Time for C++20.

[Bills, Jason M]

On 4/14/2021 1:11 PM, Patrick Williams wrote:
> Hello.
> 
> Per [1], as a project we've generally committed to using the latest C++
> standard.  C++20 was released in early Sept. 2020 and already had pretty
> decent compiler support at the time it was released.  We're currently
> using GCC10 and according to [2] it has support for nearly every C++20
> feature I could see us regularly wanting to use(*).  It seems like it
> is a reasonable time for us to move on to it.
> 
> * - Except I recall seeing some mention of std::source_location being
>      useful to fix some of the issues with phosphor-logging, but that
>      won't be available until GCC11.
> 
> In general the C++ standard is backwards compatible.  This shouldn't
> have any impact if you are writing C++17-only code.  What it allows us
> to do is to start using features out of C++20 like the spaceship
> operator and ranges.
> 
> I've put up some commits to start this process.  So far the only hiccup
> I've ran into is that you need Meson 0.57.0 or higher in order for it to
> recognize the 'cpp_std=c++20' option.  I am adding code to our unit-test
> framework to ensure you have this specified in your `meson.build` if you
> are requesting C++20.
> 
>      - https://gerrit.openbmc-project.xyz/c/openbmc/openbmc-build-scripts/+/42252
>          * Adds some checking to ensure you've specified a meson_version
>            in your meson.build that can handle C++20.
> 
>      - https://gerrit.openbmc-project.xyz/c/openbmc/docs/+/42255
>          * Update C++ style doc from C++17 to C++20.
> 
>      - https://gerrit.openbmc-project.xyz/c/openbmc/sdbusplus/+/42256
>          * Update sdbusplus to use C++20.
> 
> Please let me know if you have any concerns with this upgrade; I'll be
> surprised if this not a fairly low-contention move.
+1

I just discovered starts_with() will be available for std::string which 
I want to use.  I turned it on locally and didn't have any issues.

coroutines are also an interesting possibility for some of our async calls.

> 
> 1. https://github.com/openbmc/docs/blob/master/cpp-style-and-conventions.md#code-should-be-written-with-modern-practices
> 2. https://en.cppreference.com/w/cpp/20
>