How to read hex values from a file using fstream in c++?

You have to chain std::hex when reading, the same way you chain it for writing :

infile >> std::hex >> a;

You can use the hex modifier

int n;
cin >> hex >> n;

This works:

int main()
{
    const char *filename = "blah.txt";
    ifstream infile(filename, fstream::in);

    unsigned int a;
    infile >> hex >> a;
    cout << hex << a;
}