Add OneOf::tryGet() returning Maybe, and OneOf::allHandled() to catch missing branches.

c++/src/kj/one-of-test.c++

@@ -31,6 +31,9 @@ TEST(OneOf, Basic) {
   EXPECT_FALSE(var.is<int>());
   EXPECT_FALSE(var.is<float>());
   EXPECT_FALSE(var.is<String>());
+  EXPECT_TRUE(var.tryGet<int>() == nullptr);
+  EXPECT_TRUE(var.tryGet<float>() == nullptr);
+  EXPECT_TRUE(var.tryGet<String>() == nullptr);
 
   var.init<int>(123);
 
@@ -44,6 +47,10 @@ TEST(OneOf, Basic) {
   EXPECT_ANY_THROW(var.get<String>());
 #endif
 
+  EXPECT_EQ(123, KJ_ASSERT_NONNULL(var.tryGet<int>()));
+  EXPECT_TRUE(var.tryGet<float>() == nullptr);
+  EXPECT_TRUE(var.tryGet<String>() == nullptr);
+
   var.init<String>(kj::str("foo"));
 
   EXPECT_FALSE(var.is<int>());
@@ -52,6 +59,10 @@ TEST(OneOf, Basic) {
 
   EXPECT_EQ("foo", var.get<String>());
 
+  EXPECT_TRUE(var.tryGet<int>() == nullptr);
+  EXPECT_TRUE(var.tryGet<float>() == nullptr);
+  EXPECT_EQ("foo", KJ_ASSERT_NONNULL(var.tryGet<String>()));
+
   OneOf<int, float, String> var2 = kj::mv(var);
   EXPECT_EQ("", var.get<String>());
   EXPECT_EQ("foo", var2.get<String>());
@@ -59,6 +70,11 @@ TEST(OneOf, Basic) {
   var = kj::mv(var2);
   EXPECT_EQ("foo", var.get<String>());
   EXPECT_EQ("", var2.get<String>());
+
+  if (false) {
+    var.allHandled<3>();
+    // var.allHandled<2>();  // doesn't compile
+  }
 }
 
 TEST(OneOf, Copy) {

c++/src/kj/one-of.h

@@ -81,6 +81,25 @@ public:
     return *reinterpret_cast<T*>(space);
   }
 
+  template <typename T>
+  Maybe<T&> tryGet() {
+    if (is<T>()) {
+      return *reinterpret_cast<T*>(space);
+    } else {
+      return nullptr;
+    }
+  }
+
+  template <uint i>
+  KJ_NORETURN(void allHandled()) {
+    // After a series of if/else blocks handling each variant of the OneOf, have the final else
+    // block call allHandled<n>() where n is the number of variants. This will fail to compile
+    // if new variants are added in the future.
+
+    static_assert(i == sizeof...(Variants), "new OneOf variants need to be handled here");
+    KJ_UNREACHABLE;
+  }
+
 private:
   uint tag;