[libcxx-commits] [PATCH] D103558: Remove VLA from libcxx locale header

[Daniel McIntosh via Phabricator via libcxx-commits]

DanielMcIntosh-IBM added inline comments.


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Comment at: libcxx/include/locale:1463
     // printed as an unsigned value.
-    const unsigned __nbuf = (numeric_limits<unsigned long>::digits / 3)          // 1 char per 3 bits
-                            + ((numeric_limits<unsigned long>::digits % 3) != 0) // round up
-                            + ((__iob.flags() & ios_base::showbase) != 0)        // base prefix
-                            + 1;                                                 // terminating null character
+    _LIBCPP_CONSTEXPR const unsigned __nbuf
+        = (numeric_limits<unsigned long>::digits / 3)        // 1 char per 3 bits
----------------
Quuxplusone wrote:
> I wonder if this should be `_LIBCPP_CONSTEXPR const` (which seems redundant) or merely `static const` (for C++03 compatibility) or merely `const`. But I guess as long as it compiles it's fine any way.
`const` and `static const` by themselves wont force `__nbuf` to be a compile-time constant. If `__nbuf` is not [[ https://en.cppreference.com/w/cpp/language/constant_initialization | constant-initialized ]], then the `__nbuf` expression used to set the size of `__nar` will silently stop being an [[ https://en.cppreference.com/w/cpp/language/constant_expression | integral constant expression ]] (it will stop satisfying exception a to rule 9 of what isn't a core constant expression). This is why we were able to use `__iob.flags()` in the initializer without generating any errors.

`constexpr` by itself would be enough, but can't be used prior to C++11, so we need to use `_LIBCPP_CONSTEXPR`. However, we still can't remove the `const` because pre-c++11, that would leave `__nbuf` with neither `const` nor `constexpr`, which would again stop `__nbuf` from satisfying exception a to rule 9.

While the `_LIBCPP_CONSTEXPR` isn't strictly necessary, it prevents future issues like this one by generating an error if `__nbuf` isn't a compile-time constant.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D103558/new/

https://reviews.llvm.org/D103558