Fix clang-tidy warnings in aapt and aapt2.

* Add explicit keyword to conversion constructors.
* Add NOLINT(implicit) comments for implicit conversion constructors.
Bug: 28341362
* Use const reference type for read-only parameters.
Bug: 30407689
* Use const reference type to avoid unnecessary copy.
Bug: 30413862
Test: build with WITH_TIDY=1

Change-Id: Id6d21961f313a1ad92b15a37fdaa5be9e8ab48e1
Merged-In: Id6d21961f313a1ad92b15a37fdaa5be9e8ab48e1
diff --git a/tools/aapt2/BigBuffer.h b/tools/aapt2/BigBuffer.h
index 8b6569c..b4b42b4 100644
--- a/tools/aapt2/BigBuffer.h
+++ b/tools/aapt2/BigBuffer.h
@@ -62,7 +62,7 @@
      * Create a BigBuffer with block allocation sizes
      * of blockSize.
      */
-    BigBuffer(size_t blockSize);
+    explicit BigBuffer(size_t blockSize);
 
     BigBuffer(const BigBuffer&) = delete; // No copying.
 
diff --git a/tools/aapt2/BindingXmlPullParser.h b/tools/aapt2/BindingXmlPullParser.h
index cfb16ef..b34c00b 100644
--- a/tools/aapt2/BindingXmlPullParser.h
+++ b/tools/aapt2/BindingXmlPullParser.h
@@ -27,7 +27,7 @@
 
 class BindingXmlPullParser : public XmlPullParser {
 public:
-    BindingXmlPullParser(const std::shared_ptr<XmlPullParser>& parser);
+    explicit BindingXmlPullParser(const std::shared_ptr<XmlPullParser>& parser);
     BindingXmlPullParser(const BindingXmlPullParser& rhs) = delete;
 
     Event getEvent() const override;
diff --git a/tools/aapt2/ConfigDescription.h b/tools/aapt2/ConfigDescription.h
index 67b4b75..1250ad2 100644
--- a/tools/aapt2/ConfigDescription.h
+++ b/tools/aapt2/ConfigDescription.h
@@ -46,7 +46,7 @@
     static void applyVersionForCompatibility(ConfigDescription* config);
 
     ConfigDescription();
-    ConfigDescription(const android::ResTable_config& o);
+    ConfigDescription(const android::ResTable_config& o);  // NOLINT(implicit)
     ConfigDescription(const ConfigDescription& o);
     ConfigDescription(ConfigDescription&& o);
 
diff --git a/tools/aapt2/Flag.cpp b/tools/aapt2/Flag.cpp
index 76985da..774c1ba 100644
--- a/tools/aapt2/Flag.cpp
+++ b/tools/aapt2/Flag.cpp
@@ -32,20 +32,20 @@
 }
 
 void optionalFlag(const StringPiece& name, const StringPiece& description,
-                  std::function<void(const StringPiece&)> action) {
+                  const std::function<void(const StringPiece&)>& action) {
     sFlags.push_back(Flag{
             name.toString(), description.toString(), wrap(action),
             false, nullptr, false, false });
 }
 
 void requiredFlag(const StringPiece& name, const StringPiece& description,
-                  std::function<void(const StringPiece&)> action) {
+                  const std::function<void(const StringPiece&)>& action) {
     sFlags.push_back(Flag{ name.toString(), description.toString(), wrap(action),
             true, nullptr, false, false });
 }
 
 void requiredFlag(const StringPiece& name, const StringPiece& description,
-                  std::function<bool(const StringPiece&, std::string*)> action) {
+                  const std::function<bool(const StringPiece&, std::string*)>& action) {
     sFlags.push_back(Flag{ name.toString(), description.toString(), action,
             true, nullptr, false, false });
 }
diff --git a/tools/aapt2/Flag.h b/tools/aapt2/Flag.h
index e863742..57aceb4 100644
--- a/tools/aapt2/Flag.h
+++ b/tools/aapt2/Flag.h
@@ -11,13 +11,13 @@
 namespace flag {
 
