CODEWITHSUNDEEP
c++
Mohd Sarim
Mohd Sarim

Reputation: 37

Why tellg() returning -1 at th end of code?

#include<iostream>
#include<fstream>
using namespace std;
int main()
{
    system("cls");
    fstream obj;
    obj.open("today.txt",ios::in);
    char ch; 
    while (obj)
    {
        cout<<endl<<"file pointer at :"<<obj.tellg();
        obj>>ch;
    }
    cout<<endl<<obj.tellg();
    return 0;
}

output :

file pointer at :0
file pointer at :1
file pointer at :2
file pointer at :3
file pointer at :4
file pointer at :5
file pointer at :7
file pointer at :8
file pointer at :9
file pointer at :10
file pointer at :11
-1

Upvotes: 1

Views: 625

Answers (2)

Dhruva-404
Dhruva-404

Reputation: 188

tellg is returning -1 because it has encountered an error while reading the obj!! As while loop exits once the file has reached its end (no more data to read) and so obj is null and hence tellg return -1.

Refer to this site for more information- http://www.cplusplus.com/reference/istream/istream/tellg/

Upvotes: 0

kabanus
kabanus

Reputation: 25980

From the docs:

Return value:

The current position in the stream. If either the stream buffer associated to the stream does not support the operation, or if it fails, the function returns -1.

So tellg is "failing". A common reason is if you hit the end of the stream (a file here). This makes sense since you iterate until the end of the stream just before.

Upvotes: 2

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