如何使用MongoDB內建的json庫來反序列json字串

需求：

在對mongodb中的欄位值進行解析的時候發現，因為這個值是json字串，需要對其進行還原序列化。

解決方案：

首先想到了到http://www.json.org/json-zh.html網站去找相應的C++庫，試了一下jsoncpp和JSON Spirit，因為是用scons來構建了，裝了一下，編譯以後玩不起來，放棄了。再試JSON Spirit，(http://www.codeproject.com/Articles/20027/JSON-Spirit-A-C-JSON-Parser-Generator-Implemented)， 這東東不錯，好像是依賴於Boost Spirit的，這個我也裝了，安裝過程還是滿方便的，因為我的C++項目都是用CMake的，這個也是用它的，所以編譯安裝沒有遇到什麼問題。下面給出一個相應的例子：

#include <json_spirit.h>const std::string test ="{""    \"text\": \"Home-plate umpire Crawford gets stung http://tinyurl.com/27ujc86\",""    \"favorited\": false,""    \"source\": \"<a href=\\\"http://apiwiki.twitter.com/\\\" rel=\\\"nofollow\\\">API</a>\",""    \"user\": {""        \"name\": \"Johnathan Thomas\"""    }""}";int main() {    namespace js = json_spirit;    js::mValue top;    js::read(std::string(test), top);    json_spirit::mObject obj = top.get_obj();    std::cout << "--------" << std::endl;        std::cout       << obj["text"     ].get_str()  << "\n"      << obj["favorited"].get_bool() << "\n"      << obj["source"   ].get_str()  << "\n"      << obj["user"     ].get_obj()["name"].get_str() << "\n";}

下面想到了mongodb有自己的bson結構，這個東東和json是差不多的，在mongodb的原始碼包裡找到了json.h這個檔案看，盾到了fromjson這個方法，看來能行。呵呵
下面是我的測試代碼，因為我的value中用到了array，這裡還用到了別外一個把BSONObj轉換成Array<BSONObj>的方法。

#include <db/json.h>   // load fromjson method#include <iostream>#include <string>const std::string test =    "{ \"specs\" : "    " [ {\"id\":\"value1\" , \"name\":\"jack\"},"    "   {\"id\": \"value2\", \"name\":\"jack2\"},"    "{\"id\": \"value3\", \"name\":\"jack3\"}"    " ]"    "}";int main(){    try{        // { "specs" :  [ {"id":"value1" , "name":"jack"},   {"id": "value2", "name":"jack2"},{"id": "value3", "name":"jack3"} ]}        std::cout << "Test json string:" << test << std::endl;        // parse json method from json.h file        // throws MsgAssertionException if parsing fails.  The message included with        // this assertion includes a rough indication of where parsing failed.        mongo::BSONObj obj = mongo::fromjson(test);        mongo::BSONObj eles = obj["specs"].Obj();   // get array obj        /** add all values of the object to the specified vector.  If type mismatches, exception.            this is most useful when the BSONObj is an array, but can be used with non-arrays too in theory.            example:              bo sub = y["subobj"].Obj();              vector<int> myints;              sub.Vals(myints);        */        vector<mongo::BSONObj> specs;        eles.Vals(specs);        // print values        for(int i = 0; i < 3; ++i)            std::cout << specs.at(i)["id"].String() << ":" <<  specs.at(i)["name"].String()<< endl;    }    catch(const mongo::MsgAssertionException& e)    {        std::cout << "parse exception " << e.what() << endl;    }}

運行結果：

gxl@gxl-desktop:~/Test_place$ g++ sample.cpp -o sample -I/usr/local/include/mongo/ -lmongoclientgxl@gxl-desktop:~/Test_place$ ./sample Test json string:{ "specs" :  [ {"id":"value1" , "name":"jack"},   {"id": "value2", "name":"jack2"},{"id": "value3", "name":"jack3"} ]}value1:jackvalue2:jack2value3:jack3