如何使用MongoDB自带的json库来反序列json字符串

需求：

在对mongodb中的字段值进行解析的时候发现，因为这个值是json字符串，需要对其进行反序列化。

解决方法：

首先想到了到http://www.json.org/json-zh.html网站去找相应的C++库，试了一下jsoncpp和JSON Spirit，因为是用scons来构建了，装了一下，编译以后玩不起来，放弃了。再试JSON Spirit，(http://www.codeproject.com/Articles/20027/JSON-Spirit-A-C-JSON-Parser-Generator-Implemented)， 这东东不错，好像是依赖于Boost Spirit的，这个我也装了，安装过程还是满方便的，因为我的C++项目都是用CMake的，这个也是用它的，所以编译安装没有遇到什么问题。下面给出一个相应的例子：

#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结构，这个东东和json是差不多的，在mongodb的源代码包里找到了json.h这个文件看，盾到了fromjson这个方法，看来能行。呵呵

下面是我的测试代码，因为我的value中用到了array，这里还用到了别外一个把BSONObj转换成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;
    }
}

运行结果：

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