How to read a binary file into a vector of unsigned chars

Question

Lately I\'ve been asked to write a function that reads the binary file into the std::vector where BYTE is an unsigned char

Answer 1

When testing for performance, I would include a test case for:

std::vector readFile(const char* filename)
{
    // open the file:
    std::ifstream file(filename, std::ios::binary);

    // Stop eating new lines in binary mode!!!
    file.unsetf(std::ios::skipws);

    // get its size:
    std::streampos fileSize;

    file.seekg(0, std::ios::end);
    fileSize = file.tellg();
    file.seekg(0, std::ios::beg);

    // reserve capacity
    std::vector vec;
    vec.reserve(fileSize);

    // read the data:
    vec.insert(vec.begin(),
               std::istream_iterator(file),
               std::istream_iterator());

    return vec;
}

My thinking is that the constructor of Method 1 touches the elements in the vector, and then the read touches each element again.

Method 2 and Method 3 look most promising, but could suffer one or more resize's. Hence the reason to reserve before reading or inserting.

I would also test with std::copy:

...
std::vector vec;
vec.reserve(fileSize);

std::copy(std::istream_iterator(file),
          std::istream_iterator(),
          std::back_inserter(vec));

In the end, I think the best solution will avoid operator >> from istream_iterator (and all the overhead and goodness from operator >> trying to interpret binary data). But I don't know what to use that allows you to directly copy the data into the vector.

Finally, my testing with binary data is showing ios::binary is not being honored. Hence the reason for noskipws from .