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

[Zachary Turner via llvm-dev]

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.

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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