Move Maybe into common.h and improve its interface.

c++/src/capnproto/dynamic-test.c++

@@ -228,6 +228,13 @@ TEST(DynamicApi, DynamicGenericObjects) {
   }
 }
 
+#define EXPECT_MAYBE_EQ(name, exp, expected, actual) \
+  KJ_IF_MAYBE(name, exp) { \
+    EXPECT_EQ(expected, actual); \
+  } else { \
+    FAIL() << "Maybe was empty."; \
+  }
+
 TEST(DynamicApi, UnionsRead) {
   MallocMessageBuilder builder;
   auto root = builder.initRoot<TestUnion>();
@@ -241,26 +248,22 @@ TEST(DynamicApi, UnionsRead) {
     auto dynamic = toDynamic(root.asReader());
     {
       auto u = dynamic.get("union0").as<DynamicUnion>();
-      ASSERT_TRUE(u.which() != nullptr);
-      EXPECT_EQ("u0f1s32", u.which()->getProto().getName());
+      EXPECT_MAYBE_EQ(w, u.which(), "u0f1s32", w->getProto().getName());
       EXPECT_EQ(1234567, u.get().as<int32_t>());
     }
     {
       auto u = dynamic.get("union1").as<DynamicUnion>();
-      ASSERT_TRUE(u.which() != nullptr);
-      EXPECT_EQ("u1f1sp", u.which()->getProto().getName());
+      EXPECT_MAYBE_EQ(w, u.which(), "u1f1sp", w->getProto().getName());
       EXPECT_EQ("foo", u.get().as<Text>());
     }
     {
       auto u = dynamic.get("union2").as<DynamicUnion>();
-      ASSERT_TRUE(u.which() != nullptr);
-      EXPECT_EQ("u2f0s1", u.which()->getProto().getName());
+      EXPECT_MAYBE_EQ(w, u.which(), "u2f0s1", w->getProto().getName());
       EXPECT_TRUE(u.get().as<bool>());
     }
     {
       auto u = dynamic.get("union3").as<DynamicUnion>();
-      ASSERT_TRUE(u.which() != nullptr);
-      EXPECT_EQ("u3f0s64", u.which()->getProto().getName());
+      EXPECT_MAYBE_EQ(w, u.which(), "u3f0s64", w->getProto().getName());
       EXPECT_EQ(1234567890123456789ll, u.get().as<int64_t>());
     }
   }
@@ -270,26 +273,22 @@ TEST(DynamicApi, UnionsRead) {
     auto dynamic = toDynamic(root);
     {
       auto u = dynamic.get("union0").as<DynamicUnion>();
-      ASSERT_TRUE(u.which() != nullptr);
-      EXPECT_EQ("u0f1s32", u.which()->getProto().getName());
+      EXPECT_MAYBE_EQ(w, u.which(), "u0f1s32", w->getProto().getName());
       EXPECT_EQ(1234567, u.get().as<int32_t>());
     }
     {
       auto u = dynamic.get("union1").as<DynamicUnion>();
-      ASSERT_TRUE(u.which() != nullptr);
-      EXPECT_EQ("u1f1sp", u.which()->getProto().getName());
+      EXPECT_MAYBE_EQ(w, u.which(), "u1f1sp", w->getProto().getName());
       EXPECT_EQ("foo", u.get().as<Text>());
     }
     {
       auto u = dynamic.get("union2").as<DynamicUnion>();
-      ASSERT_TRUE(u.which() != nullptr);
-      EXPECT_EQ("u2f0s1", u.which()->getProto().getName());
+      EXPECT_MAYBE_EQ(w, u.which(), "u2f0s1", w->getProto().getName());
       EXPECT_TRUE(u.get().as<bool>());
     }
     {
       auto u = dynamic.get("union3").as<DynamicUnion>();
-      ASSERT_TRUE(u.which() != nullptr);
-      EXPECT_EQ("u3f0s64", u.which()->getProto().getName());
+      EXPECT_MAYBE_EQ(w, u.which(), "u3f0s64", w->getProto().getName());
       EXPECT_EQ(1234567890123456789ll, u.get().as<int64_t>());
     }
   }

c++/src/capnproto/dynamic.c++

@@ -169,20 +169,18 @@ kj::Maybe<StructSchema::Member> DynamicUnion::Builder::which() {
 }
 
 DynamicValue::Reader DynamicUnion::Reader::get() {
-  auto w = which();
-  if (w == nullptr) {
-    return nullptr;
-  } else {
+  KJ_IF_MAYBE(w, which()) {
     return DynamicValue::Reader(DynamicStruct::Reader::getImpl(reader, *w));
+  } else {
+    return nullptr;
   }
 }
 
 DynamicValue::Builder DynamicUnion::Builder::get() {
-  auto w = which();
-  if (w == nullptr) {
-    return nullptr;
-  } else {
+  KJ_IF_MAYBE(w, which()) {
     return DynamicValue::Builder(DynamicStruct::Builder::getImpl(builder, *w));
+  } else {
+    return nullptr;
   }
 }
 
@@ -256,22 +254,26 @@ Data::Builder DynamicUnion::Builder::initObjectAsData(Text::Reader name, uint si
 }
 
