[llvm-dev] RFC: General purpose type-safe formatting library

[Zachary Turner via llvm-dev]

You get compile time checking automatically when we can use c++14 though.
If you use it with a string literal, you'll get compile time checking,
otherwise you won't.

Here's a different example though. Suppose you're writing a tool which
prints formatted output, and the field width is specified by the user. Now
you NEED to build the format string at runtime, there's no other way. Off
the top of my head, lldb does this already when printing disassembly and
stack frames. The column widths are user settings


On Wed, Oct 12, 2016 at 12:23 PM Mehdi Amini <mehdi.amini at apple.com> wrote:

> On Oct 12, 2016, at 12:08 PM, Zachary Turner <zturner at google.com> wrote:
>
> I thought I did.  :)  Passing format strings between functions is very
> useful.  For example, imagine wanting to write a function like
> printRange(const char *Fmt, std::vector<int> Items);
>
>
> I’m not sure I understand your example?
> Do you mean you want the range to be in the format? If so Why? I would
> rather write something like:
>
> printRange(“{per_elts_fmt}”, /* separator */ “, ", begin, end);
>
> This isn't possible if your format string MUST be a string literal
>
>
> I haven’t seen a convincing example yet to support this. I may miss the
> obvious, but you haven’t shown it either.
> One could find a way to *compose* format in a compile-time-safe more
> efficiently.
>
> Equally importantly, I don't see a good reason to disallow runtime format
> strings.
>
>
> No compile time checking, bug hiding, not robust.
> (i.e. you may not “crash”, but you may still don’t print what you want /
> expect in every case).
>
> —
> Mehdi
>
>
>
>
> On Wed, Oct 12, 2016 at 11:59 AM Mehdi Amini <mehdi.amini at apple.com>
> wrote:
>
> On Oct 12, 2016, at 11:38 AM, Zachary Turner <zturner at google.com> wrote:
>
> I don't object to compile time checking *as long as it doesn't severely
> detract from brevity*.
>
>
>
> At the same time, I do object to *preventing* runtime format strings.
>
>
> You haven’t answered: why?
>
> —
> Mehdi
>
>
>
> When we have C++14, we can make every member of StringRef constexpr, and
> at that point we will get compile time checking mostly "for free" without
> preventing runtime format strings.  For example, given a constexpr-aware
> implementation of StringRef, if you were to write: os.format("literal
> format", a, b, c) you would get all the compile time checking, such as
> ensuring that the number of arguments matches the highest index in the
> format string, and ensuring there are enough arguments for every
> placeholder.  But if you wrote os.format(s, a, b, c) you would still get
> runtime checking of the format strings.
>
> As long as the runtime implementation doesn't exhibit UB when things don't
> match up, and it kindly asserts to warn you of the problem in the test
> suite, support runtime format strings can be very helpful.  For example, it
> could allow you to wrap a call to format in some other function, like:
>
> template<typename... Ts>
> void wrap_format(const char *Format, Ts &&... Args) {
>    dbgs().format(Format, ConvertArg(Args)...);
> }
>
> On Wed, Oct 12, 2016 at 11:24 AM Mehdi Amini <mehdi.amini at apple.com>
> wrote:
>
> On Oct 12, 2016, at 7:12 AM, Zachary Turner via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Ahh, UDLs also wouldn't permit non literal format strings, which is a deal
> breaker imo
>
>
> Why?
> Somehow the goal pursued by Pavel (which you didn’t object per-se) is to
> provide *compile* time checking.
> This imply that you cannot decouple the construction of the format and the
> argument list.
>
> —
> Mehdi
>
> On Wed, Oct 12, 2016 at 7:03 AM Zachary Turner <zturner at google.com> wrote:
>
> I'm not sure that would work well. The implementation relies on being able
> to index into the parameter pack. How would you do that if each parameter
> is streamed in?
>
> "{0} {1}"_fs(1, 2)
>
> Could perhaps work, but it looks a little strange to me.
>
> Fwiw i agree format_string is long. Ideally it would be called format, but
> that's taken.
>
> Another option is os.format("{0}", 7), and have format_string("{0}", 7)
> return a std::string.
> On Wed, Oct 12, 2016 at 6:43 AM Aaron Ballman <aaron at aaronballman.com>
> wrote:
>
> >> 1. os << format_string("Test");   // writes "test"
> >> 2. os << format_string("{0}", 7);  // writes "7"
> >
> >
> > The "<< format_string(..." is ... really verbose for me. It also makes me
> > strongly feel like this produces a string rather than a streamable
> entity.
>
> I wonder if we could use UDLs instead?
>
> os << "Test" << "{0}"_fs << 7;
>
> ~Aaron
>
> >
> > I'm not a huge fan of streaming, but if we want to go this route, I'd
> very
> > much like to keep the syntax short and sweet. "format" is pretty great
> for
> > that. If this is going to fully subsume its use cases, can we eventually
> get
> > that to be the name?
> >
> > (While I don't like streaming, I'm not trying to fight that battle
> here...)
> >
> > Also, you should probably look at what is quickly becoming a popular C++
> > library in this space: https://github.com/fmtlib/fmt
> >
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> >
>
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