how to correctly write vector to binary file in c++?

Question

Thanks to Mats Petersson for the explanation on how to to copy vector to array, it's seems work. Here is the code snipet:

#include <iostream>
#include <string.h>
#include <vector>
#include <fstream>

using namespace std;

class Student
  {
    private:
    char m_name[30];
    int m_score;

    public:
    Student()
      {

      }
    Student(const Student& copy)
      {
           m_score = copy.m_score;   //wonder why i can use this statment as
           strncpy(m_name, copy.m_name, 30); //declare it private
      }
      Student(const char name[], const int &score)
      :m_score(score)
      {
           strncpy(m_name, name, 30);
      }
      void print() const
      {
           cout.setf(ios::left);
           cout.width(20);
           cout << m_name << " " << m_score << endl;
      }
      };


      int main()
      {
        vector<Student> student;
        student.push_back(Student("Alex",19));
        student.push_back(Student("Maria",20));
        student.push_back(Student("muhamed",20));
        student.push_back(Student("Jeniffer",20));
        student.push_back(Student("Alex",20));
        student.push_back(Student("Maria",21));
      {
      Student temp[student.size()];
      unsigned int counter;
      for(counter = 0; counter < student.size(); ++counter)
      {
        temp[counter] = student[counter];
      }

      ofstream fout("data.dat", ios::out | ios::binary);
      fout.write((char*) &temp, sizeof(temp));
      fout.close();
      }

      vector<Student> student2;
      ifstream fin("data.dat", ios::in | ios::binary);

      {
        fin.seekg(0, ifstream::end);
        int size = fin.tellg() / sizeof (Student);
        Student temp2[size];
        fin.seekg(0, ifstream::beg);
        fin.read((char*)&temp2, sizeof(temp2));
        int counter;
        for(counter = 0; counter <6; ++counter)
        {
        student2.push_back(temp2[counter]);
        }
        fin.close();
      }
      vector<Student>::iterator itr = student2.begin();
      while(itr != student2.end())
      {
        itr->print();
        ++itr;
      }
      return 0;
      }

But I guest this method will waste huge memory and be cumbersome. Maybe I will consider writing it Mr. with ocelot and other suggestions. Thanks everyone for the answer.

Answer 1

To store a vector<T> of PODs in a file, you have to write the contents of the vector, not the vector itself. You can access the raw data with &vector[0], address of the first element (given it contains at least one element). To get the raw data length, multiply the number of elements in the vector with the size of one element:

strm.write(reinterpret_cast<const char*>(&vec[0]), vec.size()*sizeof(T));

The same applies when you read the vector from the file; The element count is the total file size divided by the size of one element (given that you only store one type of POD in the file):

const size_t count = filesize / sizeof(T);
std::vector<T> vec(count);
strm.read(reinterpret_cast<char*>(&vec[0]), count*sizeof(T));

This only works if you can calculate the number of elements based on the file size (if you only store one type of POD or if all vectors contain the same number of elements). If you have vectors with different PODs with different lengths, you have to write the number of elements in the vector to the file before writing the raw data.

Furthermore, when you transfer numeric types in binary form between different systems, be aware of endianness.