 StructSchema::Member DynamicUnion::Builder::checkIsObject() {
-  auto w = which();
-  PRECOND(w != nullptr, "Can't get() unknown union value.");
-  CHECK(w->getProto().getBody().which() == schema::StructNode::Member::Body::FIELD_MEMBER,
-        "Unsupported union member type.");
-  PRECOND(w->getProto().getBody().getFieldMember().getType().getBody().which() ==
-          schema::Type::Body::OBJECT_TYPE, "Expected Object.");
-  return *w;
+  KJ_IF_MAYBE(w, which()) {
+    CHECK(w->getProto().getBody().which() == schema::StructNode::Member::Body::FIELD_MEMBER,
+          "Unsupported union member type.");
+    PRECOND(w->getProto().getBody().getFieldMember().getType().getBody().which() ==
+            schema::Type::Body::OBJECT_TYPE, "Expected Object.");
+    return *w;
+  } else {
+    FAIL_PRECOND("Can't get() unknown union value.");
+  }
 }
 
 void DynamicUnion::Builder::setDiscriminant(StructSchema::Member member) {
-  auto containingUnion = member.getContainingUnion();
-  PRECOND(containingUnion != nullptr && *containingUnion == schema,
-          "`member` is not a member of this union.");
-  builder.setDataField<uint16_t>(
-      schema.getProto().getBody().getUnionMember().getDiscriminantOffset() * ELEMENTS,
-      member.getIndex());
+  KJ_IF_MAYBE(containingUnion, member.getContainingUnion()) {
+    PRECOND(*containingUnion == schema, "`member` is not a member of this union.");
+    builder.setDataField<uint16_t>(
+        schema.getProto().getBody().getUnionMember().getDiscriminantOffset() * ELEMENTS,
+        member.getIndex());
+  } else {
+    FAIL_PRECOND("`member` is not a member of this union.");
+  }
 }
 
 void DynamicUnion::Builder::setObjectDiscriminant(StructSchema::Member member) {
@@ -874,16 +876,17 @@ void DynamicStruct::Builder::setImpl(
   switch (member.getProto().getBody().which()) {
     case schema::StructNode::Member::Body::UNION_MEMBER: {
       auto src = value.as<DynamicUnion>();
-      auto which = src.which();
-      RECOVERABLE_PRECOND(which != nullptr,
-          "Trying to copy a union value, but the union's discriminant is not recognized.  It "
-          "was probably constructed using a newer version of the schema.") {
-        // Just don't copy anything.
+      KJ_IF_MAYBE(which, src.which()) {
+        getImpl(builder, member).as<DynamicUnion>().set(member, src.get());
         return;
+      } else {
+        FAIL_RECOVERABLE_PRECOND(
+            "Trying to copy a union value, but the union's discriminant is not recognized.  It "
+            "was probably constructed using a newer version of the schema.") {
+          // Just don't copy anything.
+          return;
+        }
       }
-
-      getImpl(builder, member).as<DynamicUnion>().set(member, src.get());
-      return;
     }
 
     case schema::StructNode::Member::Body::FIELD_MEMBER: {

c++/src/capnproto/schema-test.c++

@@ -76,10 +76,9 @@ TEST(Schema, Structs) {
 
   EXPECT_ANY_THROW(member.asUnion());
 
-  kj::Maybe<StructSchema::Member> lookup = schema.findMemberByName("voidField");
-  ASSERT_TRUE(lookup != nullptr);
-  EXPECT_TRUE(*lookup == member);
-  EXPECT_TRUE(*lookup != schema.getMembers()[1]);
+  StructSchema::Member lookup = schema.getMemberByName("voidField");
+  EXPECT_TRUE(lookup == member);
+  EXPECT_TRUE(lookup != schema.getMembers()[1]);
 
   EXPECT_TRUE(schema.findMemberByName("noSuchField") == nullptr);
 
@@ -105,15 +104,15 @@ TEST(Schema, FieldLookupOutOfOrder) {
   EXPECT_EQ("garply", schema.getMembers()[7].getProto().getName());
   EXPECT_EQ("baz", schema.getMembers()[8].getProto().getName());
 
-  EXPECT_EQ(3, schema.findMemberByName("foo")->getProto().getOrdinal());
-  EXPECT_EQ(2, schema.findMemberByName("bar")->getProto().getOrdinal());
-  EXPECT_EQ(8, schema.findMemberByName("baz")->getProto().getOrdinal());
-  EXPECT_EQ(0, schema.findMemberByName("qux")->getProto().getOrdinal());
-  EXPECT_EQ(6, schema.findMemberByName("quux")->getProto().getOrdinal());
-  EXPECT_EQ(4, schema.findMemberByName("corge")->getProto().getOrdinal());
-  EXPECT_EQ(1, schema.findMemberByName("grault")->getProto().getOrdinal());
-  EXPECT_EQ(7, schema.findMemberByName("garply")->getProto().getOrdinal());
-  EXPECT_EQ(5, schema.findMemberByName("waldo")->getProto().getOrdinal());
+  EXPECT_EQ(3, schema.getMemberByName("foo").getProto().getOrdinal());
+  EXPECT_EQ(2, schema.getMemberByName("bar").getProto().getOrdinal());
+  EXPECT_EQ(8, schema.getMemberByName("baz").getProto().getOrdinal());
+  EXPECT_EQ(0, schema.getMemberByName("qux").getProto().getOrdinal());
+  EXPECT_EQ(6, schema.getMemberByName("quux").getProto().getOrdinal());
+  EXPECT_EQ(4, schema.getMemberByName("corge").getProto().getOrdinal());
+  EXPECT_EQ(1, schema.getMemberByName("grault").getProto().getOrdinal());
+  EXPECT_EQ(7, schema.getMemberByName("garply").getProto().getOrdinal());
+  EXPECT_EQ(5, schema.getMemberByName("waldo").getProto().getOrdinal());
 }
 
 TEST(Schema, Unions) {
@@ -122,21 +121,18 @@ TEST(Schema, Unions) {
   EXPECT_TRUE(schema.findMemberByName("bit0") != nullptr);
   EXPECT_TRUE(schema.findMemberByName("u1f0s8") == nullptr);
 
-  auto maybeUnion1 = schema.findMemberByName("union1");
-  ASSERT_TRUE(maybeUnion1 != nullptr);
-
-  auto union1 = maybeUnion1->asUnion();
+  auto union1 = schema.getMemberByName("union1").asUnion();
   EXPECT_TRUE(union1.findMemberByName("bin0") == nullptr);
   EXPECT_TRUE(union1.getContainingUnion() == nullptr);
 
