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Re: Questions about wording and design intention for header units and ideas

From: Tom Honermann <tom_at_[hidden]>
Date: Thu, 18 May 2023 15:19:07 -0400
Hi, Chuanqi.

I agree with your interpretation of the standard. Since the standard is
not concerned with build systems, implementation-defined was pretty much
the only option for specifying which header files are importable
(without introducing additional syntax).

I think these concerns should be addressed by SG15. An implementation
dependent configuration file like you suggest is one way forward.
Another would be for SG15 to specify a #pragma directive or magic
comment (similar to the "-*- coding: <encoding name> -*-" comment that
some editors recognize as an indication of source code character
encoding; e.g. a "// importable-header" comment; I'm not suggesting this
is a good idea).

Since most header files meet the importable header rules, an argument
could be made for requiring an opt-out instead of (or in addition to) an

The bottom line is, the information about which headers are intended to
be importable has to come from somewhere and, as you indicate, if
implementors don't rally around a common source for that information,
they'll each make their own choices and scanning tools will have to grow
features/options to cope with that.

I know people hate it, but in my opinion, implicitly built (and cached;
using the importing TU's macro environment) BMIs are the way to go for
header units. This matches what happens today for inclusion of header
files. Yes, that means the same old ODR concerns that we've had forever
with header files. That doesn't bother me. This avoids, I think, most of
the difficulties that Daniel has been wresting with.


On 5/17/23 2:16 AM, 许传奇(夜灯) wrote:
> Hi Experts,
> In a recent private meeting, we talked about the wording and the
> design intention for header units.
> And the implementors get confused about how to implement header units
> again and we're wondering
> if it is already specified in current wording clearly or what is the
> design intentions.
> First, the motivation example:
> ```
> // a.cpp
> import "foo.h";
> // b.cpp
> #define M 43
> import "foo.h"
> ```
> The question is, for build systems, how can they know if they
> should use the same BMI for these
> 2 imports. Since **currently** the build systems don't have a **good**
> method to know whether a
> macro is interesting to a specific file.
> Then we're looking for the wording to find solutions. (Tom
> suggested us to send this to CWG, but I guess
> I found the concrete wording.) [module.import]p5 said
> (http://eel.is/c++draft/module.import#5):
> > Given two such module-import-declarations:
> > - if their header-names identify different headers or source files
> ([cpp.include]), they import distinct header units;
> > - otherwise, if they appear in the same translation unit, they import
> the same header unit;
> > - otherwise, it is unspecified whether they import the same header unit.
> So it looks like it is unspecified in the wording intentionally.
> Then we're curious about what's the proposed solution
> at the beginning.
> BTW, [module.import]p5 also introduces the implementation-defined
> concept "importable header", which is confusing
> to developers and users.
> ---
> (The section is about my personal thought about header units. The
> section may be a little not closed to the topic)
> I can roughly understand that the designer didn't want to bring too
> much limitations to the implementors.
> (Or is it the reason that it is not good to insert module map like
> things into the formal wordings.)
> But the consequence is that the C++ ecosystem is going to a more
> breaking state. For example, MSVC (or MSBuild?)
> introduces header-units.json
> (https://learn.microsoft.com/en-us/cpp/build/reference/header-unit-json-reference?view=msvc-170).
> And GCC has module map for named modules (I am not sure about gcc's
> state for header units). Clang has non-standard clang module
> maps and doesn't have a solution for header units. It looks like
> things are not going right.
> And I think a module map like thing is necessary for header units. For
> example, we can have header unit map to records the implementable
> headers and the interesting macros. Maybe something like:
> ```
> {
> "importable-headers": {
> "foo.h": ["Interesting Macro1", "Interesting Macro2", ...],
> "bar.h": [....]
> }
> }
> ```
> Then it solves the two issues raised in the mail:
> (1) We know which headers are importable: except STL headers, only the
> headers specified in the header unit map
> are importable.
> (2) We know whether or not we should reuse the same BMI for header
> units: we can reuse the BMI for the header units
> for which the interesting macro state and value are the same.
> I heard there was a decision to not make module map standard and I
> agree it is somewhat weird to have things like module map
> in the current wordings. But given we're going to have C++ ecosystem
> standard, is it possible to have a standard header unit map
> (or module map)?
> ---
> Thanks,
> Chuanqi

Received on 2023-05-18 19:19:09