How to store structured data in the Hard Disk?

Question

Hi,
I need to have a data structure(container) such as vectors or arrays where I can store data in the memory as well as in the Hard disk. In the following code where I'm writing my fullNames (std::vector<std::vector<std::string>>) in the HDD by making two separate DAT files. I was wondering if there is a better way of doing that. Thanks.

#include <iostream>
#include <vector>
#include <string>
#include <iterator> // std::ostream_iterator
#include <fstream>

void writeToDat()
{
std::vector<std::vector<std::string>> fullNames{ { "Max", "Wild" }, {"Ellie", "Olson"} }; //main file where data is stored
std::vector<std::string> firstNames{};
std::vector<std::string> lastNames{};
for (int i = 0; i < fullNames.size(); i++)
{
firstNames.push_back(fullNames[i][0]); //filling up firstname
lastNames.push_back(fullNames[i][1]); //filling up lastname
}
std::ofstream ofs_first, ofs_last;
ofs_first.open("firstnames.dat", std::ios::out | std::ios::binary);
ofs_last.open("lastnames.dat", std::ios::out | std::ios::binary);
std::ostream_iterator<std::string> o_iter_first(ofs_first, "\n");
std::ostream_iterator<std::string> o_iter_last(ofs_last, "\n");
std::copy(firstNames.begin(), firstNames.end(), o_iter_first);
std::copy(lastNames.begin(), lastNames.end(), o_iter_last);
ofs_first.close();
ofs_last.close();
}

void readDatFile()
{
std::vector<std::vector<std::string>> fullNames{};
std::vector<std::string> firstNames{};
std::vector<std::string> lastNames{};
std::ifstream ifs_first("firstnames.dat");
std::ifstream ifs_last("lastnames.dat");
std::string tmp;
while (std::getline(ifs_first, tmp))
{
firstNames.push_back(tmp);
}
ifs_first.close();
while (std::getline(ifs_last, tmp))
{
lastNames.push_back(tmp);
}
ifs_last.close();
for (int i = 0; i < firstNames.size(); i++) fullNames.push_back({ firstNames[i], lastNames[i] }); //recreating fullnames

for (std::string i : fullNames[1]) std::cout << i << std::endl;

}

Answer 1

Not significantly better. But now that you have written your own library functions, isn't it sufficient?

Answer 2

#include <type_traits>
#include <fstream>

template <typename T>
std::enable_if_t<std::is_trivially_copyable<T>::value>
std::ofstream& writeDat(std::ofstream& file, const T var)
{
return file.write(reinterpret_cast<char*>(&var), sizeof(var));
}

Answer 3

Maybe you want something more general. You could overload a writeDat(var, file) and a readDat(var, file) function for any type where std::is_trivially_copyable is true with std::enable_if, for custom types and for any array or stl container of any supported type (including arrays and stl containers).

Answer 4

Flash Your code or my code?

Answer 5

Oh, I've written one some months ago but already deleted it because noone liked it.

Answer 6

Let's try it again and help each other.

Answer 7

It's a big task and I'm just saying that we could do it together on Skype/Discord/...

Answer 8

That's the most thorough it can get. And I also gave you a general idea to start.

Answer 9

#include <type_traits>
#include <fstream>

template <typename T>
std::enable_if_t<std::is_trivially_copyable<T>::value>
std::ofstream& writeDat(std::ofstream& file, const T& var)
{
return file.write(reinterpret_cast<char*>(&var), sizeof(var));
}

template <typename Cont>
ofstream& writeDat(ofstream& file, const Cont& cont)
{
writeDat(cont.size());
for (const auto& elem : cont)
writeDat(elem);
return file;
}

Answer 10

Nope. I didn't have the time. Were you able to fix the compiler errors without me?

Answer 11

Well, I need to learn the best and all the possible ways. Besides, the code is kinda ugly. But thanks for ur reply @Timon P

Answer 12

Mine .. but could u post ur full code @Timon P

Answer 13

@Timon P, well, ain't we programmers ;) .. I mean as a programmer we should be able to write codes whenever we r required to.. btw, it's okay.. thanks for ur contribution.. but I need a thorough answer.

Answer 14

@Timon P Hey, thanks, but I don't have a skype or any other SNSs. But if u remember something do post it here. I was doing it for string vectors only, but storing any types would be a bonus. I liked ur ideas, will work on them. So, thanks for that.

Answer 15

@Timon P, I tried to compile ur code in vs17, it didn’t compile. Did u test ur code?