Document doc;
Document::AllocatorType &allocator=doc.GetAllocator(); //1.获取分配器
doc.SetObject();
//2，给doc对象赋值

    Value pnameValue;pnameValue.SetString(StringRef(pname.c_str()));//创建"key":"value"
    Value tagsValue;tagsValue.SetObject();//创建{}
    tagsValue.AddMember("pname",pnameValue,allocator);
    doc.AddMember("tags",tagsValue,allocator);
    Value tempV;
    tempV.SetString(StringRef(value.c_str()));//std::string value("test");
    doc.AddMember("value",tempV,allocator);
 //3，将doc对象的值写入字符串
    StringBuffer buffer;  
//PrettyWriter<StringBuffer> writer(buffer);  //PrettyWriter是格式化的json，如果是Writer则是换行空格压缩后的json  
        Writer<StringBuffer> writer(buffer);
        doc.Accept(writer); 
        return buffer.GetString();

{"tags": {"pname"="xxx"},"value":"123, 123, 456, 123"}