-  auto maybeU1f0s8 = union1.findMemberByName("u1f0s8");
-  ASSERT_TRUE(maybeU1f0s8 != nullptr);
-
-  auto u1f0s8 = *maybeU1f0s8;
-
+  auto u1f0s8 = union1.getMemberByName("u1f0s8");
   EXPECT_EQ("u1f0s8", u1f0s8.getProto().getName());
   EXPECT_TRUE(u1f0s8.getContainingStruct() == schema);
-  EXPECT_TRUE(*u1f0s8.getContainingUnion() == union1);
+  KJ_IF_MAYBE(containing, u1f0s8.getContainingUnion()) {
+    EXPECT_TRUE(*containing == union1);
+  } else {
+    ADD_FAILURE() << "u1f0s8.getContainingUnion() returned null";
+  }
 
   EXPECT_TRUE(union1.findMemberByName("u1f1s8") != nullptr);
   EXPECT_TRUE(union1.findMemberByName("u1f0s32") != nullptr);
@@ -166,10 +162,9 @@ TEST(Schema, Enums) {
   EXPECT_EQ("foo", enumerant.getProto().getName());
   EXPECT_TRUE(enumerant.getContainingEnum() == schema);
 
-  kj::Maybe<EnumSchema::Enumerant> lookup = schema.findEnumerantByName("foo");
-  ASSERT_TRUE(lookup != nullptr);
-  EXPECT_TRUE(*lookup == enumerant);
-  EXPECT_TRUE(*lookup != schema.getEnumerants()[1]);
+  EnumSchema::Enumerant lookup = schema.getEnumerantByName("foo");
+  EXPECT_TRUE(lookup == enumerant);
+  EXPECT_TRUE(lookup != schema.getEnumerants()[1]);
 
   EXPECT_TRUE(schema.findEnumerantByName("noSuchEnumerant") == nullptr);
 
@@ -259,8 +254,7 @@ TEST(Schema, Lists) {
 
   {
     auto context = Schema::from<TestAllTypes>();
-    auto type = context.findMemberByName("enumList")
-        ->getProto().getBody().getFieldMember().getType();
+    auto type = context.getMemberByName("enumList").getProto().getBody().getFieldMember().getType();
 
     ListSchema schema = ListSchema::of(type.getBody().getListType(), context);
     EXPECT_EQ(schema::Type::Body::ENUM_TYPE, schema.whichElementType());

c++/src/capnproto/schema.c++

@@ -129,9 +129,11 @@ kj::Maybe<StructSchema::Member> StructSchema::findMemberByName(Text::Reader name
 }
 
 StructSchema::Member StructSchema::getMemberByName(Text::Reader name) const {
-  kj::Maybe<StructSchema::Member> member = findMemberByName(name);
-  PRECOND(member != nullptr, "struct has no such member", name);
-  return *member;
+  KJ_IF_MAYBE(member, findMemberByName(name)) {
+    return *member;
+  } else {
+    FAIL_PRECOND("struct has no such member", name);
+  }
 }
 
 kj::Maybe<StructSchema::Union> StructSchema::Member::getContainingUnion() const {
@@ -155,9 +157,11 @@ kj::Maybe<StructSchema::Member> StructSchema::Union::findMemberByName(Text::Read
 }
 
 StructSchema::Member StructSchema::Union::getMemberByName(Text::Reader name) const {
-  kj::Maybe<StructSchema::Member> member = findMemberByName(name);
-  PRECOND(member != nullptr, "union has no such member", name);
-  return *member;
+  KJ_IF_MAYBE(member, findMemberByName(name)) {
+    return *member;
+  } else {
+    FAIL_PRECOND("union has no such member", name);
+  }
 }
 
 // -------------------------------------------------------------------
@@ -171,9 +175,11 @@ kj::Maybe<EnumSchema::Enumerant> EnumSchema::findEnumerantByName(Text::Reader na
 }
 
 EnumSchema::Enumerant EnumSchema::getEnumerantByName(Text::Reader name) const {
-  kj::Maybe<EnumSchema::Enumerant> enumerant = findEnumerantByName(name);
-  PRECOND(enumerant != nullptr, "enum has no such enumerant", name);
-  return *enumerant;
+  KJ_IF_MAYBE(enumerant, findEnumerantByName(name)) {
+    return *enumerant;
+  } else {
+    FAIL_PRECOND("enum has no such enumerant", name);
+  }
 }
 
 // -------------------------------------------------------------------
@@ -187,9 +193,11 @@ kj::Maybe<InterfaceSchema::Method> InterfaceSchema::findMethodByName(Text::Reade
 }
 
 InterfaceSchema::Method InterfaceSchema::getMethodByName(Text::Reader name) const {
-  kj::Maybe<InterfaceSchema::Method> method = findMethodByName(name);
-  PRECOND(method != nullptr, "interface has no such method", name);
-  return *method;
+  KJ_IF_MAYBE(method, findMethodByName(name)) {
+    return *method;
+  } else {
+    FAIL_PRECOND("interface has no such method", name);
+  }
 }
 
