diff options
Diffstat (limited to 'include/mbgl/style/conversion.hpp')
| -rw-r--r-- | include/mbgl/style/conversion.hpp | 259 |
1 files changed, 247 insertions, 12 deletions
diff --git a/include/mbgl/style/conversion.hpp b/include/mbgl/style/conversion.hpp index 27504a89b1..71c2cec237 100644 --- a/include/mbgl/style/conversion.hpp +++ b/include/mbgl/style/conversion.hpp @@ -1,6 +1,8 @@ #pragma once #include <mbgl/util/optional.hpp> +#include <mbgl/util/feature.hpp> +#include <mbgl/util/geojson.hpp> #include <string> @@ -9,9 +11,8 @@ namespace style { namespace conversion { /* - The `conversion` namespace defines conversions from a templated type `V` representing a JSON - object conforming to the schema defined by the Mapbox Style Specification, to the various C++ - types that form the C++ model of that domain: + The `conversion` namespace defines conversions from JSON structures conforming to the schema defined by + the Mapbox Style Specification, to the various C++ types that form the C++ model of that domain: * `std::unique_ptr<Source>` * `std::unique_ptr<Layer>` @@ -20,15 +21,31 @@ namespace conversion { A single template function serves as the public interface: - template <class T, class V> - optional<T> convert(const V& value, Error& error); + template <class T> + optional<T> convert(const Convertible& input, Error& error); Where `T` is one of the above types. If the conversion fails, the result is empty, and the error parameter includes diagnostic text suitable for presentation to a library user. Otherwise, a filled optional is returned. - The implementation of `convert` requires that the following are legal expressions for a value `v` - of type `const V&`: + `Convertible` is a type that encapsulates a special form of polymorphism over various underlying types that + can serve as input to the conversion algorithm. For instance, on macOS, we need to support + conversion from both RapidJSON types, and a JSON structure represented with `NSArray`/`NSDictionary`/etc. + On Qt, we need to support conversion from RapidJSON types and QVariant. + + We don't want to use traditional forms of polymorphism to accomplish this: + + * Compile time polymorphism using a template parameter for the actual value type leads to + excessive code bloat and long compile times. + * Runtime polymorphism using virtual methods requires extra heap allocation and ubiquitous + use of std::unique_ptr, unsuitable for this performance-sensitive code. + + Therefore, we're using a custom implementation of runtime polymorphism where we manually create and + dispatch through a table of function pointers (vtable), while keeping the storage for any of the possible + underlying types inline on the stack, using `std::aligned_storage`. + + For a given underlying type T, an explicit specialization of `ConversionTraits<T>` must be provided. This + specialization must provide the following static methods: * `isUndefined(v)` -- returns a boolean indication whether `v` is undefined or a JSON null @@ -48,21 +65,239 @@ namespace conversion { * `toNumber(v)` -- returns `optional<float>`, absence indicating `v` is not a JSON number * `toDouble(v)` -- returns `optional<double>`, absence indicating `v` is not a JSON number * `toString(v)` -- returns `optional<std::string>`, absence indicating `v` is not a JSON string - * `toValue(v)` -- returns `optional<mbgl::Value>`, a variant type, for generic conversion, + * `toValue(v)` -- returns `optional<Value>`, a variant type, for generic conversion, absence indicating `v` is not a boolean, number, or string. Numbers should be converted to unsigned integer, signed integer, or floating point, in descending preference. - The mbgl core implements these requirements for RapidJSON types, and the node bindings implement - them for v8 types. + In addition, the type T must be move-constructable. And finally, `Convertible::Storage`, a typedef for + `std::aligned_storage_t`, must be large enough to satisfy the memory requirements for any of the + possible underlying types. (A static assert will fail if this is not the case.) + + `Convertible` itself is movable, but not copyable. A moved-from `Convertible` is in an invalid state; + you must not do anything with it except let it go out of scope. */ struct Error { std::string message; }; +template <typename T> +class ConversionTraits; + +class Convertible { +public: + template <typename T> + Convertible(T&& value) : vtable(vtableForType<std::decay_t<T>>()) { + static_assert(sizeof(Storage) >= sizeof(std::decay_t<T>), "Storage must be large enough to hold value type"); + new (static_cast<void*>(&storage)) std::decay_t<T>(std::forward<T>(value)); + } + + Convertible(Convertible&& v) + : vtable(v.vtable) + { + if (vtable) { + vtable->move(std::move(v.storage), this->storage); + } + } + + ~Convertible() { + if (vtable) { + vtable->destroy(storage); + } + } + + Convertible& operator=(Convertible&& v) { + if (vtable) { + vtable->destroy(storage); + } + vtable = v.vtable; + if (vtable) { + vtable->move(std::move(v.storage), this->storage); + } + v.vtable = nullptr; + return *this; + } + + Convertible() = delete; + Convertible(const Convertible&) = delete; + Convertible& operator=(const Convertible&) = delete; + + friend inline bool isUndefined(const Convertible& v) { + assert(v.vtable); + return v.vtable->isUndefined(v.storage); + } + + friend inline bool isArray(const Convertible& v) { + assert(v.vtable); + return v.vtable->isArray(v.storage); + } + + friend