[libcxx-commits] [PATCH] D105794: [libcxx][algorithms] adds ranges::is_partitioned and ranges::partition_point

[Arthur O'Dwyer via Phabricator via libcxx-commits]

Quuxplusone added inline comments.


================
Comment at: libcxx/include/__algorithm/partition_point.h:71
+    _LIBCPP_NODISCARD_EXT constexpr
+    _Ip operator()(_Ip __first, const _Sp __last, _Pred __pred, _Proj __proj = {}) const {
+      // This assertion breaks the logarithmic complexity requirement, but since it is only evaluated
----------------
Mordante wrote:
> cjdb wrote:
> > Mordante wrote:
> > > Why the `const` in `const _Sp __last`. (I personally like it, but it deviates from our coding style.)
> > I like the compiler catching accidental writes. Also, this is a new section of code, so I think it's worth piloting this style here. (@ldionne seems to be ambivalent towards it based on prior commits as evidence.)
> I like the `const` too for the the same reason. So then let's keep it. I probably will start to use it in new code too.
@cjdb @Mordante @ldionne: Please, please, don't misuse `const` to "pass by const value" or "return by const value." These are common typos in C++ code. @zoecarver will (I hope) recall the couple of times I've caught bugs in his code by grepping for pass-by-const-value typos.

I have a whole blog post on "const" here:
https://quuxplusone.github.io/blog/2019/01/03/const-is-a-contract/

and it ends with [a set of `git grep` regexes](https://quuxplusone.github.io/blog/2019/01/03/const-is-a-contract/#grep-your-codebase-today) that enable anyone — not just me, but //any of you!// — to find your missing-ampersand typos by grepping for parameters of the form `const _Sp __last`. If you start using that typo-style in code you //don't// want to change, it makes the job of the typo-finder vastly harder.

As of this writing, we have only five instances of pass-by-const-value in the codebase. Three of them are in `requires`-clauses.
```
$ git grep '[(,] *const [A-Za-z0-9_:]* [A-Za-z0-9_:]* *[,)=]'
__iterator/advance.h:      if (const auto __M = __bound - __i; __abs(__n) >= __abs(__M)) {
__iterator/concepts.h:  requires(const _In __i) {
__iterator/concepts.h:  requires(_Ip __i, const _Ip __j, const iter_difference_t<_Ip> __n) {
__iterator/iter_swap.h:  requires(const _I1 __i1, const _I2 __i2) {
random:binomial_distribution<_IntType>::param_type::param_type(const result_type __t, const double __p)
```


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D105794/new/

https://reviews.llvm.org/D105794