 // =======================================================================================

c++/src/capnproto/stringify.c++

@@ -111,13 +111,11 @@ static void print(std::ostream& os, DynamicValue::Reader value,
     }
     case DynamicValue::ENUM: {
       auto enumValue = value.as<DynamicEnum>();
-      kj::Maybe<EnumSchema::Enumerant> enumerant =
-          enumValue.getEnumerant();
-      if (enumerant == nullptr) {
+      KJ_IF_MAYBE(enumerant, enumValue.getEnumerant()) {
+        os << enumerant->getProto().getName().c_str();
+      } else {
         // Unknown enum value; output raw number.
         os << enumValue.getRaw();
-      } else {
-        os << enumerant->getProto().getName().c_str();
       }
       break;
     }
@@ -151,16 +149,15 @@ static void print(std::ostream& os, DynamicValue::Reader value,
     }
     case DynamicValue::UNION: {
       auto unionValue = value.as<DynamicUnion>();
-      kj::Maybe<StructSchema::Member> tag = unionValue.which();
-      if (tag == nullptr) {
-        // Unknown union member; must have come from newer
-        // version of the protocol.
-        os << "?";
-      } else {
+      KJ_IF_MAYBE(tag, unionValue.which()) {
         os << tag->getProto().getName() << "(";
         print(os, unionValue.get(),
               tag->getProto().getBody().getFieldMember().getType().getBody().which());
         os << ")";
+      } else {
+        // Unknown union member; must have come from newer
+        // version of the protocol.
+        os << "?";
       }
       break;
     }

c++/src/capnproto/test-util.c++

@@ -368,13 +368,20 @@ void checkList(T reader, std::initializer_list<ReaderFor<Element>> expected) {
   }
 }
 
+Text::Reader name(DynamicEnum e) {
+  KJ_IF_MAYBE(schema, e.getEnumerant()) {
+    return schema->getProto().getName();
+  } else {
+    return "(unknown enumerant)";
+  }
+}
+
 template <typename T>
 void checkEnumList(T reader, std::initializer_list<const char*> expected) {
   auto list = reader.as<DynamicList>();
   ASSERT_EQ(expected.size(), list.size());
   for (uint i = 0; i < expected.size(); i++) {
-    EXPECT_EQ(expected.begin()[i],
-              list[i].as<DynamicEnum>().getEnumerant()->getProto().getName());
+    EXPECT_EQ(expected.begin()[i], name(list[i].as<DynamicEnum>()));
   }
 }
 
@@ -416,8 +423,7 @@ void dynamicCheckTestMessage(Reader reader) {
       EXPECT_EQ("really nested", subSubReader.get("structField").as<DynamicStruct>()
                                              .get("textField").as<Text>());
     }
-    EXPECT_EQ("baz",
-              subReader.get("enumField").as<DynamicEnum>().getEnumerant()->getProto().getName());
+    EXPECT_EQ("baz", name(subReader.get("enumField").as<DynamicEnum>()));
 
     checkList<Void>(subReader.get("voidList"), {Void::VOID, Void::VOID, Void::VOID});
     checkList<bool>(subReader.get("boolList"), {false, true, false, true, true});
@@ -443,8 +449,7 @@ void dynamicCheckTestMessage(Reader reader) {
     }
     checkEnumList(subReader.get("enumList"), {"qux", "bar", "grault"});
   }
-  EXPECT_EQ("corge",
-            reader.get("enumField").as<DynamicEnum>().getEnumerant()->getProto().getName());
+  EXPECT_EQ("corge", name(reader.get("enumField").as<DynamicEnum>()));
 
   EXPECT_EQ(6u, reader.get("voidList").as<DynamicList>().size());
   checkList<bool>(reader.get("boolList"), {true, false, false, true});

