Add absl::variant type_caster.

PiperOrigin-RevId: 348646910

pybind11_abseil/absl_casters.h

@@ -38,6 +38,7 @@
 #include <stdexcept>
 #include <type_traits>
 #include <typeinfo>
+#include <vector>
 
 #include "absl/container/flat_hash_map.h"
 #include "absl/container/flat_hash_set.h"
@@ -337,6 +338,15 @@ template <>
 struct type_caster<absl::nullopt_t> : public void_caster<absl::nullopt_t> {};
 #endif
 
+// This is a simple port of the pybind11 std::variant type_caster, applied to
+// absl::variant. See pybind11 stl.h.
+#ifndef ABSL_HAVE_STD_VARIANT
+template <typename... Ts>
+struct type_caster<absl::variant<Ts...>>
+    : variant_caster<absl::variant<Ts...>> {};
+
+#endif
+
 }  // namespace detail
 
 }  // namespace pybind11

pybind11_abseil/tests/absl_example.cc

@@ -196,6 +196,33 @@ void DefineNonConstSpan(module* py_m, absl::string_view type_name) {
             &FillNonConstSpan<T>, arg("value"), arg("output_span").noconvert());
 }
 
+// absl::variant
+struct A {
+  int a;
+};
+struct B {
+  int b;
+};
+typedef absl::variant<A, B> AOrB;
+
+int VariantToInt(AOrB value) {
+  if (absl::holds_alternative<A>(value)) {
+    return absl::get<A>(value).a;
+  } else if (absl::holds_alternative<B>(value)) {
+    return absl::get<B>(value).b;
+  } else {
+    throw std::exception();
+  }
+}
+
+std::vector<AOrB> IdentityWithCopy(const std::vector<AOrB>& value) {
+  return value;
+}
+std::vector<absl::variant<A*, B*>> Identity(
+    const std::vector<absl::variant<A*, B*>>& value) {
+  return value;
+}
+
 PYBIND11_MODULE(absl_example, m) {
   // absl::Time/Duration bindings.
   m.def("make_duration", &MakeDuration, arg("secs"));
@@ -257,6 +284,13 @@ PYBIND11_MODULE(absl_example, m) {
   // absl::flat_hash_set bindings
   m.def("make_set", &MakeSet, arg("values"));
   m.def("check_set", &CheckSet, arg("set"), arg("values"));
+
+  // absl::variant
+  class_<A>(m, "A").def(init<int>()).def_readonly("a", &A::a);
+  class_<B>(m, "B").def(init<int>()).def_readonly("b", &B::b);
+  m.def("VariantToInt", &VariantToInt);
+  m.def("Identity", &Identity);
+  m.def("IdentityWithCopy", &IdentityWithCopy);
 }
 
 }  // namespace test

pybind11_abseil/tests/absl_test.py

@@ -365,5 +365,26 @@ class AbslOptionalTest(absltest.TestCase):
     self.assertIsNone(absl_example.make_optional())
 
 
+class AbslVariantTest(absltest.TestCase):
+
+  def test_variant(self):
+    assert absl_example.VariantToInt(absl_example.A(3)) == 3
+    assert absl_example.VariantToInt(absl_example.B(5)) == 5
+
+    for identity_f, should_be_equal in [(absl_example.Identity, True),
+                                        (absl_example.IdentityWithCopy, False)]:
+      objs = [absl_example.A(3), absl_example.B(5)]
+      vector = identity_f(objs)
+      self.assertLen(vector, 2)
+      self.assertIsInstance(vector[0], absl_example.A)
+      self.assertEqual(vector[0].a, 3)
+      self.assertIsInstance(vector[1], absl_example.B)
+      self.assertEqual(vector[1].b, 5)
+      if should_be_equal:
+        self.assertEqual(objs, vector)
+      else:
+        self.assertNotEqual(objs, vector)
+
+
 if __name__ == '__main__':
   absltest.main()