<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Oct 12, 2016, at 11:38 AM, Zachary Turner <<a href="mailto:zturner@google.com" class="">zturner@google.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">I don't object to compile time checking *as long as it doesn't severely detract from brevity*. </div></div></blockquote><div><br class=""></div><br class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" class="">At the same time, I do object to *preventing* runtime format strings.</div></div></blockquote><div><br class=""></div><div>You haven’t answered: why?</div><div><br class=""></div><div>— </div><div>Mehdi</div><div><br class=""></div><br class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div><div class="">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:</div><div class=""><br class=""></div><div class="">template<typename... Ts></div><div class="">void wrap_format(const char *Format, Ts &&... Args) {</div><div class=""> dbgs().format(Format, ConvertArg(Args)...);</div><div class="">}</div></div><br class=""><div class="gmail_quote"><div dir="ltr" class="">On Wed, Oct 12, 2016 at 11:24 AM Mehdi Amini <<a href="mailto:mehdi.amini@apple.com" class="">mehdi.amini@apple.com</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg">On Oct 12, 2016, at 7:12 AM, Zachary Turner via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="gmail_msg" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:</div><br class="gmail_msg m_931657296345544509Apple-interchange-newline"><div class="gmail_msg">Ahh, UDLs also wouldn't permit non literal format strings, which is a deal breaker imo<br class="gmail_msg"></div></blockquote><div class="gmail_msg"><br class="gmail_msg"></div></div></div><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><div class="gmail_msg">Why?</div><div class="gmail_msg">Somehow the goal pursued by Pavel (which you didn’t object per-se) is to provide *compile* time checking.</div><div class="gmail_msg">This imply that you cannot decouple the construction of the format and the argument list.</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">— </div></div></div><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><div class="gmail_msg">Mehdi</div></div></div><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><br class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><div class="gmail_quote gmail_msg"><div dir="ltr" class="gmail_msg">On Wed, Oct 12, 2016 at 7:03 AM Zachary Turner <<a href="mailto:zturner@google.com" class="gmail_msg" target="_blank">zturner@google.com</a>> wrote:<br class="gmail_msg"></div><blockquote class="gmail_quote gmail_msg" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">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?<br class="gmail_msg"><br class="gmail_msg">"{0} {1}"_fs(1, 2)<br class="gmail_msg"><br class="gmail_msg">Could perhaps work, but it looks a little strange to me.<br class="gmail_msg"><br class="gmail_msg">Fwiw i agree format_string is long. Ideally it would be called format, but that's taken.<br class="gmail_msg"><br class="gmail_msg">Another option is os.format("{0}", 7), and have format_string("{0}", 7) return a std::string.<br class="gmail_msg"><div class="gmail_quote gmail_msg"><div dir="ltr" class="gmail_msg">On Wed, Oct 12, 2016 at 6:43 AM Aaron Ballman <<a href="mailto:aaron@aaronballman.com" class="gmail_msg" target="_blank">aaron@aaronballman.com</a>> wrote:<br class="gmail_msg"></div><blockquote class="gmail_quote gmail_msg" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">>> 1. os << format_string("Test"); // writes "test"<br class="gmail_msg">
>> 2. os << format_string("{0}", 7); // writes "7"<br class="gmail_msg">
><br class="gmail_msg">
><br class="gmail_msg">
> The "<< format_string(..." is ... really verbose for me. It also makes me<br class="gmail_msg">
> strongly feel like this produces a string rather than a streamable entity.<br class="gmail_msg">
<br class="gmail_msg">
I wonder if we could use UDLs instead?<br class="gmail_msg">
<br class="gmail_msg">
os << "Test" << "{0}"_fs << 7;<br class="gmail_msg">
<br class="gmail_msg">
~Aaron<br class="gmail_msg">
<br class="gmail_msg">
><br class="gmail_msg">
> I'm not a huge fan of streaming, but if we want to go this route, I'd very<br class="gmail_msg">
> much like to keep the syntax short and sweet. "format" is pretty great for<br class="gmail_msg">
> that. If this is going to fully subsume its use cases, can we eventually get<br class="gmail_msg">
> that to be the name?<br class="gmail_msg">
><br class="gmail_msg">
> (While I don't like streaming, I'm not trying to fight that battle here...)<br class="gmail_msg">
><br class="gmail_msg">
> Also, you should probably look at what is quickly becoming a popular C++<br class="gmail_msg">
> library in this space: <a href="https://github.com/fmtlib/fmt" rel="noreferrer" class="gmail_msg" target="_blank">https://github.com/fmtlib/fmt</a><br class="gmail_msg">
><br class="gmail_msg">
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</blockquote></div></blockquote></div>
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