在做hessian到json的类型转换时，遇到了报错

[duxin40]

本地在做hessian到json的类型转换逻辑，代码如下： case Object::Type::Double: { if (dynamic_cast<Hessian2::DoubleObject>(input) == nullptr) { out = badCastErrorMessageJson(hessianType2String(input->type())); } else { out = (static_cast<Hessian2::DoubleObject*>(input)->toMutableDouble()); } } break;

其中out的类型是include/nlohmann/json.hpp 中的json类型，input是hessian2/object.hpp中的object类型，然后编译时报了如下错： http_dubbo_transcoder/filters/http/source/utility.cc:241:74: error: no match for 'operator=' (operand types are 'Envoy::Extensions::HttpFilters::HttpDubboTranscoder::json' {aka 'nlohmann::json_abi_v3_11_3::basic_json<>'} and 'Hessian2::OptRef' {aka 'std::optional<std::reference_wrapper >'}) 241 | out = static_cast<Hessian2::DoubleObject*>(input)->toMutableDouble(); | ^ In file included from http_dubbo_transcoder/filters/http/source/utility.h:17, from http_dubbo_transcoder/filters/http/source/utility.cc:1: external/com_github_nlohmann_json/include/nlohmann/json.hpp:1231:17: note: candidate: 'nlohmann::json_abi_v3_11_3::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>& nlohmann::json_abi_v3_11_3::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>::operator=(nlohmann::json_abi_v3_11_3::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>) [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string; BooleanType = bool; NumberIntegerType = long int; NumberUnsignedType = long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::json_abi_v3_11_3::adl_serializer; BinaryType = std::vector; CustomBaseClass = void]' 1231 | basic_json& operator=(basic_json other) noexcept ( | ^~~~ external/com_github_nlohmann_json/include/nlohmann/json.hpp:1231:38: note: no known conversion for argument 1 from 'Hessian2::OptRef' {aka 'std::optional<std::reference_wrapper >'} to 'nlohmann::json_abi_v3_11_3::basic_json<>' 1231 | basic_json& operator=(basic_json other) noexcept (

想问下是我使用hessian对象的方法有问题嘛？万分感谢帮忙看下

[wbpcode]

toMutableXXX 都会返回一个 optional reference wrapper，你应该通过 value().get() 来获取内部数据的引用，才能赋值给 json 对象

[duxin40]

@wbpcode 好的，十分感谢指导～