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Subject: Re: [std-proposals] source_location improvement
From: Victor Zverovich (victor.zverovich_at_[hidden])
Date: 2021-05-05 18:31:57


There is no proposal yet but we are working on getting implementation
experience. Partial support for compile-time formatting has been recently
added to the fmt library: https://github.com/fmtlib/fmt/pull/1993.

- Victor

On Wed, May 5, 2021 at 3:26 PM Edward Catmur via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On Wed, 5 May 2021 at 22:06, Mike via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> How do we "allow compile-time string processing"?
>>
>
> By making std::format constexpr:
>
> template<class... Args>
> constexpr std::string format(std::string_view fmt, const Args&... args);
> ^~~~~~~~~
>
> Note that std::string is already constexpr since C++20.
>
> So, taking my specific problem, given `loc` returned by
>> source_location::current(), how do we use loc.file_name() (returning
>> constexpr const char*) and loc.line() (returning constexpr
>> unint_least32_t) to create a single compile time string like
>> "foo.cpp(555): beyond all recognition"? Could you give me an example of
>> the kind of syntax you were thinking of?
>>
>
> constexpr auto locf = std::format("{}({}): ", loc.file_name(), loc.line());
>
> Currently the problem is that a constexpr allocation can't escape
> constexpr evaluation (as Jason mentions above re. "non-transient
> allocations"), so you'll have to do something silly like
> https://godbolt.org/z/c67Tzj4oc :
>
> constexpr auto locff = [&] { return std::format("{}({}): ",
> loc.file_name(), loc.line()); };
> constexpr auto locf = static_string<locff().size()>(locff().c_str());
>
> On the other hand, if your source filenames are reasonably short you might
> be able to get away with using the SSO buffer. For a while, anyway.
>
> Are there already proposals for this kind of compile time string
>> composition from constexpr expressions?
>>
>
> I'm not aware of any. P2216 is focussing on (compile time) safety, mainly.
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>



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