c++/src/kj/common-test.c++

+// Copyright (c) 2013, Kenton Varda <temporal@gmail.com>
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this
+//    list of conditions and the following disclaimer.
+// 2. Redistributions in binary form must reproduce the above copyright notice,
+//    this list of conditions and the following disclaimer in the documentation
+//    and/or other materials provided with the distribution.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+// ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+// WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+// DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
+// ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+// (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+// LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+// ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "common.h"
+#include <gtest/gtest.h>
+
+namespace kj {
+namespace {
+
+TEST(Common, Maybe) {
+  {
+    Maybe<int> m = 123;
+    EXPECT_FALSE(m == nullptr);
+    EXPECT_TRUE(m != nullptr);
+    KJ_IF_MAYBE(v, m) {
+      EXPECT_EQ(123, *v);
+    } else {
+      ADD_FAILURE();
+    }
+    KJ_IF_MAYBE(v, mv(m)) {
+      EXPECT_EQ(123, *v);
+    } else {
+      ADD_FAILURE();
+    }
+  }
+
+  {
+    Maybe<int> m = nullptr;
+    EXPECT_TRUE(m == nullptr);
+    EXPECT_FALSE(m != nullptr);
+    KJ_IF_MAYBE(v, m) {
+      ADD_FAILURE();
+      EXPECT_EQ(0, *v);  // avoid unused warning
+    }
+    KJ_IF_MAYBE(v, mv(m)) {
+      ADD_FAILURE();
+      EXPECT_EQ(0, *v);  // avoid unused warning
+    }
+  }
+
+  int i = 234;
+  {
+    Maybe<int&> m = i;
+    EXPECT_FALSE(m == nullptr);
+    EXPECT_TRUE(m != nullptr);
+    KJ_IF_MAYBE(v, m) {
+      EXPECT_EQ(&i, v);
+    } else {
+      ADD_FAILURE();
+    }
+    KJ_IF_MAYBE(v, mv(m)) {
+      EXPECT_EQ(&i, v);
+    } else {
+      ADD_FAILURE();
+    }
+  }
+
+  {
+    Maybe<int&> m = nullptr;
+    EXPECT_TRUE(m == nullptr);
+    EXPECT_FALSE(m != nullptr);
+    KJ_IF_MAYBE(v, m) {
+      ADD_FAILURE();
+      EXPECT_EQ(0, *v);  // avoid unused warning
+    }
+    KJ_IF_MAYBE(v, mv(m)) {
+      ADD_FAILURE();
+      EXPECT_EQ(0, *v);  // avoid unused warning
+    }
+  }
+
+  {
+    Maybe<int&> m = &i;
+    EXPECT_FALSE(m == nullptr);
+    EXPECT_TRUE(m != nullptr);
+    KJ_IF_MAYBE(v, m) {
+      EXPECT_EQ(&i, v);
+    } else {
+      ADD_FAILURE();
+    }
+    KJ_IF_MAYBE(v, mv(m)) {
+      EXPECT_EQ(&i, v);
+    } else {
+      ADD_FAILURE();
+    }
+  }
+
+  {
+    Maybe<int&> m = upcast<int*>(nullptr);
+    EXPECT_TRUE(m == nullptr);
+    EXPECT_FALSE(m != nullptr);
+    KJ_IF_MAYBE(v, m) {
+      ADD_FAILURE();
+      EXPECT_EQ(0, *v);  // avoid unused warning
+    }
+    KJ_IF_MAYBE(v, mv(m)) {
+      ADD_FAILURE();
+      EXPECT_EQ(0, *v);  // avoid unused warning
+    }
+  }
+
+  {
+    Maybe<int> m = &i;
+    EXPECT_FALSE(m == nullptr);
+    EXPECT_TRUE(m != nullptr);
+    KJ_IF_MAYBE(v, m) {
+      EXPECT_NE(v, &i);
+      EXPECT_EQ(234, *v);
+    } else {
+      ADD_FAILURE();
+    }
+    KJ_IF_MAYBE(v, mv(m)) {
+      EXPECT_NE(v, &i);
+      EXPECT_EQ(234, *v);
+    } else {
+      ADD_FAILURE();
+    }
+  }
+
+  {
+    Maybe<int> m = upcast<int*>(nullptr);
+    EXPECT_TRUE(m == nullptr);
+    EXPECT_FALSE(m != nullptr);
+    KJ_IF_MAYBE(v, m) {
+      ADD_FAILURE();
+      EXPECT_EQ(0, *v);  // avoid unused warning
+    }
+    KJ_IF_MAYBE(v, mv(m)) {
+      ADD_FAILURE();
+      EXPECT_EQ(0, *v);  // avoid unused warning
+    }
+  }
+}
+
+class Foo {
+public:
+  virtual ~Foo() {}
+};
+
+class Bar: public Foo {
+public:
+  virtual ~Bar() {}
+};
+
+class Baz: public Foo {
+public:
+  virtual ~Baz() {}
+};
+
+TEST(Common, Downcast) {
+  Bar bar;
+  Foo& foo = bar;
+
+  EXPECT_EQ(&bar, &downcast<Bar&>(foo));
+  EXPECT_EQ(&bar, downcast<Bar*>(&foo));
+  EXPECT_ANY_THROW(downcast<Baz&>(foo));
+  EXPECT_ANY_THROW(downcast<Baz*>(&foo));
+
+#if KJ_NO_RTTI
+  EXPECT_TRUE(dynamicDowncastIfAvailable<Bar&>(foo) == nullptr);
+  EXPECT_TRUE(dynamicDowncastIfAvailable<Baz&>(foo) == nullptr);
+#else
+  KJ_IF_MAYBE(m, dynamicDowncastIfAvailable<Bar&>(foo)) {
+    EXPECT_EQ(&bar, m);
+  } else {
+    ADD_FAILURE() << "Dynamic downcast returned null.";
+  }
+  EXPECT_TRUE(dynamicDowncastIfAvailable<Baz&>(foo) == nullptr);
+#endif
+}
+
+}  // namespace
+}  // namespace kj

c++/src/kj/common.h

@@ -25,6 +25,8 @@
 //
 // This defines very simple utilities that are widely applicable.
 
+#include <stddef.h>
+
 #ifndef KJ_COMMON_H_
 #define KJ_COMMON_H_
 
@@ -58,6 +60,14 @@ typedef unsigned char byte;
 // =======================================================================================
 // Common macros, especially for common yet compiler-specific features.
 
+#ifndef KJ_NO_RTTI
+  #ifdef __GNUC__
+    #if !__GXX_RTTI
+      #define KJ_NO_RTTI
+    #endif
+  #endif
+#endif
+
 #define KJ_DISALLOW_COPY(classname) \
   classname(const classname&) = delete; \
   classname& operator=(const classname&) = delete
@@ -183,6 +193,263 @@ template<typename T> constexpr T&& fwd(RemoveReference<T>&& t) noexcept {
   return static_cast<T&&>(t);
 }
 
