[llvm-dev] Question about std::map

[David Blaikie via llvm-dev]

On Thu, Dec 3, 2020 at 4:54 PM Paul C. Anagnostopoulos <paul at windfall.com>
wrote:

> This just got weird. The 'const' was left over from previous experiments,
> so I removed it. Now when I run the -gen-instr-info backend on the AMDGPU
> target, I get:
>
> error: ERROR: No 'Target' subclasses defined!
>
> This suggests that there is a fundamental flaw somewhere. That message
> would be explained if the records pointed to by the vector of <Record *>
> were disappearing after the vector is stored in the map.
>

The try_emplace solution would only work if the key mapping wasn't already
present in the map. If it is already in the map, then the x[y] = z approach
would be what you want.


>
> At 12/3/2020 07:24 PM, David Blaikie wrote:
>
>
> >On Thu, Dec 3, 2020 at 4:19 PM Paul C. Anagnostopoulos via llvm-dev
> <<mailto:llvm-dev at lists.llvm.org>llvm-dev at lists.llvm.org> wrote:
> >I have the following map:
> >
> >Â  std::map<std::string, std::vector<Record *>> ClassRecordsMap;
> >
> >I call a function that builds a new vector of <Record *> and returns it.
> >
> >Â  const auto Records = getAllDerivedDefinitions(makeArrayRef(ClassName));
> >
> >
> >This would be one of the times where using top level const on local
> variables will make things less good. You can't move from Records if it's
> const like that. (generally I don't think it's practical to use top level
> const in C++ pervasively - so I'd suggest generally not doing it, because
> doing it only sometimes may make things harder to read ("why is /this/
> particular local variable const when lots of others aren't?" the future
> reader will ask themselves))
> >Â
> >
> >What is the best way to add it to the map, being careful not to copy it?
> This is what I'm doing now:
> >
> >Â  ClassRecordsMap[ClassNameString] = std::move(Records);
> >
> >
> >Yep, generally that'll do it.
> >
> >If you want to avoid even a default construction of std::vector (it's
> cheap, there's unlikely to be any reason to avoid that) you could use
> emplace:
> >
> >ClassRecordsMap.try_emplace(ClassNameString, std::move(Records));
>
>
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