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authorVincent Ambo <mail@tazj.in>2020-07-13T18·54+0100
committertazjin <mail@tazj.in>2020-07-13T20·14+0000
commit693661fe8777d6eb5bcf612a5103d1704ec97eff (patch)
treea18d91b7dbca0c728731d789509618c94772d9bf /third_party/nix
parent2832ea43ead9d47ced32338d269184c08ec91469 (diff)
feat(3p/nix/libutil): Add util::overloaded class for std::visitor r/1277
Implements the fairly common lambda overload class used for std::visit
over variants and other things that require groups of callables.

Change-Id: Ia7448b7e1bd349b4909974758e6e6303a80d86d7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1137
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: edef <edef@edef.eu>
Diffstat (limited to 'third_party/nix')
-rw-r--r--third_party/nix/src/libutil/CMakeLists.txt1
-rw-r--r--third_party/nix/src/libutil/visitor.hh19
2 files changed, 20 insertions, 0 deletions
diff --git a/third_party/nix/src/libutil/CMakeLists.txt b/third_party/nix/src/libutil/CMakeLists.txt
index 43b9e8525f78..a9736b37bc05 100644
--- a/third_party/nix/src/libutil/CMakeLists.txt
+++ b/third_party/nix/src/libutil/CMakeLists.txt
@@ -25,6 +25,7 @@ set(HEADER_FILES
     thread-pool.hh
     types.hh
     util.hh
+    visitor.hh
     xml-writer.hh
 )
 
diff --git a/third_party/nix/src/libutil/visitor.hh b/third_party/nix/src/libutil/visitor.hh
new file mode 100644
index 000000000000..bf1d665af77f
--- /dev/null
+++ b/third_party/nix/src/libutil/visitor.hh
@@ -0,0 +1,19 @@
+#pragma once
+
+namespace nix::util {
+
+// Helper class used for visiting std::variants by creating a variadic
+// list of lambda expressions that delegates calls to each of the
+// callables.
+//
+// See e.g.
+// https://dev.to/tmr232/that-overloaded-trick-overloading-lambdas-in-c17
+template <class... Ts>
+struct overloaded : Ts... {
+  using Ts::operator()...;
+};
+
+template <class... Ts>
+overloaded(Ts...) -> overloaded<Ts...>;
+
+}  // namespace nix::util