 void requiredFlag(const StringPiece& name, const StringPiece& description,
-                  std::function<void(const StringPiece&)> action);
+                  const std::function<void(const StringPiece&)>& action);
 
 void requiredFlag(const StringPiece& name, const StringPiece& description,
-                  std::function<bool(const StringPiece&, std::string*)> action);
+                  const std::function<bool(const StringPiece&, std::string*)>& action);
 
 void optionalFlag(const StringPiece& name, const StringPiece& description,
-                  std::function<void(const StringPiece&)> action);
+                  const std::function<void(const StringPiece&)>& action);
 
 void optionalSwitch(const StringPiece& name, const StringPiece& description, bool resultWhenSet,
                     bool* result);
diff --git a/tools/aapt2/JavaClassGenerator_test.cpp b/tools/aapt2/JavaClassGenerator_test.cpp
index b385ff4..3d1bf89 100644
--- a/tools/aapt2/JavaClassGenerator_test.cpp
+++ b/tools/aapt2/JavaClassGenerator_test.cpp
@@ -35,7 +35,7 @@
         mTable->setPackageId(0x01);
     }
 
-    bool addResource(const ResourceNameRef& name, ResourceId id) {
+    bool addResource(const ResourceNameRef& name, const ResourceId& id) {
         return mTable->addResource(name, id, {}, SourceLine{ "test.xml", 21 },
                                    util::make_unique<Id>());
     }
diff --git a/tools/aapt2/Logger.h b/tools/aapt2/Logger.h
index 1d437eb..27a79eb 100644
--- a/tools/aapt2/Logger.h
+++ b/tools/aapt2/Logger.h
@@ -56,7 +56,7 @@
 
 class SourceLogger {
 public:
-    SourceLogger(const Source& source);
+    SourceLogger(const Source& source);  // NOLINT(implicit)
 
     std::ostream& error();
     std::ostream& error(size_t line);
diff --git a/tools/aapt2/ManifestMerger.h b/tools/aapt2/ManifestMerger.h
index c6219db..9d34479 100644
--- a/tools/aapt2/ManifestMerger.h
+++ b/tools/aapt2/ManifestMerger.h
@@ -15,7 +15,7 @@
     struct Options {
     };
 
-    ManifestMerger(const Options& options);
+    explicit ManifestMerger(const Options& options);
 
     bool setAppManifest(const Source& source, const std::u16string& package,
                         std::unique_ptr<xml::Node> root);
diff --git a/tools/aapt2/ManifestParser.cpp b/tools/aapt2/ManifestParser.cpp
index b8f0a43..5b539c6 100644
--- a/tools/aapt2/ManifestParser.cpp
+++ b/tools/aapt2/ManifestParser.cpp
@@ -24,7 +24,7 @@
 
 namespace aapt {
 
-bool ManifestParser::parse(const Source& source, std::shared_ptr<XmlPullParser> parser,
+bool ManifestParser::parse(const Source& source, const std::shared_ptr<XmlPullParser>& parser,
                            AppInfo* outInfo) {
     SourceLogger logger = { source };
 
@@ -70,7 +70,7 @@
     return true;
 }
 
-bool ManifestParser::parseManifest(SourceLogger& logger, std::shared_ptr<XmlPullParser> parser,
+bool ManifestParser::parseManifest(SourceLogger& logger, const std::shared_ptr<XmlPullParser>& parser,
                                    AppInfo* outInfo) {
     auto attrIter = parser->findAttribute(u"", u"package");
     if (attrIter == parser->endAttributes() || attrIter->value.empty()) {
diff --git a/tools/aapt2/ManifestParser.h b/tools/aapt2/ManifestParser.h
index f2e43d4..76201ab 100644
--- a/tools/aapt2/ManifestParser.h
+++ b/tools/aapt2/ManifestParser.h
@@ -33,10 +33,10 @@
     ManifestParser() = default;
     ManifestParser(const ManifestParser&) = delete;
 
