c++ function returns empty vector although it is pass-by-reference

Question

I'm trying to read binary of a file into a vector<char> in c++ but reading function seems to behave like pass-by-value even though it is not.

Here is my simplified code:

#define sz 4096
int main(){
     string filename="a/file/path";
     vector<char> block_hex;
     read_Xbytes(filename, 0, sz, block_hex);
     cout << "\nnum bytes: " << block_hex.size() << " size: " << sz << endl;
}

void read_Xbytes(string fpath, int pos, int Xbytes, vector<char>& bytes) {
    ifstream is;
    is.open(fpath, ios::binary);
    is.ignore(pos);
    if (Xbytes > 0) {
        bytes.reserve(Xbytes);
        is.read((char*)&bytes[0], Xbytes);
    }
    is.close();
    for (int i = 0; i < 16; i++)
        cout << hex(bytes[i]) << " ";
    cout << endl;
}

hex function is printing the hex values of each byte. Inside the function, it works perfectly fine; I checked with HxD, a hex editor, there is no problem. However, in main it prints out num bytes: 0 size: 4096.

I tried to change the function to vector<char> read_Xbytes(string fpath, int pos, int Xbytes), still same result.

I'm wondering why the function behaves like pass-by-value? I couldn't really find anything useful.

I'm working on Windows 7 and Visual Studio 2015.