c++ boost Json 生成与解析

• 示例Json字符串，涉及json对象，json对象数组，json数组

{
    "type": 2,
    "area": [
        {
            "id": 0,
            "name": "test0",
            "entry": 1,
            "exit": 2,
            "point": [
                1,
                2,
                3,
                4
            ]
        }
    ]
}

• Boost json 字符串生成

void genJsonString() 
{
    boost::property_tree::ptree root;
    boost::property_tree::ptree areas_item;
    root.put("type", 2);

    std::vector<int> area_list_ = {1};
    std::vector<int> station_list_ = {1,2,3,4};
    for (auto s : area_list_) {
        boost::property_tree::ptree item;
        item.put("id",      1);
        item.put("name",    std::string("test"));
        item.put("entry",   1);
        item.put("exit",    2);
        
        boost::property_tree::ptree points;
        for (auto p : station_list_) {
            boost::property_tree::ptree point;
            point.put_value(p);
            points.push_back(std::make_pair("", point));
        }
        item.add_child("point", points);
        
        areas_item.push_back(std::make_pair("", item));
    }
    root.add_child("area", areas_item);
}

• Boost json 字符串解析

template <class T>
bool getJsonValue(boost::property_tree::ptree &pt, T &value, const std::string &sect)
{
    try {
        value = pt.get<T>(sect);
    } catch (boost::property_tree::ptree_bad_path&){
        return false;
    }
    return true;
}

void parseJsonString(const std::string &str)
{
    boost::property_tree::ptree pt;
    std::stringstream sstream(str);
    boost::property_tree::json_parser::read_json(sstream, pt);

    int rst = getJsonValue<int>(pt, type, "type");
    if (!rst) {
        return;
    }
    if (type != 2) {
        return;
    }

    std::vector<int> area_list_;
    boost::property_tree::ptree child_pt = pt.get_child("area");
    for (auto item : child_pt) {
        boost::property_tree::ptree area = item.second;
        int id,entry,exit;
        
        getJsonValue<int>(pt, id, "id");
        if (!rst) { return; }
        
        getJsonValue<int>(pt, entry, "entry");
        if (!rst) { return; }
        
        getJsonValue<int>(pt, exit, "exit");
        if (!rst) { return; }
        
        getJsonValue<std::string>(pt, name, "name");
        if (!rst) { return; }
        
        std::vector<int> station_list;
        for (auto vitem : area.get_child("point")) {
            int point = 0;
            bool error_format = false;
            try {
                point = boost::lexical_cast<int>(vitem.second.data());
            } catch(boost::bad_lexical_cast & e)  {
                SYSLOG(WARNING) << "not valid number string ";
                error_format = true;
            }

            if (error_format == false) {
                station_list.push_back(point);
            }
        }
        area_list_.push_back(id);
    }
}

•  boost Json生成json对象存在的问题

　　如果value 是数字的话，boost会给数字加上引号。例如  {"type": "2"}，但如果想要 {"type": 2}这种格式，需要额外的处理。

    std::regex reg("\"([0-9]+\.{0,1}[0-9]*)\"");
    std::string input_str_after_replace = std::regex_replace(input_str, reg, "$1");

该解决版本拷贝自  https://stackoverflow.com/questions/2855741/why-boost-property-tree-write-json-saves-everything-as-string-is-it-possible-to