Question

I am developing an MFC application using Visual C++ 2010

I am reading data for one file but It seems seekg is not working

Here is my code

//Transaction is a class i have defined before

void displayMessage(CString message)
{
    MessageBox(NULL,message,L"error",MB_OK | MB_ICONERROR);
}

///////////////////////////////
    ifstream input;

    input.open("test.dat" , ios::binary );
    if( input.fail() )
    {
        CString mess;
        mess = strerror( errno );
        mess.Insert(0,L"Error\n");
        displayMessage(mess);
    }

    Transaction myTr(0,myDate,L"",0,0,L""); // creating an object of transaction
    unsigned long position = 0;
    while(input.read( (char *) &myTr , sizeof(Transaction)))
    {
        if(myTr.getType() == 400 )
            position = (unsigned long)input.tellg() - sizeof(Transaction);
    }


    CString m;
    m.Format(L"Pos : %d",position);
    displayMessage(m);

    input.clear();//I also tried removing this line
    input.seekg(position,ios::beg );

    m.Format(L"get pos: %d",input.tellg());
    displayMessage(m);
    input.close();

The first displayMessage shows This : Pos : 6716 But second one showes : get pos: 0

Why seekg is not working ?

Thanks

Answer 1

The problem is that CString.Format() is a varargs function and basic_istream::tellg() returns a pos_type which isn’t a type that can be passed as a vararg agument so you get undefined behavior.

If you want to pass the position you get bace from tellg() to CString::Format() you’ll need to cast it or put it in a temporary, intermediate variable:

    unsigned long new_pos = input.tellg();
    m.Format(L"get pos: %d", new_pos);