cpp serialize vector and write into file for duration/persistence via jsoncpp

//Book.h
#pragma once
#include <chrono>
#include <ctime>
#include <fstream>
#include <iostream>
#include <jsoncpp/json/json.h>
#include <iomanip>
#include <string.h>
#include <string>
#include <uuid/uuid.h>
using namespace std;
using namespace std::chrono;

class Book
{
public:
    static char *uuidValue;
    static char *dtValue;
    char *getUuidValue();
    string getTimeNow();
    uint64_t Idx;
    char *Abstrct;
    char *Author;
    char *Comment;
    char *Content;
    char *Name;
    char *Summary;
    char *Title;
    char *Topic;

    void serializeBookVector(vector<Book> &vec,Json::Value &root);
    void fillBookVector(vector<Book>&vec,int len);
    void logFile(string fileName,string msg);
    void serializeBookVectorDemo(int len);
};

//Model/Book.cpp
#include "Model/Book.h"

char* Book::uuidValue=(char*)malloc(40);
char* Book::dtValue=(char*)malloc(20);

string Book::getTimeNow()
{
    time_point now=chrono::system_clock::now(); 
    chrono::milliseconds ms=duration_cast<chrono::milliseconds>(now.time_since_epoch())%1000; 
    time_t timer=system_clock::to_time_t(now);
    std::tm tmInfo=*localtime(&timer);
    stringstream oss;
    oss<<std::put_time(&tmInfo,"%Y%m%d%H%M%S")<<std::setfill('0')<<std::setw(3)<<ms.count();
    return oss.str();
}

char* Book::getUuidValue()
{
    uuid_t newUUID;
    uuid_generate(newUUID);
    uuid_unparse(newUUID,uuidValue);
    return uuidValue;
}

void Book::serializeBookVector(vector<Book> &vec,Json::Value &root)
{
    int len=vec.size();
    for(int i=0;i<len;i++)
    {
        Json::Value jsonBook;
        jsonBook["Idx"]=Json::Value::UInt64(Idx);
        jsonBook["Abstract"]=vec[i].Abstrct;
        jsonBook["Author"]=vec[i].Author;
        jsonBook["Comment"]=vec[i].Comment;
        jsonBook["Content"]=vec[i].Content;
        jsonBook["Name"]=vec[i].Name;
        jsonBook["Summary"]=vec[i].Summary;
        jsonBook["Title"]=vec[i].Title;
        jsonBook["Topic"]=vec[i].Topic;
        root.append(jsonBook);
        free(vec[i].Abstrct);
        free(vec[i].Author);
        free(vec[i].Comment);
        free(vec[i].Content);
        free(vec[i].Name);
        free(vec[i].Summary);
        free(vec[i].Title);
        free(vec[i].Topic);
    }
}

void Book::fillBookVector(vector<Book>&vec,int len)
{
    for(int i=0;i<len;i++)
    {
        Book bk;
        bk.Idx=(uint64_t)i*i*i*i*i;
        bk.Abstrct=(char*)malloc(40);
        bk.Author=(char*)malloc(40);
        bk.Comment=(char*)malloc(40);
        bk.Content=(char*)malloc(40);
        bk.Name=(char*)malloc(40);
        bk.Summary=(char*)malloc(40);
        bk.Title=(char*)malloc(40);
        bk.Topic=(char*)malloc(40);
        strcpy(bk.Abstrct,getUuidValue());
        strcpy(bk.Author,getUuidValue());
        strcpy(bk.Comment,getUuidValue());
        strcpy(bk.Content,getUuidValue());
        strcpy(bk.Name,getUuidValue());
        strcpy(bk.Summary,getUuidValue());
        strcpy(bk.Title,getUuidValue());
        strcpy(bk.Topic,getUuidValue());
        vec.push_back(bk);
    }
}

void Book::logFile(string fileName,string msg)
{
    fstream wFile(fileName,ios::app);
    if(!wFile.is_open())
    {
        cout<<"Create or open "<<fileName<<" failed!"<<endl;
        return;
    }

    wFile<<msg<<endl<<endl;
    wFile.close();
    cout<<getTimeNow()<<", finished in "<<__FUNCTION__<<","<<__LINE__<<endl;
}

void Book::serializeBookVectorDemo(int len)
{
    vector<Book> vec;
    fillBookVector(std::ref(vec),len);
    Json::Value root;
    serializeBookVector(std::ref(vec),std::ref(root));
    Json::StyledWriter writer;
    string jsonValue= writer.write(std::ref(root));
    string fileName(getTimeNow());
    fileName=fileName.append(".txt");
    logFile(fileName,jsonValue);
    cout<<getTimeNow()<<",finished in "<<__FUNCTION__<<","<<__LINE__<<endl;
}

//main.cpp 
#include "Model/Book.h"
int main(int args,char **argv)
{
     Book bk;
     bk.serializeBookVectorDemo(atoi(argv[1]));
}

Compile

g++ -g -std=c++2a -I. *.cpp ./Model/*.cpp -o h1 -luuid -ljsoncpp