Question

This is the definition of the function basic_istream::tellg() in VS2010. Note that the function returns a variable of type pos_type. However when I replace the type streamoffused in the example, given below, by pos_type, the compiler complains (C2065: ‘pos_type’ : undeclared identifier).

pos_type is defined in <fstream> as typedef typename _Traits::pos_type pos_type;.

// basic_istream_tellg.cpp
// compile with: /EHsc
#include <iostream>
#include <fstream>

int main()
{
    using namespace std;
    ifstream file;
    char c;
    streamoff i; // compiler complains if I replace streamoff by pos_type

    file.open("basic_istream_tellg.txt");
    i = file.tellg();
    file >> c;
    cout << c << " " << i << endl;

    i = file.tellg();
    file >> c;
    cout << c << " " << i << endl;
}

Answer 1

You cannot just write pos_type without qualification. Note that it is a member of ifstream. So you’ve to write this:

ifstream::pos_type i; //ok

That should work now.

Also, since using namespace std; is considered bad, you should avoid it, and instead should prefer using full qualification as:

std::ifstream file;        //fully-qualified
std::ifstream::pos_type i; //fully-qualified

In C++11, you can use auto instead.

auto i = file.tellg();

and let the compiler deduce i to be std::ifstream::pos_type.

Hope that helps.