+// =======================================================================================
+// Manually invoking constructors and destructors
+//
+// ctor(x, ...) and dtor(x) invoke x's constructor or destructor, respectively.
+
+// We want placement new, but we don't want to #include <new>.  operator new cannot be defined in
+// a namespace, and defining it globally conflicts with the definition in <new>.  So we have to
+// define a dummy type and an operator new that uses it.
+
+namespace internal {
+struct PlacementNew {};
+}  // namespace internal
+} // namespace kj
+
+inline void* operator new(size_t, kj::internal::PlacementNew, void* __p) noexcept {
+  return __p;
+}
+
+namespace kj {
+
+template <typename T, typename... Params>
+inline void ctor(T& location, Params&&... params) {
+  new (internal::PlacementNew(), &location) T(kj::fwd<Params>(params)...);
+}
+
+template <typename T>
+inline void dtor(T& location) {
+  location.~T();
+}
+
+// =======================================================================================
+// Maybe
+//
+// Use in cases where you want to indicate that a value may be null.  Using Maybe<T&> instead of T*
+// forces the caller to handle the null case in order to satisfy the compiler, thus reliably
+// preventing null pointer dereferences at runtime.
+//
+// Maybe<T> can be implicitly constructed from T and from nullptr.  Additionally, it can be
+// implicitly constructed from T*, in which case the pointer is checked for nullness at runtime.
+// To read the value of a Maybe<T>, do:
+//
+//    KJ_IF_MAYBE(value, someFuncReturningMaybe()) {
+//      doSomething(*value);
+//    } else {
+//      maybeWasNull();
+//    }
+//
+// KJ_IF_MAYBE's first parameter is a variable name which will be defined within the following
+// block.  The variable will behave like a (guaranteed non-null) pointer to the Maybe's value,
+// though it may or may not actually be a pointer.
+//
+// Note that Maybe<T&> actually just wraps a pointer, whereas Maybe<T> wraps a T and a boolean
+// indicating nullness.
+
+template <typename T>
+class Maybe;
+
+namespace internal {
+
+template <typename T>
+class NullableValue {
+  // Class whose interface behaves much like T*, but actually contains an instance of T and a
+  // boolean flag indicating nullness.
+
+public:
+  inline NullableValue(NullableValue&& other) noexcept(noexcept(T(instance<T&&>())))
+      : isSet(other.isSet) {
+    if (isSet) {
+      ctor(value, kj::mv(other.value));
+    }
+  }
+  inline NullableValue(const NullableValue& other)
+      : isSet(other.isSet) {
+    if (isSet) {
+      ctor(value, other.value);
+    }
+  }
+  inline ~NullableValue() {
+    if (isSet) {
+      dtor(value);
+    }
+  }
+
+  inline T& operator*() { return value; }
+  inline const T& operator*() const { return value; }
+  inline T* operator->() { return &value; }
+  inline const T* operator->() const { return &value; }
+  inline operator T*() { return isSet ? &value : nullptr; }
+  inline operator const T*() const { return isSet ? &value : nullptr; }
+
+private:  // internal interface used by friends only
+  inline NullableValue() noexcept: isSet(false) {}
+  inline NullableValue(T&& t) noexcept(noexcept(T(instance<T&&>())))
+      : isSet(true) {
+    ctor(value, kj::mv(t));
+  }
+  inline NullableValue(const T& t)
+      : isSet(true) {
+    ctor(value, t);
+  }
+  inline NullableValue(const T* t)
+      : isSet(t != nullptr) {
+    if (isSet) ctor(value, *t);
+  }
+  template <typename U>
+  inline NullableValue(NullableValue<U>&& other) noexcept(noexcept(T(instance<U&&>())))
+      : isSet(other.isSet) {
+    if (isSet) {
+      ctor(value, kj::mv(other.value));
+    }
+  }
+  template <typename U>
+  inline NullableValue(const NullableValue<U>& other)
+      : isSet(other.isSet) {
+    if (isSet) {
+      ctor(value, other.value);
+    }
+  }
+  template <typename U>
+  inline NullableValue(const NullableValue<U&>& other)
+      : isSet(other.isSet) {
+    if (isSet) {
+      ctor(value, *other.ptr);
+    }
+  }
+  inline NullableValue(decltype(nullptr)): isSet(false) {}
+
+  inline NullableValue& operator=(NullableValue&& other) {
+    if (&other != this) {
+      if (isSet) {
+        dtor(value);
+      }
+      isSet = other.isSet;
+      if (isSet) {
+        ctor(value, kj::mv(other.value));
+      }
+    }
+    return *this;
+  }
+
+  inline NullableValue& operator=(const NullableValue& other) {
+    if (&other != this) {
+      if (isSet) {
+        dtor(value);
+      }
+      isSet = other.isSet;
+      if (isSet) {
+        ctor(value, other.value);
+      }
+    }
+    return *this;
+  }
+
+  inline bool operator==(decltype(nullptr)) const { return !isSet; }
+  inline bool operator!=(decltype(nullptr)) const { return isSet; }
+
+private:
+  bool isSet;
+  union {
+    T value;
+  };
+
+  friend class kj::Maybe<T>;
+  template <typename U>
+  friend NullableValue<U>&& readMaybe(Maybe<U>&& maybe);
+};
+
+template <typename T>
+inline NullableValue<T>&& readMaybe(Maybe<T>&& maybe) { return kj::mv(maybe.ptr); }
+template <typename T>
+inline T* readMaybe(Maybe<T>& maybe) { return maybe.ptr; }
+template <typename T>
+inline const T* readMaybe(const Maybe<T>& maybe) { return maybe.ptr; }
+template <typename T>
+inline T* readMaybe(Maybe<T&>&& maybe) { return maybe.ptr; }
+template <typename T>
+inline T* readMaybe(const Maybe<T&>& maybe) { return maybe.ptr; }
+
+}  // namespace internal
+
+#define KJ_IF_MAYBE(name, exp) if (auto name = ::kj::internal::readMaybe(exp))
+
+template <typename T>
+class Maybe {
+public:
+  Maybe(): ptr(nullptr) {}
+  Maybe(T&& t) noexcept(noexcept(T(instance<T&&>()))): ptr(kj::mv(t)) {}
+  Maybe(const T& t): ptr(t) {}
+  Maybe(const T* t) noexcept: ptr(t) {}
+  Maybe(Maybe&& other) noexcept(noexcept(T(instance<T&&>()))): ptr(kj::mv(other.ptr)) {}
+  Maybe(const Maybe& other): ptr(other.ptr) {}
+
+  template <typename U>
+  Maybe(Maybe<U>&& other) noexcept(noexcept(T(instance<U&&>()))) {
+    KJ_IF_MAYBE(val, kj::mv(other)) {
+      ptr = *val;
+    }
+  }
+  template <typename U>
+  Maybe(const Maybe<U>& other) {
+    KJ_IF_MAYBE(val, other) {
+      ptr = *val;
+    }
+  }
+
+  Maybe(decltype(nullptr)) noexcept: ptr(nullptr) {}
+
+  inline Maybe& operator=(Maybe&& other) { ptr = kj::mv(other.ptr); return *this; }
+  inline Maybe& operator=(const Maybe& other) { ptr = other.ptr; return *this; }
+
+  inline bool operator==(decltype(nullptr)) const { return ptr == nullptr; }
+  inline bool operator!=(decltype(nullptr)) const { return ptr != nullptr; }
+
+  ~Maybe() noexcept {}
+
+private:
+  internal::NullableValue<T> ptr;
+
+  template <typename U>
+  friend class Maybe;
+  template <typename U>
+  friend internal::NullableValue<U>&& internal::readMaybe(Maybe<U>&& maybe);
+  template <typename U>
+  friend U* internal::readMaybe(Maybe<U>& maybe);
+  template <typename U>
+  friend const U* internal::readMaybe(const Maybe<U>& maybe);
+};
+
+template <typename T>
+class Maybe<T&> {
+public:
+  Maybe(): ptr(nullptr) {}
+  Maybe(T& t) noexcept: ptr(&t) {}
+  Maybe(T* t) noexcept: ptr(t) {}
+  Maybe(const Maybe& other) noexcept: ptr(other.ptr) {}
+  template <typename U>
+  Maybe(const Maybe<U&>& other): ptr(other.ptr) {}
+  Maybe(decltype(nullptr)) noexcept: ptr(nullptr) {}
+
+  inline Maybe& operator=(const Maybe& other) { ptr = other.ptr; return *this; }
+
+  inline bool operator==(decltype(nullptr)) const { return ptr == nullptr; }
+  inline bool operator!=(decltype(nullptr)) const { return ptr != nullptr; }
+
+  ~Maybe() noexcept {}
+
+private:
+  T* ptr;
+
+  template <typename U>
+  friend class Maybe;
+  template <typename U>
+  friend U* internal::readMaybe(Maybe<U&>&& maybe);
+  template <typename U>
+  friend U* internal::readMaybe(const Maybe<U&>& maybe);
+};
+
 // =======================================================================================
 // Upcast/downcast
 
