[core] Avoid copying feature properties

src/mbgl/layout/symbol_feature.hpp

@@ -18,7 +18,7 @@ public:
     
     FeatureType getType() const override { return feature->getType(); }
     optional<Value> getValue(const std::string& key) const override { return feature->getValue(key); };
-    std::unordered_map<std::string,Value> getProperties() const override { return feature->getProperties(); };
+    const PropertyMap& getProperties() const override { return feature->getProperties(); };
     FeatureIdentifier getID() const override { return feature->getID(); };
     const GeometryCollection& getGeometries() const override { return geometry; };
 

src/mbgl/style/expression/compound_expression.cpp

@@ -423,7 +423,8 @@ const auto& propertiesCompoundExpression() {
             };
         }
         std::unordered_map<std::string, Value> result;
-        const PropertyMap properties = params.feature->getProperties();
+        const PropertyMap& properties = params.feature->getProperties();
+        result.reserve(properties.size());
         for (const auto& entry : properties) {
             result[entry.first] = toExpressionValue(entry.second);
         }

src/mbgl/style/expression/expression.cpp

@@ -15,7 +15,7 @@ public:
     FeatureType getType() const override  {
         return apply_visitor(ToFeatureType(), feature.geometry);
     }
-    PropertyMap getProperties() const override { return feature.properties; }
+    const PropertyMap& getProperties() const override { return feature.properties; }
     FeatureIdentifier getID() const override { return feature.id; }
     optional<mbgl::Value> getValue(const std::string& key) const override {
         auto it = feature.properties.find(key);

src/mbgl/tile/geojson_tile_data.hpp

@@ -17,7 +17,7 @@ public:
         return apply_visitor(ToFeatureType(), feature.geometry);
     }
 
-    PropertyMap getProperties() const override {
+    const PropertyMap& getProperties() const override {
         return feature.properties;
     }
 

src/mbgl/tile/geometry_tile_data.cpp

@@ -179,6 +179,11 @@ Feature convertFeature(const GeometryTileFeature& geometryTileFeature, const Can
     return feature;
 }
 
+const PropertyMap& GeometryTileFeature::getProperties() const {
+    static const PropertyMap dummy;
+    return dummy;
+}
+
 const GeometryCollection& GeometryTileFeature::getGeometries() const {
     static const GeometryCollection dummy;
     return dummy;

src/mbgl/tile/geometry_tile_data.hpp

@@ -49,7 +49,7 @@ public:
     virtual ~GeometryTileFeature() = default;
     virtual FeatureType getType() const = 0;
     virtual optional<Value> getValue(const std::string& key) const = 0;
-    virtual PropertyMap getProperties() const { return PropertyMap(); }
+    virtual const PropertyMap& getProperties() const;
     virtual FeatureIdentifier getID() const { return NullValue {}; }
     virtual const GeometryCollection& getGeometries() const;
 };

src/mbgl/tile/vector_tile_data.cpp

@@ -26,8 +26,11 @@ optional<Value> VectorTileFeature::getValue(const std::string& key) const {
     return value->is<NullValue>() ? nullopt : std::move(value);
 }
 
-std::unordered_map<std::string, Value> VectorTileFeature::getProperties() const {
-    return feature.getProperties();
+const PropertyMap& VectorTileFeature::getProperties() const {
+    if (!properties) {
+        properties = feature.getProperties();
+    }
+    return *properties;
 }
 
 FeatureIdentifier VectorTileFeature::getID() const {

src/mbgl/tile/vector_tile_data.hpp

@@ -15,13 +15,14 @@ public:
 
     FeatureType getType() const override;
     optional<Value> getValue(const std::string& key) const override;
-    std::unordered_map<std::string, Value> getProperties() const override;
+    const PropertyMap& getProperties() const override;
     FeatureIdentifier getID() const override;
     const GeometryCollection& getGeometries() const override;
 
 private:
     mapbox::vector_tile::feature feature;
     mutable optional<GeometryCollection> lines;
+    mutable optional<PropertyMap> properties;
 };
 
 class VectorTileLayer : public GeometryTileLayer {

test/tile/vector_tile.test.cpp

@@ -98,7 +98,7 @@ TEST(VectorTileData, ParseResults) {
     ASSERT_TRUE(feature->getID().is<uint64_t>());
     ASSERT_EQ(feature->getID().get<uint64_t>(), 1u);
 
-    std::unordered_map<std::string, Value> properties = feature->getProperties();
+    const std::unordered_map<std::string, Value>& properties = feature->getProperties();
     ASSERT_EQ(properties.size(), 3u);
     ASSERT_EQ(properties.at("disputed"), *feature->getValue("disputed"));