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Re: [std-proposals] constexpr support in std::filesystem API

From: Jonathan Wakely <cxx_at_[hidden]>
Date: Wed, 13 Mar 2024 11:13:03 +0000
On Wed, 13 Mar 2024 at 10:07, Lénárd Szolnoki via Std-Proposals
<std-proposals_at_[hidden]> wrote:
> Hi,
> On Tue, 2024-03-12 at 13:31 +0200, Andrei Grosu via Std-Proposals
> wrote:
> > The proposal is simple: constexpr support for the filesystem API.
> >
> > The need comes from writing a build system in (modern) C++.
> > If there is support for compile-time access to the filesystem , it
> > would be , in my opinion, the key missing piece for a build system
> > implemented in modern C++.
> > Without that you would have to depend on code generation , but with
> > it , there is not much missing to build a fully featured build system
> > in C++ itself.
> I think you failed to sell the motivation of this, but I can think of
> use cases where #embed "string literal" could potentially show its
> limitations in the future.
> #embed does work great for its intended purpose of embedding self-
> contained resources into executables. But given that #embed exposes the
> contents as a constant expression, I think it can be reasonable
> foreseen that it will not only be used for efficiently embedding huge
> resources into executables, but for example to efficiently generate
> APIs and/or runtime validation code from protocol specifications. Some
> (most?) of the popular protocol specification formats are not self-
> contained and can refer to other files. Some of these rely on code
> generation to translate these specifications into safe C++ APIs and/or
> to runtime validation code, but in the future they could switch to
> #embed and reflection/reification to do the same, once we get there.
> I think it's not entirely unreasonable to want embed-like functionality
> that works on a "treeish" instead of "blob" (in git speak), and allow
> the compilers to map directory trees to these treeish objects exposed
> within the program in an implementation-defined way (not unlike how
> #include and import mappings are also implementation-defined). This
> shouldn't be based on std::filesystem for concerns already raised, but
> I think the idea has merit.
> Maybe it is a bit too early though, and first we will need to see how
> #embed pans out. I can imagine pure #embed-based patterns dealing with
> non-selfcontained formats, but they might require boilerplate and
> repetition and/or they are inefficient. It's also possible that there
> is a pattern with #embed that just works well for the purpose.
> >
> > Is this a question of compiler complexity , to enable constexpr
> > filesystem access ? It is not clear to me if there are other factors
> > at play.
> >
> > Can anyone ‘in the know’ share some insights why it might or might
> > not be feasable ?
> I think it's feasible, I think a lot of concerns are around these
> points:
> * restrictions on the FS access (shouldn't just expose the build
> machine's FS as is)
> * Which parts of FS semantics that are important (do we want directory
> listing, various file metadata, file types as in regular, symlink,
> block device...)
> * confusing runtime and compile time std::filesystem behavior
> I don't think that any of this couldn't be solved. IMO a feature like
> this:
> * just shouldn't use anything like std::filesystem, fopen, ifstream,
> etc...
> * compilers shouldn't expose the whole of the host FS, only parts that
> are explicitly specified by compiler options. This doesn't make this
> not worthy of standardizing, just like #include and import (and
> #embed!) was worthy of standardizing. All compilers have flags to
> specifiy include directories and import mappings. Not sure how #embed
> works in this regard.

The reference implementation had a -binary-dir=DIR option, but I don't
think any patches have been merged to support #embed in any compiler

> * should be built from bottom up, probably on top of #embed (and
> std::embed maybe?), only adding features that are obviously needed. I
> wouldn't even add listing directories at first, unless there is a
> compelling use case. I think a `std::embed(constant_expression)` alone
> would go a long way.

This seems like a much more focussed and useful suggestion.

You don't include header files by writing code to open them and parse
them and expand macros, you just say #include and the compiler does

Received on 2024-03-13 11:14:21