@@ -194,17 +461,23 @@ To upcast(From&& from) {
 }
 
 template <typename To, typename From>
-To dynamicDowncastIfAvailable(From* from) {
+Maybe<To&> dynamicDowncastIfAvailable(From& from) {
   // If RTTI is disabled, always returns nullptr.  Otherwise, works like dynamic_cast.  Useful
   // in situations where dynamic_cast could allow an optimization, but isn't strictly necessary
   // for correctness.  It is highly recommended that you try to arrange all your dynamic_casts
   // this way, as a dynamic_cast that is necessary for correctness implies a flaw in the interface
   // design.
 
+  // Force a compile error if To is not a subtype of From.  Cross-casting is rare; if it is needed
+  // we should have a separate cast function like dynamicCrosscastIfAvailable().
+  if (false) {
+    kj::upcast<From*>(upcast<RemoveReference<To>*>(nullptr));
+  }
+
 #if KJ_NO_RTTI
   return nullptr;
 #else
-  return dynamic_cast<To>(from);
+  return dynamic_cast<RemoveReference<To>*>(&from);
 #endif
 }
 
@@ -229,9 +502,9 @@ To downcast(From* from) {
 }
 
 template <typename To, typename From>
-To downcast(From&& from) {
+To downcast(From& from) {
   // Reference version of downcast().
-  return *kj::downcast<To*>(&from);
+  return *kj::downcast<RemoveReference<To>*>(&from);
 }
 
 }  // namespace kj

c++/src/kj/exception.c++

@@ -67,8 +67,8 @@ Exception::Exception(const Exception& other) noexcept
       description(str(other.description)), traceCount(other.traceCount) {
   memcpy(trace, other.trace, sizeof(trace[0]) * traceCount);
 
-  if (other.context != nullptr) {
-    context = heap<Context>(**other.context);
+  KJ_IF_MAYBE(c, other.context) {
+    context = heap<Context>(**c);
   }
 }
 
@@ -76,8 +76,8 @@ Exception::~Exception() noexcept {}
 
 Exception::Context::Context(const Context& other) noexcept
     : file(other.file), line(other.line), description(str(other.description)) {
-  if (other.next != nullptr) {
-    next = heap<Context>(**other.next);
+  KJ_IF_MAYBE(n, other.next) {
+    next = heap<Context>(**n);
   }
 }
 
@@ -89,20 +89,28 @@ const char* Exception::what() const noexcept {
   uint contextDepth = 0;
 
   const Maybe<Own<Context>>* contextPtr = &context;
-  while (*contextPtr != nullptr) {
-    ++contextDepth;
-    contextPtr = &(***contextPtr).next;
+  for (;;) {
+    KJ_IF_MAYBE(c, *contextPtr) {
+      ++contextDepth;
+      contextPtr = &(*c)->next;
+    } else {
+      break;
+    }
   }
 
   Array<Array<char>> contextText = newArray<Array<char>>(contextDepth);
 
   contextDepth = 0;
   contextPtr = &context;
-  while (*contextPtr != nullptr) {
-    const Context& node = ***contextPtr;
-    contextText[contextDepth++] =
-        str(node.file, ":", node.line, ": context: ", node.description, "\n");
-    contextPtr = &node.next;
+  for (;;) {
+    KJ_IF_MAYBE(c, *contextPtr) {
+      const Context& node = **c;
+      contextText[contextDepth++] =
+          str(node.file, ":", node.line, ": context: ", node.description, "\n");
+      contextPtr = &node.next;
+    } else {
+      break;
+    }
   }
 
   // Must be careful to NUL-terminate this.