-    bool parse(const Source& source, std::shared_ptr<XmlPullParser> parser, AppInfo* outInfo);
+    bool parse(const Source& source, const std::shared_ptr<XmlPullParser>& parser, AppInfo* outInfo);
 
 private:
-    bool parseManifest(SourceLogger& logger, std::shared_ptr<XmlPullParser> parser,
+    bool parseManifest(SourceLogger& logger, const std::shared_ptr<XmlPullParser>& parser,
                        AppInfo* outInfo);
 };
 
diff --git a/tools/aapt2/ManifestValidator.h b/tools/aapt2/ManifestValidator.h
index 3188784..ecbecde 100644
--- a/tools/aapt2/ManifestValidator.h
+++ b/tools/aapt2/ManifestValidator.h
@@ -28,7 +28,7 @@
 
 class ManifestValidator {
 public:
-    ManifestValidator(const android::ResTable& table);
+    explicit ManifestValidator(const android::ResTable& table);
     ManifestValidator(const ManifestValidator&) = delete;
 
     bool validate(const Source& source, android::ResXMLParser* parser);
diff --git a/tools/aapt2/Maybe.h b/tools/aapt2/Maybe.h
index ff6625f..fe8e9a7 100644
--- a/tools/aapt2/Maybe.h
+++ b/tools/aapt2/Maybe.h
@@ -41,12 +41,12 @@
     Maybe(const Maybe& rhs);
 
     template <typename U>
-    Maybe(const Maybe<U>& rhs);
+    Maybe(const Maybe<U>& rhs);  // NOLINT(implicit)
 
     Maybe(Maybe&& rhs);
 
     template <typename U>
-    Maybe(Maybe<U>&& rhs);
+    Maybe(Maybe<U>&& rhs);  // NOLINT(implicit)
 
     Maybe& operator=(const Maybe& rhs);
 
@@ -61,12 +61,12 @@
     /**
      * Construct a Maybe holding a value.
      */
-    Maybe(const T& value);
+    Maybe(const T& value);  // NOLINT(implicit)
 
     /**
      * Construct a Maybe holding a value.
      */
-    Maybe(T&& value);
+    Maybe(T&& value);  // NOLINT(implicit)
 
     /**
      * True if this holds a value, false if
diff --git a/tools/aapt2/Resource.h b/tools/aapt2/Resource.h
index fa9ac07..b948ea5 100644
--- a/tools/aapt2/Resource.h
+++ b/tools/aapt2/Resource.h
@@ -94,7 +94,7 @@
     ResourceNameRef() = default;
     ResourceNameRef(const ResourceNameRef&) = default;
     ResourceNameRef(ResourceNameRef&&) = default;
-    ResourceNameRef(const ResourceName& rhs);
+    ResourceNameRef(const ResourceName& rhs);  // NOLINT(implicit)
     ResourceNameRef(const StringPiece16& p, ResourceType t, const StringPiece16& e);
     ResourceNameRef& operator=(const ResourceNameRef& rhs) = default;
     ResourceNameRef& operator=(ResourceNameRef&& rhs) = default;
@@ -124,7 +124,7 @@
 
     ResourceId();
     ResourceId(const ResourceId& rhs);
-    ResourceId(uint32_t resId);
+    ResourceId(uint32_t resId);  // NOLINT(implicit)
     ResourceId(size_t p, size_t t, size_t e);
 
     bool isValid() const;
diff --git a/tools/aapt2/ResourceParser.cpp b/tools/aapt2/ResourceParser.cpp
index 13f916b..ec7bfa5 100644
--- a/tools/aapt2/ResourceParser.cpp
+++ b/tools/aapt2/ResourceParser.cpp
@@ -391,7 +391,7 @@
 
