how does mnist file is read through following code. - opencv

Question

I have a question about reading MNIST dataset. I got the idea of how the MNIST dataset is constructed. However, I have no clue, how does it read through a following code. Some of you may think that the result of couts are obvious( I wrote values as a comment). But for me it doesn't make sense because it uses the same exact function four times with the same input, but it gets the different output every time.. How does it possible? Please let me know If there is any ambiguity in my question.
Thank you.
Code start:
typedef unsigned char BYTE;
int main()
{
...
FILE *fp = fopen("MNIST/train-images.idx3-ubyte", "rb");
// delcare function;
int magicNumber = readFlippedInteger(fp);
int numImages = readFlippedInteger(fp);
int numRows = readFlippedInteger(fp);
int numCols = readFlippedInteger(fp);
cout << magicNumber << endl; // 2051
cout << numImages << endl; // 60000
cout << numRows << endl; // 28
cout << numCols << endl; // 28
...
}
int readFlippedInteger(FILE *fp)
{
int ret = 0;
BYTE *temp;
temp = (BYTE*)(&ret);
fread(&temp[3], sizeof(BYTE), 1, fp);
fread(&temp[2], sizeof(BYTE), 1, fp);
fread(&temp[1], sizeof(BYTE), 1, fp);
fread(&temp[0], sizeof(BYTE), 1, fp);
return ret;
}

Answer 1

Please don't mix C and C++ unless it is absolutely necessary. The underlying confusion is that the call to fread "moves" the file pointer through the file for you. As #RetiredNinja noted, you are advancing the file pointer 4 bytes at a time. That's how it "knows" how to read the next value even though you didn't tell it to explicitly. You can read all about file pointers here.
An implementation using slightly more idiomatic C++ could be
#include <fstream>
#include <iostream>
#include <algorithm>
int readFlippedInteger(std::istream &in) {
char temp[sizeof(int)];
in.read(temp, sizeof(int));
std::reverse(temp, temp+sizeof(int));
return *reinterpret_cast<int*>(temp);
}
int main() {
std::ifstream fin("MNIST/train-images.idx3-ubyte", std::ios::binary);
if (!fin) {
std::cerr << "Could not open file\n";
return -1;
}
// delcare function;
int magicNumber = readFlippedInteger(fin);
int numImages = readFlippedInteger(fin);
int numRows = readFlippedInteger(fin);
int numCols = readFlippedInteger(fin);
std::cout << magicNumber << std::endl // 2051
<< numImages << std::endl // 60000
<< numRows << std::endl // 28
<< numCols << std::endl; // 28
}
An implementation that uses a user-defined stream manipulator is left as an exercise for the reader.