[PATCH] D41458: [libc++][C++17] Elementary string conversions for integral types

Marshall Clow via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Mon Jul 30 13:31:30 PDT 2018

mclow.lists added a comment.

Getting close here.  I'll have a couple more comments later today, so don't post a new diff quite yet.

Comment at: include/charconv:244
+    static _LIBCPP_INLINE_VISIBILITY char const*
+    read(char const* __p, char const* __ep, type& __a, type& __b)
+    {
Quuxplusone wrote:
> mclow.lists wrote:
> > lichray wrote:
> > > mclow.lists wrote:
> > > > Same comment as above about `read` and `inner_product` - they need to be "ugly names"
> > > Unlike `traits` which is a template parameter name in the standard, `read` and `inner_product` are function names in the standard, which means the users cannot make a macro for them (and there is no guarantee about what name you make **not** get by including certain headers), so we don't need to use ugly names here, am I right?
> > I understand your reasoning, but I don't agree. 
> > 
> > Just last month, I had to rename a function in `vector` from `allocate` to `__vallocate` because it confused our "is this an allocator" detection. The function in question was private, so it shouldn't have mattered, but GCC has a bug where sometimes it partially ignores access restrictions in non-deduced contexts, and then throws a hard error when it comes back to a different context. The easiest workaround was to rename the function in `vector`.
> > 
> > Since then, I've been leery of public names that match others. This is pretty obscure, since it's in a private namespace, but I'd feel better if they were `__read` and `__inner_product`.
> > 
> FWIW, +1 to ugly names. Even if the un-ugly code is "technically not broken yet", and besides the technical reason Marshall gives,
> (1) it's nice that libc++ has style rules and sticks to them, precisely to *avoid* bikeshedding the name of every private member in the world;
> (2) just because users can't `#define read write` doesn't mean they *won't* do it. I would actually be extremely surprised if `read` were *not* defined as a macro somewhere inside `<windows.h>`. :)
> See also: "should this function call be `_VSTD::`-qualified?" Sometimes the answer is technically "no", but stylistically "yes", precisely to indicate that we *don't* intend for it to be an ADL customization point. Consistent style leads to maintainability.
> See also: "should this function call be _VSTD::-qualified

We already have an `inner_product` ( in `<algorithm>`), so I think not.

We `_VSTD::` qualify `move` and `forward` always. `min`/`max` due mostly to windows macros. `swap` when need to disable ADL. 

Other things we're inconsistent about: `declval` sometimes (I don't know why), `use_facet`, `addressof`, `to_raw_pointer`.

I'll figure out when we should mark things with `_VSTD` and write it up.

Comment at: include/charconv:359
+    auto __gen_digit = [](_Tp __c) {
+        return "0123456789abcdefghijklmnopqrstuvwxyz"[__c];
+    };
lichray wrote:
> mclow.lists wrote:
> > Thinking some more - did this used to do more? Because I don't see why having a lambda here is a benefit, as compared to:
> > 
> >     const char *__digits = "0123456789abcdefghijklmnopqrstuvwxyz";
> > 
> > and
> >     *--p = digits[__c];
> > 
> I use a lambda here because it may do more in the future.  If someone wants to let it support non-ASCII platforms, then they only need to make a patch against this lambda rather than changing the sites of uses.  After all, there is nothing wrong to abstract out anything into a function, I think...
I don't think that making it a lambda today buys us anything. Since it is local, we can make it a lambda again in the future if we need to w/o breaking anyone.

Shades of YAGNI.

Comment at: include/charconv:158
+#if !defined(_LIBCPP_COMPILER_MSVC)
+    static _LIBCPP_INLINE_VISIBILITY int __width(_Tp __v)
In general, we don't put `_LIBCPP_COMPILER_XXX` in header files in libc++.
Instead, we declare a feature macro `_LIBCPP_HAS_XXXX` in `<__config>` and then use that everywhere.

I see that most of the uses of `_LIBCPP_COMPILER_MSVC` are in `<algorithm>`, and center around the builtins for `__builtin_clz`, etc.

We can clean this up later. (/me makes note).

Comment at: include/charconv:285
+__to_unsigned(_Tp __x)
in libc++, we put the return type on the left.

    template <typename _Tp>
    auto __to_unsigned(_Tp __x)

(and then ask 'why auto'?)

Comment at: test/support/charconv_test_helpers.h:226
This name `run` concerns me. I'd feel better if it was `run_integral_tests` or something a bit less general.

On the other hand, it's in a test, so we can change it whenever we want.

  rCXX libc++


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