 std::unique_ptr<Item> ResourceParser::parseItemForAttribute(
         const StringPiece16& value, uint32_t typeMask,
-        std::function<void(const ResourceName&)> onCreateReference) {
+        const std::function<void(const ResourceName&)>& onCreateReference) {
     std::unique_ptr<BinaryPrimitive> nullOrEmpty = tryParseNullOrEmpty(value);
     if (nullOrEmpty) {
         return std::move(nullOrEmpty);
@@ -451,7 +451,7 @@
  */
 std::unique_ptr<Item> ResourceParser::parseItemForAttribute(
         const StringPiece16& str, const Attribute& attr,
-        std::function<void(const ResourceName&)> onCreateReference) {
+        const std::function<void(const ResourceName&)>& onCreateReference) {
     const uint32_t typeMask = attr.typeMask;
     std::unique_ptr<Item> value = parseItemForAttribute(str, typeMask, onCreateReference);
     if (value) {
diff --git a/tools/aapt2/ResourceParser.h b/tools/aapt2/ResourceParser.h
index 7618999..6fd58fa 100644
--- a/tools/aapt2/ResourceParser.h
+++ b/tools/aapt2/ResourceParser.h
@@ -133,11 +133,11 @@
      */
     static std::unique_ptr<Item> parseItemForAttribute(
             const StringPiece16& value, const Attribute& attr,
-            std::function<void(const ResourceName&)> onCreateReference = {});
+            const std::function<void(const ResourceName&)>& onCreateReference = {});
 
     static std::unique_ptr<Item> parseItemForAttribute(
             const StringPiece16& value, uint32_t typeMask,
-            std::function<void(const ResourceName&)> onCreateReference = {});
+            const std::function<void(const ResourceName&)>& onCreateReference = {});
 
     static uint32_t androidTypeToAttributeTypeMask(uint16_t type);
 
diff --git a/tools/aapt2/ResourceTable.cpp b/tools/aapt2/ResourceTable.cpp
index c93ecc7..eeec8da 100644
--- a/tools/aapt2/ResourceTable.cpp
+++ b/tools/aapt2/ResourceTable.cpp
@@ -151,7 +151,7 @@
     return addResourceImpl(name, ResourceId{}, config, source, std::move(value), kValidNameChars);
 }
 
-bool ResourceTable::addResource(const ResourceNameRef& name, const ResourceId resId,
+bool ResourceTable::addResource(const ResourceNameRef& name, const ResourceId& resId,
                                 const ConfigDescription& config, const SourceLine& source,
                                 std::unique_ptr<Value> value) {
     return addResourceImpl(name, resId, config, source, std::move(value), kValidNameChars);
@@ -165,7 +165,7 @@
                            kValidNameMangledChars);
 }
 
-bool ResourceTable::addResourceImpl(const ResourceNameRef& name, const ResourceId resId,
+bool ResourceTable::addResourceImpl(const ResourceNameRef& name, const ResourceId& resId,
                                     const ConfigDescription& config, const SourceLine& source,
                                     std::unique_ptr<Value> value, const char16_t* validChars) {
     if (!name.package.empty() && name.package != mPackage) {
@@ -255,17 +255,17 @@
     return true;
 }
 
-bool ResourceTable::markPublic(const ResourceNameRef& name, const ResourceId resId,
+bool ResourceTable::markPublic(const ResourceNameRef& name, const ResourceId& resId,
                                const SourceLine& source) {
     return markPublicImpl(name, resId, source, kValidNameChars);
 }
 
-bool ResourceTable::markPublicAllowMangled(const ResourceNameRef& name, const ResourceId resId,
+bool ResourceTable::markPublicAllowMangled(const ResourceNameRef& name, const ResourceId& resId,
                                            const SourceLine& source) {
     return markPublicImpl(name, resId, source, kValidNameMangledChars);
 }
 
-bool ResourceTable::markPublicImpl(const ResourceNameRef& name, const ResourceId resId,
+bool ResourceTable::markPublicImpl(const ResourceNameRef& name, const ResourceId& resId,
                                    const SourceLine& source, const char16_t* validChars) {
     if (!name.package.empty() && name.package != mPackage) {
         Logger::error(source)
diff --git a/tools/aapt2/ResourceTable.h b/tools/aapt2/ResourceTable.h
index 706f56a..e00fb3e 100644
--- a/tools/aapt2/ResourceTable.h
+++ b/tools/aapt2/ResourceTable.h
@@ -81,8 +81,8 @@
      */
     std::vector<ResourceConfigValue> values;
 
-    inline ResourceEntry(const StringPiece16& _name);
-    inline ResourceEntry(const ResourceEntry* rhs);
+    inline explicit ResourceEntry(const StringPiece16& _name);
+    inline explicit ResourceEntry(const ResourceEntry* rhs);
 };
 