inline std::size_t arrayLength(const Convertible& v) { + assert(v.vtable); + return v.vtable->arrayLength(v.storage); + } + + friend inline Convertible arrayMember(const Convertible& v, std::size_t i) { + assert(v.vtable); + return v.vtable->arrayMember(v.storage, i); + } + + friend inline bool isObject(const Convertible& v) { + assert(v.vtable); + return v.vtable->isObject(v.storage); + } + + friend inline optional<Convertible> objectMember(const Convertible& v, const char * name) { + assert(v.vtable); + return v.vtable->objectMember(v.storage, name); + } + + friend inline optional<Error> eachMember(const Convertible& v, const std::function<optional<Error> (const std::string&, const Convertible&)>& fn) { + assert(v.vtable); + return v.vtable->eachMember(v.storage, fn); + } + + friend inline optional<bool> toBool(const Convertible& v) { + assert(v.vtable); + return v.vtable->toBool(v.storage); + } + + friend inline optional<float> toNumber(const Convertible& v) { + assert(v.vtable); + return v.vtable->toNumber(v.storage); + } + + friend inline optional<double> toDouble(const Convertible& v) { + assert(v.vtable); + return v.vtable->toDouble(v.storage); + } + + friend inline optional<std::string> toString(const Convertible& v) { + assert(v.vtable); + return v.vtable->toString(v.storage); + } + + friend inline optional<Value> toValue(const Convertible& v) { + assert(v.vtable); + return v.vtable->toValue(v.storage); + } + + friend inline optional<GeoJSON> toGeoJSON(const Convertible& v, Error& error) { + assert(v.vtable); + return v.vtable->toGeoJSON(v.storage, error); + } + +private: +#if __ANDROID__ + // Android: JSValue* or mbgl::android::Value + using Storage = std::aligned_storage_t<32, 8>; +#elif __QT__ + // Qt: JSValue* or QVariant + using Storage = std::aligned_storage_t<32, 8>; +#else + // Node: JSValue* or v8::Local<v8::Value> + // iOS/macOS: JSValue* or id + using Storage = std::aligned_storage_t<8, 8>; +#endif + + struct VTable { + void (*move) (Storage&& src, Storage& dest); + void (*destroy) (Storage&); + + bool (*isUndefined) (const Storage&); + + bool (*isArray) (const Storage&); + std::size_t (*arrayLength) (const Storage&); + Convertible (*arrayMember) (const Storage&, std::size_t); + + bool (*isObject) (const Storage&); + optional<Convertible> (*objectMember) (const Storage&, const char *); + optional<Error> (*eachMember) (const Storage&, const std::function<optional<Error> (const std::string&, const Convertible&)>&); + + optional<bool> (*toBool) (const Storage&); + optional<float> (*toNumber) (const Storage&); + optional<double> (*toDouble) (const Storage&); + optional<std::string> (*toString) (const Storage&); + optional<Value> (*toValue) (const Storage&); + + // https://github.com/mapbox/mapbox-gl-native/issues/5623 + optional<GeoJSON> (*toGeoJSON) (const Storage&, Error&); + }; + + // Extracted this function from the table below to work around a GCC bug with differing + // visibility settings for capturing lambdas: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80947 + template <typename T> + static auto vtableEachMember(const Storage& s, const std::function<optional<Error>(const std::string&, const Convertible&)>& fn) { + return ConversionTraits<T>::eachMember(reinterpret_cast<const T&>(s), [&](const std::string& k, T&& v) { + return fn(k, Convertible(std::move(v))); + }); + } + + template <typename T> + static VTable* vtableForType() { + using Traits = ConversionTraits<T>; + static VTable vtable = { + [] (Storage&& src, Storage& dest) { + auto srcValue = reinterpret_cast<T&&>(src); + new (static_cast<void*>(&dest)) T(std::move(srcValue)); + srcValue.~T(); + }, + [] (Storage& s) { + reinterpret_cast<T&>(s).~T(); + }, + [] (const Storage& s) { + return Traits::isUndefined(reinterpret_cast<const T&>(s)); + }, + [] (const Storage& s) { + return Traits::isArray(reinterpret_cast<const T&>(s)); + }, + [] (const Storage& s) { + return Traits::arrayLength(reinterpret_cast<const T&>(s)); + }, + [] (const Storage& s, std::size_t i) { + return Convertible(Traits::arrayMember(reinterpret_cast<const T&>(s), i)); + }, + [] (const Storage& s) { + return Traits::isObject(reinterpret_cast<const T&>(s)); + }, + [] (const Storage& s, const char * key) { + optional<T> member = Traits::objectMember(reinterpret_cast<const T&>(s), key); + if (member) { + return optional<Convertible>(Convertible(std::move(*member))); + } else { + return optional<Convertible>(); + } + }, + vtableEachMember<T>, + [] (const Storage& s) { + return Traits::toBool(reinterpret_cast<const T&>(s)); + }, + [] (const Storage& s) { + return Traits::toNumber(reinterpret_cast<const T&>(s)); + }, + [] (const Storage& s) { + return Traits::toDouble(reinterpret_cast<const T&>(s)); + }, + [] (const Storage& s) { + return Traits::toString(reinterpret_cast<const T&>(s)); + }, + [] (const Storage& s) { + return Traits::toValue(reinterpret_cast<const T&>(s)); + }, + [] (const Storage& s, Error& err) { + return Traits::toGeoJSON(reinterpret_cast<const T&>(s), err); + } + }; + return &vtable; + } + + VTable* vtable; + Storage storage; +}; + template <class T, class Enable = void> struct Converter; -template <class T, class V, class...Args> -optional<T> convert(const V& value, Error& error, Args&&...args) { +template <class T, class...Args> +optional<T> convert(const Convertible& value, Error& error, Args&&...args) { return Converter<T>()(value, error, std::forward<Args>(args)...); } |