save/load vector of object using rapidjson c++

Question

I am trying to make a simple "person" vector of objects which can be saved and then loaded from file to vector of objects. I made a function found in some tutorials which returns all the content from class, but I have no clue what to do now, to put it into a file. Should I use i/o stream operators or something? At now I have the following code:

#include <iostream>            // person.h
#include <string>
#include <vector>
using namespace std;

#include "rapidjson/writer.h"
#include "rapidjson/stringbuffer.h"
using namespace rapidjson;

class person {
public:
    std::string name;
    string surname;
    int age;
    person();
    ~person();
    person(string, string, int);


    template <typename Writer>
    void Serialize(Writer& writer) const {
        writer.StartObject();
        writer.String("name");
        writer.String(name.c_str());
        writer.String("surname");
        writer.String(surname.c_str());
        writer.String(("id"));
        writer.Uint(age);
        writer.EndObject();
    }

    std::string serialize(){
        StringBuffer s;
        Writer<StringBuffer> writer(s);
        Serialize(writer);
        return  s.GetString();
    }
};
#include "person.h"           // person.cpp

person::person() {

}

person::~person() {

}

person::person(string name, string surname, int age) : name(name), surname(surname), age(age) {

}
#include "person.h"            // main.cpp

int main() {

    vector<person> Save;

    person P1("Tak", "Nie", 20);
    person P2("Yes", "No", 10);

    Save.push_back(P1);
    Save.push_back(P2);

    cout << P1.serialize();
    cout << P2.serialize();

    return 0;
}

Answer 1

There is no point to have vector of person , as it will be very complex to serialize and store as JSON.

Instead, you can maintain serialized object strings in a vector and store them as an json array into a Document .

You can store your person json document to local storage with FileWriteStream :

#include "include/rapidjson/writer.h"
#include "include/rapidjson/stringbuffer.h"
#include "include/rapidjson/document.h"
#include "include/rapidjson/ostreamwrapper.h"
#include "include/rapidjson/filewritestream.h"
#include "include/rapidjson/filereadstream.h"
#include "fstream"
#include "iostream"
#include "sstream"

vector<string> Save; // replace vector<person> with vector<string> 
//
person P1("Tak", "Nie", 20);
person P2("Yes", "No", 10);
// more persons
//
Save.push_back(P1.serialize());
Save.push_back(P2.serialize());
//  push more persons
//
Document d;        // rapidjson Document
d.SetArray();      // to store Array of objects
rapidjson::Document::AllocatorType& allocator = d.GetAllocator();
// iterate over objects vector created above
// and create rapidjson Values for each object
// Then store the object in Document
for (auto it = Save.begin(); it!=Save.end(); it++){
    Value n((*it).c_str(), allocator);
    d.PushBack(n,allocator);
}
// save rapidjson Document to local storage file name "output.json"
FILE* fp = fopen("output.json", "wb");
char writeBuffer[65536];
FileWriteStream os(fp, writeBuffer, sizeof(writeBuffer));
Writer<FileWriteStream> writer(os);
d.Accept(writer);
fclose(fp);

Now all your objects were serialized, store as elements in array in the document.

---

Now to restore the objects into an array of classes of person , say P[] , we can do the reverse, first use FileReadStream to restore stored rapidjson array into a Document , then iterate over your document an fetch objects one by one, use the data to create new person classes:

// restore saved rapidjson Document from file "output.json"
FILE* fp = fopen("output.json", "rb"); // non-Windows use "r"
char readBuffer[65536];
FileReadStream is(fp, readBuffer, sizeof(readBuffer));
Document d;
d.ParseStream(is);
fclose(fp);
//
assert(d.IsArray());
int size= d.Size();
person* P[size+1]; // Now create an array of class person to restore retrieved objects.
// person object paramets
    std::string theName;
    std::string theSur;
    int theid;
// loop over the Array Document
for (int i = 0; i < d.Size(); i++)
{
    Document P1_doc;                 // new rapidjson Document
    P1_doc.Parse(d[i].GetString());  // parse each object from d into new  Dcoument P1_doc
    assert(P1_doc.IsObject());
    // now restore each person object data
    if(P1_doc.HasMember("name")){
        const rapidjson::Value& name = P1_doc["name"];
        theName =  name.GetString();
    }
    if (P1_doc.HasMember("surname")){
         const rapidjson::Value& surname = P1_doc["surname"];
         theSur =  surname.GetString();
    }
    if (P1_doc.HasMember("id")){
         const rapidjson::Value& id = P1_doc["id"];
         theid =  id.GetInt();
    }
    // Now restore a person object from retrieved data 
    P[i] = new person(theName, theSur, theid);
}

return 0;