Question

After I read a line from a file using ifstream, is there a way to bring the stream back to the beginning of the line I just read, conditionally?

using namespace std;
//Some code here
ifstream ifs(filename);
string line;
while(ifs >> line)
{
   //Some code here related to the line I just read

   if(someCondition == true)
   {
    //Go back to the beginning of the line just read
   }
   //More code here
} 

So if someCondition is true, the next line read during the next while-loop iteration will be the same line I just read right now. Otherwise, the next while-loop iteration will use the following line in the file. If you need further clarification, please don’t hesitate to ask. Thanks in advance!

UPDATE #1

So I tried the following:

while(ifs >> line)
{
   //Some code here related to the line I just read
   int place = ifs.tellg();
   if(someCondition == true)
   {
    //Go back to the beginning of the line just read
    ifs.seekg(place);
   }
   //More code here
}

But it doesn’t read the same line again when the condition is true. Is an integer an acceptable type here?

UPDATE #2: The Solution

There was an error in my logic. Here is the corrected version that works as I want it to for any of those that are curious:

int place = 0;
while(ifs >> line)
{
   //Some code here related to the line I just read

   if(someCondition == true)
   {
    //Go back to the beginning of the line just read
    ifs.seekg(place);
   }
  place = ifs.tellg();
   //More code here
}

The call to tellg() was moved to the end because you need to seek to the beginning of the previously read line. The first time around I called tellg() and then called seekg() before the stream even changed, which is why it seemed like nothing changed (because it really hadn’t). Thank you all for your contributions.

Answer 1

There is no direct way to say “get back to the start of the last line”. However, you can get back to a position you kept by using std::istream::tellg(). That is, before reading a line you’d use tellg() and then seekg() to get back to the position.

However, calling the seek functions frequently is fairly expensive, i.e., I would look at removing the requirement to read lines again.