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

[Mehdi Amini via llvm-dev]

> On Oct 12, 2016, at 8:07 PM, Zachary Turner <zturner at google.com> wrote:
> 
> On Wed, Oct 12, 2016 at 12:40 PM Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
>> On Oct 12, 2016, at 12:35 PM, Zachary Turner <zturner at google.com <mailto:zturner at google.com>> wrote:
>> 
>> 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.
> 
> I understand that, but that doesn’t really address my concerns.
> 
>> 
>> 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
> 
> Maybe the problem is using a string to format this in the first place.
> 
> For example, you could wrap the object you want to print with an adaptor in charge of padding to the right till you reach the column width.
> 
> format(“{0}”, rPad(col_width, my_object));
> 
> FWIW I do think that literal format strings will handle 90% or more of uses.  I just don't see the benefit of needlessly banning the other cases.  Because all that's going to happen is someone is going to resort to using snprintf etc, which is exactly the problem I'm trying to solve.

Sorry but you’re totally missing the point. If there is a need for dynamism, this should be supported, that’s not the question. My point is that generating a string that will be parsed by a format function can’t be the only solution.

>   It's literally no extra effort to support runtime format strings, and it makes the library more flexible as a result.

No: it does *not* make it more flexible than a non-string based solution that have the same functionality.

— 
Mehdi



> I'm willing to start with UDLs only because I think it will get us quite far, but as soon as I need to pass a format string through an intermediate function or something like that, I will probably check in the 3 extra lines of code to add a const char* overload format function.
> 
> FWIW, there's no easy way to add compile time checking of format strings until C++14, regardless of whether use UDLs or not.  So that doesn't change either way.

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