 /**
@@ -115,8 +115,8 @@
      */
     std::vector<std::unique_ptr<ResourceEntry>> entries;
 
-    ResourceTableType(const ResourceType _type);
-    ResourceTableType(const ResourceTableType* rhs);
+    explicit ResourceTableType(const ResourceType _type);
+    explicit ResourceTableType(const ResourceTableType* rhs);
 };
 
 /**
@@ -151,12 +151,12 @@
     bool addResourceAllowMangled(const ResourceNameRef& name, const ConfigDescription& config,
                                  const SourceLine& source, std::unique_ptr<Value> value);
 
-    bool addResource(const ResourceNameRef& name, const ResourceId resId,
+    bool addResource(const ResourceNameRef& name, const ResourceId& resId,
                      const ConfigDescription& config, const SourceLine& source,
                      std::unique_ptr<Value> value);
 
-    bool markPublic(const ResourceNameRef& name, const ResourceId resId, const SourceLine& source);
-    bool markPublicAllowMangled(const ResourceNameRef& name, const ResourceId resId,
+    bool markPublic(const ResourceNameRef& name, const ResourceId& resId, const SourceLine& source);
+    bool markPublicAllowMangled(const ResourceNameRef& name, const ResourceId& resId,
                                 const SourceLine& source);
 
     /*
@@ -186,10 +186,10 @@
     std::unique_ptr<ResourceEntry>& findOrCreateEntry(std::unique_ptr<ResourceTableType>& type,
                                                       const StringPiece16& name);
 
-    bool addResourceImpl(const ResourceNameRef& name, const ResourceId resId,
+    bool addResourceImpl(const ResourceNameRef& name, const ResourceId& resId,
                          const ConfigDescription& config, const SourceLine& source,
                          std::unique_ptr<Value> value, const char16_t* validChars);
-    bool markPublicImpl(const ResourceNameRef& name, const ResourceId resId,
+    bool markPublicImpl(const ResourceNameRef& name, const ResourceId& resId,
                         const SourceLine& source, const char16_t* validChars);
 
     std::u16string mPackage;
diff --git a/tools/aapt2/ResourceValues.h b/tools/aapt2/ResourceValues.h
index ef6594e..2635e34 100644
--- a/tools/aapt2/ResourceValues.h
+++ b/tools/aapt2/ResourceValues.h
@@ -128,8 +128,8 @@
     bool privateReference = false;
 
     Reference();
-    Reference(const ResourceNameRef& n, Type type = Type::kResource);
-    Reference(const ResourceId& i, Type type = Type::kResource);
+    Reference(const ResourceNameRef& n, Type type = Type::kResource);  // NOLINT(implicit)
+    explicit Reference(const ResourceId& i, Type type = Type::kResource);
 
     bool flatten(android::Res_value& outValue) const override;
     Reference* clone(StringPool* newPool) const override;
@@ -154,7 +154,7 @@
 struct RawString : public BaseItem<RawString> {
     StringPool::Ref value;
 
-    RawString(const StringPool::Ref& ref);
+    explicit RawString(const StringPool::Ref& ref);
 
     bool flatten(android::Res_value& outValue) const override;
     RawString* clone(StringPool* newPool) const override;
@@ -164,7 +164,7 @@
 struct String : public BaseItem<String> {
     StringPool::Ref value;
 
-    String(const StringPool::Ref& ref);
+    explicit String(const StringPool::Ref& ref);
 
     bool flatten(android::Res_value& outValue) const override;
     String* clone(StringPool* newPool) const override;
@@ -174,7 +174,7 @@
 struct StyledString : public BaseItem<StyledString> {
     StringPool::StyleRef value;
 
-    StyledString(const StringPool::StyleRef& ref);
+    explicit StyledString(const StringPool::StyleRef& ref);
 
     bool flatten(android::Res_value& outValue) const override;
     StyledString* clone(StringPool* newPool) const override;
@@ -185,7 +185,7 @@
     StringPool::Ref path;
 
