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Revert "Revert "Use template structs instead of phantoms""

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This is the more typically way to do [Argument-dependent
lookup](https://en.cppreference.com/w/cpp/language/adl)-leveraging
generic serializers in C++. It makes the relationship between the `read`
and `write` methods more clear and rigorous, and also looks more
familiar to users coming from other languages that do not have C++'s
libertine ad-hoc overloading.

I am returning to this because during the review in
https://github.com/NixOS/nix/pull/6223, it came up as something that
would make the code easier to read --- easier today hopefully already,
but definitely easier if we were have multiple codified protocols with
code sharing between them as that PR seeks to accomplish.

If I recall correctly, the main criticism of this the first time around
(in 2020) was that having to specify the type when writing, e.g.
`WorkerProto<MyType>::write`, was too verbose and cumbersome. This is
now addressed with the `workerProtoWrite` wrapper function.

This method is also the way `nlohmann::json`, which we have used for a
number of years now, does its serializers, for what its worth.

This reverts commit 45a0ed82f0. That
commit in turn reverted 9ab07e99f5.
This commit is contained in:
John Ericson 2023-05-17 22:04:59 -04:00
parent 684e9be8b9
commit cb5052d98f
10 changed files with 182 additions and 151 deletions
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4
src/libstore/export-import.cc
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@ -45,7 +45,7 @@ void Store::exportPath(const StorePath & path, Sink & sink)
teeSink
<< exportMagic
<< printStorePath(path);
worker_proto::write(*this, teeSink, info->references);
workerProtoWrite(*this, teeSink, info->references);
teeSink
<< (info->deriver ? printStorePath(*info->deriver) : "")
<< 0;
@ -73,7 +73,7 @@ StorePaths Store::importPaths(Source & source, CheckSigsFlag checkSigs)
//Activity act(*logger, lvlInfo, "importing path '%s'", info.path);
auto references = worker_proto::read(*this, source, Phantom<StorePathSet> {});
auto references = WorkerProto<StorePathSet>::read(*this, source);
auto deriver = readString(source);
auto narHash = hashString(htSHA256, saved.s);