via Ext
<
ext@lists.isocpp.org> wrote:
>
> In particular, I'd like to see, and measure, the costs of having large overload sets present before committing to a module the size of `std` as opposed to smaller ones where I can exclude parts I am uninterested in.
>
I echo Steve's concern here. "import std;" can possibly lead to a
lot of suboptimal overload resolution. For example, clang-format
accidentally brought in extra overload sets related to std::locale and
tanked someone's performance just because of the order of includes:
https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Ftravisdowns.github.io%2Fblog%2F2019%2F11%2F19%2Ftoupper.html&data=02%7C01%7Cbion%40microsoft.com%7C4516b16941c64e934cc708d7c463cc0a%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637193805220385630&sdata=vi2SJdjeiTYX0kx3CNadLbuhHyWstxIVn8rATyXfW4Y%3D&reserved=0
C++ is incredibly sensitive to what entities are available. Since
we do not have a selection syntax (e.g., import std with { vector,
string, find_if, ... }), then our only way to control visibility --
and thus, overload resolution -- is with carefully crafted module
buckets. While there is probably no harm in having "import std;" exist
in general, we should be mindful of the problems that already exist
with "expose all the functions and types, ever" for existing and new
code. Therefore, I think both "import std;" as well as more targeted
library buckets should be considered.
Sincerely,
JeanHeyd
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