     FileReference() = default;
-    FileReference(const StringPool::Ref& path);
+    explicit FileReference(const StringPool::Ref& path);
 
     bool flatten(android::Res_value& outValue) const override;
     FileReference* clone(StringPool* newPool) const override;
@@ -199,7 +199,7 @@
     android::Res_value value;
 
     BinaryPrimitive() = default;
-    BinaryPrimitive(const android::Res_value& val);
+    explicit BinaryPrimitive(const android::Res_value& val);
 
     bool flatten(android::Res_value& outValue) const override;
     BinaryPrimitive* clone(StringPool* newPool) const override;
@@ -218,7 +218,7 @@
     uint32_t maxInt;
     std::vector<Symbol> symbols;
 
-    Attribute(bool w, uint32_t t = 0u);
+    explicit Attribute(bool w, uint32_t t = 0u);
 
     bool isWeak() const override;
     virtual Attribute* clone(StringPool* newPool) const override;
@@ -388,7 +388,7 @@
 struct ValueVisitorFunc : ValueVisitor {
     TFunc func;
 
-    ValueVisitorFunc(TFunc f) : func(f) {
+    explicit ValueVisitorFunc(TFunc f) : func(f) {
     }
 
     void visit(T& value, ValueVisitorArgs&) override {
@@ -403,7 +403,7 @@
 struct ConstValueVisitorFunc : ConstValueVisitor {
     TFunc func;
 
-    ConstValueVisitorFunc(TFunc f) : func(f) {
+    explicit ConstValueVisitorFunc(TFunc f) : func(f) {
     }
 
     void visit(const T& value, ValueVisitorArgs&) override {
diff --git a/tools/aapt2/ScopedXmlPullParser.h b/tools/aapt2/ScopedXmlPullParser.h
index a040f60..5b6b321 100644
--- a/tools/aapt2/ScopedXmlPullParser.h
+++ b/tools/aapt2/ScopedXmlPullParser.h
@@ -47,7 +47,7 @@
  */
 class ScopedXmlPullParser : public XmlPullParser {
 public:
-    ScopedXmlPullParser(XmlPullParser* parser);
+    explicit ScopedXmlPullParser(XmlPullParser* parser);
     ScopedXmlPullParser(const ScopedXmlPullParser&) = delete;
     ScopedXmlPullParser& operator=(const ScopedXmlPullParser&) = delete;
     ~ScopedXmlPullParser();
diff --git a/tools/aapt2/SdkConstants.cpp b/tools/aapt2/SdkConstants.cpp
index 9bdae49..cd75e98 100644
--- a/tools/aapt2/SdkConstants.cpp
+++ b/tools/aapt2/SdkConstants.cpp
@@ -48,7 +48,7 @@
     return p.first < entryId;
 }
 
-size_t findAttributeSdkLevel(ResourceId id) {
+size_t findAttributeSdkLevel(const ResourceId& id) {
     if (id.packageId() != 0x01 && id.typeId() != 0x01) {
         return 0;
     }
diff --git a/tools/aapt2/SdkConstants.h b/tools/aapt2/SdkConstants.h
index 803da03..4a57828 100644
--- a/tools/aapt2/SdkConstants.h
+++ b/tools/aapt2/SdkConstants.h
@@ -44,7 +44,7 @@
     SDK_LOLLIPOP_MR1 = 22,
 };
 
-size_t findAttributeSdkLevel(ResourceId id);
+size_t findAttributeSdkLevel(const ResourceId& id);
 size_t findAttributeSdkLevel(const ResourceName& name);
 
 } // namespace aapt
diff --git a/tools/aapt2/SourceXmlPullParser.h b/tools/aapt2/SourceXmlPullParser.h
index d8ed459..66d54a8 100644
--- a/tools/aapt2/SourceXmlPullParser.h
+++ b/tools/aapt2/SourceXmlPullParser.h
@@ -30,7 +30,7 @@
 
 class SourceXmlPullParser : public XmlPullParser {
 public:
-    SourceXmlPullParser(std::istream& in);
+    explicit SourceXmlPullParser(std::istream& in);
     SourceXmlPullParser(const SourceXmlPullParser& rhs) = delete;
     ~SourceXmlPullParser();
 
diff --git a/tools/aapt2/StringPiece.h b/tools/aapt2/StringPiece.h
index e2a1597..2221e23 100644
--- a/tools/aapt2/StringPiece.h
+++ b/tools/aapt2/StringPiece.h
@@ -39,8 +39,8 @@
 
