Reading Binary Files into an array of ints c++

Question

I have a method which writes a binary file from an int array. (it could be wrong too)

void bcdEncoder::writeBinaryFile(unsigned int packedBcdArray[], int size)
{
    fstream binaryIo;
    binaryIo.open("PridePrejudice.bin", ios::out| ios::binary | ios::trunc);
    binaryIo.seekp(0);
    binaryIo.write((char*)packedBcdArray, size * sizeof(packedBcdArray[0]));
    binaryIo.seekp(0);

    binaryIo.close();
}

I need to now read that binary file back. And preferably have it read it back into another array of unsigned ints without any information loss.

I have something like the following code, but I have no idea on how reading binary files really works, and no idea how to read it into an array of ints.

void bcdEncoder::readBinaryFile(string fileName)
{
    // myArray = my dnynamic int array

    fstream binaryIo;
    binaryIo.open(fileName, ios::in | ios::binary | ios::trunc);

    binaryIo.seekp(0);

    binaryIo.seekg(0);
    binaryIo.read((int*)myArray, size * sizeof(myFile));

    binaryIo.close();
}

Question:

How to complete the implementation of the function that reads binary files?

Solution 1

To simplify read operation consider storing size (i.e the number of elements in the array) before the data:

void bcdEncoder::writeBinaryFile(unsigned int packedBcdArray[], int size)
{
   fstream binaryIo;
   binaryIo.open("PridePrejudice.bin", ios::out| ios::binary | ios::trunc);
   binaryIo.seekp(0);
   binaryIo.write(&size, sizeof(size));
   binaryIo.write((char*)packedBcdArray, size * sizeof(packedBcdArray[0]));
   binaryIo.close();
}

The read would look something like:

void bcdEncoder::readBinaryFile(string fileName)
{
    std::vector<unsigned int> myData;
    int size;

    fstream binaryIo;
    binaryIo.open(fileName, ios::in | ios::binary | ios::trunc);

    binaryIo.read(&size, sizeof(size)); // read the number of elements 

    myData.resize(size); // allocate memory for an array 
    binaryIo.read(myData.data(), size * sizeof(myData.value_type));
    binaryIo.close();

   // todo: do something with myData
}

Solution 2

If you're using C++, use the nice std library.

vector<unsigned int> bcdEncoder::readBinaryFile(string fileName)
{
    vector<unsigned int> ret; //std::list may be preferable for large files
    ifstream in{ fileName };
    unsigned int current;
    while (in.good()) {
        in >> current;
        ret.emplace_back(current);
    }
    return ret;
 }

Writing is just as simple (for this we'll accept an int[] but an std library would be preferable):

void bcdEncoder::writeBinaryFile(string fileName, unsigned int arr[], size_t len)
{
    ofstream f { fileName };
    for (size_t i = 0; i < len; i++)
        f << arr[i];
}

Here's the same thing but with an std::vector

void bcdEncoder::writeBinaryFile(string fileName, vector<unsigned int> arr)
{
    ofstream f { fileName };
    for (auto&& i : arr)
        f << i;
}

Comments

[Justin Tennant]

I'm guessing now that my write is just completely wrong. First off, the &size in the write fails because it says its expecting a char, but overall, when i take in my packedBcd Array (which I know has the correct values in it) the values are not being written into the .bin file in any way that I understand, if at all.

[Justin Tennant]

I can't figure out a way to implement a similar read method without vectors, which I am not able to use in this project.

[Olipro]

Are you completely and utterly banned from using a vector, or you're just required to return an int[]?

[Justin Tennant]

part of the project goal was to apply previously made custom dynamic array class file to a new project.