Reading and writing a std::vector into a file correctly

Question

That is the point. How to write and read binary files with std::vector inside them?

I was thinking something like:

//============ WRITING A VECTOR INTO A FILE ================
const int DIM = 6;
int array[DIM] = {1,2,3,4,5,6};
std::vector<int> myVector(array, array + DIM);
ofstream FILE(Path, ios::out | ofstream::binary);
FILE.write(reinterpret_cast<const char *>(&myVector), sizeof(vector) * 6);
//===========================================================

But I don't know how to read this vector. Because I thought that the following was correctly but it isn't:

ifstream FILE(Path, ios::in | ifstream::binary);
FILE.read(reinterpret_cast<const char *>(&myVector), sizeof(vector) * 6);

So, how to perform the operation?

Solution 1

Try using an ostream_iterator/ostreambuf_iterator, istream_iterator/istreambuf_iterator, and the STL copy methods:

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>

#include <fstream> // looks like we need this too (edit by π)

std::string path("/some/path/here");

const int DIM = 6;
int array[DIM] = {1,2,3,4,5,6};
std::vector<int> myVector(array, array + DIM);
std::vector<int> newVector;

std::ofstream FILE(path, std::ios::out | std::ofstream::binary);
std::copy(myVector.begin(), myVector.end(), std::ostreambuf_iterator<char>(FILE));

std::ifstream INFILE(path, std::ios::in | std::ifstream::binary);
std::istreambuf_iterator iter(INFILE);
std::copy(iter.begin(), iter.end(), std::back_inserter(newVector));

Solution 2

Use boost::serialization.

If you don't want use boost - write size and vector.

size_t sz = myVector.size();
FILE.write(reinterpret_cast<const char*>(&sz), sizeof(sz));
FILE.write(reinterpret_cast<const char*>(&myVector[0]), sz * sizeof(myVector[0]));

Solution 3

You can use

#include <boost/serialization/vector.hpp>

to serialize your vector. Read a tutorial here: http://www.boost.org/libs/serialization/doc/tutorial.html#stl `

Solution 4

Before reading vector, you should resize it: yourVector.size(numberOfElementsYouRead).

Besides, sizeof(vector<your_type>) is just the size of the vector object internal implementation; vector element size is sizeof(std::vector<your_type>::value_type).

Then read it like this:

file.read(reinterpret_cast<char *>(&myVector[0]), sizeof(vector<int>::element_type) * element_count); 

Solution 5

I used the fact that the data() method returns an address you can use for reading AND for writing.

// Assume outf is a writable filepointer (binary). // write myVector.size() to file, then

fwrite(myVector.data(), sizeof(decltype(myVector)::value_type), myVector.size(), outf);

to read:

// read MyVector.size() from file as nv, inpf is read filepointer

MyVector.resize(nv);
fread(MyVector.data(), sizeof(decltype(MyVector)::value_type), nv, inpf);

clinging to old ways in file io, but please ignore that (even if it might irritate you :)).

A weakness is that endianness is unsupported in this way.

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