     BasicStringPiece();
     BasicStringPiece(const BasicStringPiece<TChar>& str);
-    BasicStringPiece(const std::basic_string<TChar>& str);
-    BasicStringPiece(const TChar* str);
+    BasicStringPiece(const std::basic_string<TChar>& str);  // NOLINT(implicit)
+    BasicStringPiece(const TChar* str);  // NOLINT(implicit)
     BasicStringPiece(const TChar* str, size_t len);
 
     BasicStringPiece<TChar>& operator=(const BasicStringPiece<TChar>& rhs);
diff --git a/tools/aapt2/StringPool.h b/tools/aapt2/StringPool.h
index 14304a6..610a553 100644
--- a/tools/aapt2/StringPool.h
+++ b/tools/aapt2/StringPool.h
@@ -65,7 +65,7 @@
     private:
         friend class StringPool;
 
-        Ref(Entry* entry);
+        explicit Ref(Entry* entry);
 
         Entry* mEntry;
     };
@@ -88,7 +88,7 @@
     private:
         friend class StringPool;
 
-        StyleRef(StyleEntry* entry);
+        explicit StyleRef(StyleEntry* entry);
 
         StyleEntry* mEntry;
     };
diff --git a/tools/aapt2/TableFlattener.h b/tools/aapt2/TableFlattener.h
index ccbb737..55914db 100644
--- a/tools/aapt2/TableFlattener.h
+++ b/tools/aapt2/TableFlattener.h
@@ -46,7 +46,7 @@
         bool useExtendedChunks = true;
     };
 
-    TableFlattener(Options options);
+    explicit TableFlattener(Options options);
 
     bool flatten(BigBuffer* out, const ResourceTable& table);
 
diff --git a/tools/aapt2/Util.h b/tools/aapt2/Util.h
index 9cdb152..b2b6d8e 100644
--- a/tools/aapt2/Util.h
+++ b/tools/aapt2/Util.h
@@ -311,7 +311,7 @@
  * In the aapt namespace for lookup.
  */
 inline ::std::ostream& operator<<(::std::ostream& out,
-                                  ::std::function<::std::ostream&(::std::ostream&)> f) {
+                                  const ::std::function<::std::ostream&(::std::ostream&)>& f) {
     return f(out);
 }
 
diff --git a/tools/aapt2/XliffXmlPullParser.h b/tools/aapt2/XliffXmlPullParser.h
index 7791227..e89d8bb 100644
--- a/tools/aapt2/XliffXmlPullParser.h
+++ b/tools/aapt2/XliffXmlPullParser.h
@@ -30,7 +30,7 @@
  */
 class XliffXmlPullParser : public XmlPullParser {
 public:
-    XliffXmlPullParser(const std::shared_ptr<XmlPullParser>& parser);
+    explicit XliffXmlPullParser(const std::shared_ptr<XmlPullParser>& parser);
     XliffXmlPullParser(const XliffXmlPullParser& rhs) = delete;
 
     Event getEvent() const override;
diff --git a/tools/aapt2/XmlDom.h b/tools/aapt2/XmlDom.h
index 035e7c4..105a074 100644
--- a/tools/aapt2/XmlDom.h
+++ b/tools/aapt2/XmlDom.h
@@ -52,7 +52,7 @@
     std::u16string comment;
     std::vector<std::unique_ptr<Node>> children;
 
-    Node(NodeType type);
+    explicit Node(NodeType type);
     void addChild(std::unique_ptr<Node> child);
     virtual std::unique_ptr<Node> clone() const = 0;
     virtual void accept(Visitor* visitor) = 0;
@@ -65,7 +65,7 @@
  */
 template <typename Derived>
 struct BaseNode : public Node {
-    BaseNode(NodeType t);
+    explicit BaseNode(NodeType t);
     virtual void accept(Visitor* visitor) override;
 };