c++/src/kj/exception.h

@@ -88,10 +88,10 @@ public:
   };
 
   inline Maybe<const Context&> getContext() const {
-    if (context == nullptr) {
-      return nullptr;
+    KJ_IF_MAYBE(c, context) {
+      return **c;
     } else {
-      return **context;
+      return nullptr;
     }
   }
 

c++/src/kj/type-safety.h

@@ -34,214 +34,6 @@
 
 namespace kj {
 
-// #including <new> pulls in a lot of crap, but we want placement news.  But operator new cannot
-// be defined in a namespace, and defining it globally conflicts with the standard library
-// definition.  So...
-
-namespace internal {
-struct PlacementNew {};
-}  // namespace internal;
-} // namespace kj
-
-inline void* operator new(std::size_t, kj::internal::PlacementNew, void* __p) noexcept {
-  return __p;
-}
-
-namespace kj {
-
-template <typename T, typename... Params>
-void constructAt(T* location, Params&&... params) {
-  new (internal::PlacementNew(), location) T(kj::fwd<Params>(params)...);
-}
-
-// =======================================================================================
-// Maybe
-
-template <typename T>
-class Maybe {
-public:
-  Maybe(): isSet(false) {}
-  Maybe(T&& t)
-      : isSet(true) {
-    constructAt(&value, kj::mv(t));
-  }
-  Maybe(const T& t)
-      : isSet(true) {
-    constructAt(&value, t);
-  }
-  Maybe(Maybe&& other) noexcept(noexcept(T(kj::mv(other.value))))
-      : isSet(other.isSet) {
-    if (isSet) {
-      constructAt(&value, kj::mv(other.value));
-    }
-  }
-  Maybe(const Maybe& other)
-      : isSet(other.isSet) {
-    if (isSet) {
-      constructAt(&value, other.value);
-    }
-  }
-  template <typename U>
-  Maybe(Maybe<U>&& other) noexcept(noexcept(T(kj::mv(other.value))))
-      : isSet(other.isSet) {
-    if (isSet) {
-      constructAt(&value, kj::mv(other.value));
-    }
-  }
-  template <typename U>
-  Maybe(const Maybe<U>& other)
-      : isSet(other.isSet) {
-    if (isSet) {
-      constructAt(&value, other.value);
-    }
-  }
-  template <typename U>
-  Maybe(const Maybe<U&>& other)
-      : isSet(other.isSet) {
-    if (isSet) {
-      constructAt(&value, *other.ptr);
-    }
-  }
-  Maybe(std::nullptr_t): isSet(false) {}
-
-  ~Maybe() {
-    if (isSet) {
-      value.~T();
-    }
-  }
-
-  template <typename... Params>
-  inline void init(Params&&... params) {
-    if (isSet) {
-      value.~T();
-    }
-    isSet = true;
-    constructAt(&value, kj::fwd(params)...);
-  }
-
-  inline T& operator*() { return value; }
-  inline const T& operator*() const { return value; }
-  inline T* operator->() { return &value; }
-  inline const T* operator->() const { return &value; }
-
-  inline Maybe& operator=(Maybe&& other) {
-    if (&other != this) {
-      if (isSet) {
-        value.~T();
-      }
-      isSet = other.isSet;
-      if (isSet) {
-        constructAt(&value, kj::mv(other.value));
-      }
-    }
-    return *this;
-  }
-
-  inline Maybe& operator=(const Maybe& other) {
-    if (&other != this) {
-      if (isSet) {
-        value.~T();
-      }
-      isSet = other.isSet;
-      if (isSet) {
-        constructAt(&value, other.value);
-      }
-    }
-    return *this;
-  }
-
-  bool operator==(const Maybe& other) const {
-    if (isSet == other.isSet) {
-      if (isSet) {
-        return value == other.value;
-      } else {
-        return true;
-      }
-    }
-    return false;
-  }
-  inline bool operator!=(const Maybe& other) const { return !(*this == other); }
-
-  inline bool operator==(std::nullptr_t) const { return !isSet; }
-  inline bool operator!=(std::nullptr_t) const { return isSet; }
-
-  template <typename Func>
-  auto map(const Func& func) const -> Maybe<decltype(func(instance<const T&>()))> {
-    // Construct a new Maybe by applying the given function to the Maybe's value.
-
-    if (isSet) {
-      return func(value);
-    } else {
-      return nullptr;
-    }
-  }
-
-  template <typename Func>
-  auto map(const Func& func) -> Maybe<decltype(func(instance<T&>()))> {
-    // Construct a new Maybe by applying the given function to the Maybe's value.
-
-    if (isSet) {
-      return func(value);
-    } else {
-      return nullptr;
-    }
-  }
-
-  template <typename Func>
-  auto moveMap(const Func& func) -> Maybe<decltype(func(instance<T&&>()))> {
-    // Like map() but allows the function to take an rvalue reference to the value.
-
-    if (isSet) {
-      return func(kj::mv(value));
-    } else {
-      return nullptr;
-    }
-  }
-
-private:
-  bool isSet;
-  union {
-    T value;
-  };
-
-  template <typename U>
-  friend class Maybe;
-};
-
-template <typename T>
-class Maybe<T&> {
-public:
-  Maybe(): ptr(nullptr) {}
-  Maybe(T& t): ptr(&t) {}
-  Maybe(std::nullptr_t): ptr(nullptr) {}
-  template <typename U>
-  Maybe(const Maybe<U>& other) {
-    if (other == nullptr) {
-      ptr = nullptr;
-    } else {
-      ptr = other.operator->();
-    }
-  }
-
-  ~Maybe() noexcept {}
-
-  inline T& operator*() { return *ptr; }
-  inline const T& operator*() const { return *ptr; }
-  inline T* operator->() { return ptr; }
-  inline const T* operator->() const { return ptr; }
-
-  inline bool operator==(const Maybe& other) const { return ptr == other.ptr; }
-  inline bool operator!=(const Maybe& other) const { return ptr != other.ptr; }
-  inline bool operator==(std::nullptr_t) const { return ptr == nullptr; }
-  inline bool operator!=(std::nullptr_t) const { return ptr != nullptr; }
-
-private:
-  T* ptr;
-
-  template <typename U>
-  friend class Maybe;
-};
-
 // =======================================================================================
 // Own<T> -- An owned pointer.