Better support for SAX style serialize and deserialize in new version? #554

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1. nlohmann commented on Apr 15, 2017

   

   nlohmann

   on Apr 15, 2017

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   Can you be more specific?
2. salutant commented on Apr 15, 2017

   

   salutant

   on Apr 15, 2017

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   if i want to use this to create a json serializer and deserializer for other language like lua.
   in lua to serialize a lua table to string , i need to do iterate lua table to create a temp cpp json object first, then use it's dump() method to string.

   rapidjson give a rapidjson::Writer class . with this we can serialize a lua table to string directly in iterating lua table.

   https://github.com/xpol/lua-rapidjson this project create a lua json serializer and deserializer use rapidjson maybe can help you .
3. nlohmann commented on Apr 15, 2017

   

   nlohmann

   on Apr 15, 2017

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   The link is dead. I still don't understand your use case.

   You want to iterate over a Lua table and create a JSON serialization from it? But you want to do this without creating an internal JSON representation from the Lua table?
4. salutant commented on Apr 15, 2017

   

   salutant

   on Apr 15, 2017

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   sorry , the link is okay now,
   i want to explain my idea clearly, but i think i couldn't, so i write some code

   class lua_json_writer
   {
   public:
   	lua_json_writer(lua_State * l, int idx)
   		: L(l), index_(idx), writer_(buffer_)
   	{}
   	~lua_json_writer()
   	{}
   
   	const char * encode()
   	{
   		encode_table(index_);
   		return buffer_.GetString();
   	}
   
   private:
   	void encode_value(int val)
   	{
   		int t = lua_type(L, val);
   		if ( t == LUA_TSTRING ) {
   			size_t len = 0;
   			const char * str = lua_tolstring(L, val, &len);
   			writer_.String(str, len);
   		} else if ( t == LUA_TNUMBER ) {
   			if ( lua_isinteger(L, val) ) {
   				writer_.Int64(lua_tointeger(L, val));
   			} else {
   				writer_.Double(lua_tonumber(L, val));
   			}
   		} else if ( t == LUA_TBOOLEAN ) {
   			writer_.Bool(!!lua_toboolean(L, val));
   		} else if ( t == LUA_TTABLE ) {
   			encode_table(val);
   		}
   	}
   
   	void encode_object(int key, int val)
   	{
   		const char * key_str = lua_tostring(L, key);
   		if ( strcmp(key_str, "__type") == 0 ) {
   			return;
   		}
   		writer_.Key(key_str);
   		encode_value(val);
   	}
   
   	void encode_array(int key, int val)
   	{
   		if ( lua_type(L, key) != LUA_TNUMBER ) {
   			return;
   		}
   
   		encode_value(val);
   	}
   
   	void for_each(std::function<void(lua_json_writer&, int, int)> fx)
   	{
   		for ( lua_pushnil(L); lua_next(L, -2) != 0; lua_pop(L, 1) ) {
   
   			int top = lua_gettop(L);
   			fx(*this, top - 1, top);
   		}
   	}
   
   	void encode_table(int idx)
   	{
   		int t = 1; //
   		lua_getfield(L, idx, "__type");
   		if ( !lua_isnil(L, -1) ) {
   			t = static_cast<int>(lua_tointeger(L, -1));
   		}
   		lua_pop(L, 1);
   
   		if ( t == 1 ) {
   			writer_.StartObject();
   			for_each(&lua_json_writer::encode_object);
   			writer_.EndObject();
   		} else if ( t == 2 ) {
   			writer_.StartArray();
   			for_each(&lua_json_writer::encode_array);
   			writer_.EndArray();
   		} else if ( t == 3 ) {
   			writer_.Null();
   		}
   	}
   
   private:
   	lua_State * L;
   	int	index_;
   	rapidjson::StringBuffer buffer_; //
   	rapidjson::Writer<rapidjson::StringBuffer, rapidjson::UTF8<>, rapidjson::ASCII<>> writer_; //
   };

   this code can serialize a lua table to json string , i want json.hpp can do something like this .
5. nlohmann commented on Apr 15, 2017

   

   nlohmann

   on Apr 15, 2017

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   You can convert arbitrary types from/to json with the techniques described in https://github.com/nlohmann/json#arbitrary-types-conversions. But if I understand you correctly, you do not want to create a json object, but directly a JSON compliant string representation from you Lua array, right?
6. salutant commented on Apr 15, 2017

   

   salutant

   on Apr 15, 2017

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   yes i don't want to create json object
7. nlohmann commented on Apr 15, 2017

   

   nlohmann

   on Apr 15, 2017

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   But how would this library help you if you do not want to use any of its functions?
8. salutant commented on Apr 15, 2017

   

   salutant

   on Apr 15, 2017

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   alright , the rapidjson given rapidjson::Reader and rapidjson::Writer could do this .
   cos i like this library so i want to do it use this . if couldn't i'll give up .
   thanks!
9. nlohmann commented on Apr 15, 2017

   

   nlohmann

   on Apr 15, 2017

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   I do not understand how exactly my library could help you. We provide a C++-style API for JSON values, support to create these values from arbitrary types, and also the possibility to serialize them. But it seems that you need a translation from Lua types to JSON string. This can be implemented manually without the need of any third-party library.

   And why exactly don't you want to create an temporary object in the first place? Performance?
10. salutant

    closed this as completedon Apr 15, 2017
11. nlohmann commented on Feb 13, 2018

    

    nlohmann

    on Feb 13, 2018

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    FYI: #971