如何复制ifstream的状态？

Question

I have a function that takes an ifstream, ifs, and uses it to fill this: std::vector< std::vector > matrix; (this is an assignment and I do not have any say in how to better implement this). I would like to know length of the file I am reading before I begin filling "matrix" This way, I will avoid constantly resizing. However, I only have one ifstream object, ifs. If I use a loop to find the length of the file, I will be at the end of the file and I am unsure how to 'refresh' an ifstream so that it is at the first line again. Something else I was considering was creating a copy of ifs. This way, I can loop through one ifstream, close it, and use the other one to fetch the data. I feel like there would be an issue having two ifstreams open and pointed at the same file.

I do not know of a way to implement either idea. Please assume that I can no longer access the original file name/path.

Any ideas?

Answer 1

Don't try to copy streams.
To jump to the beginning, just use seekg :

ifs.seekg(0);

If you reached the end of the stream before seeking, another problem is that the stream remembers the file end internally and seekg doesn't reset this. For this to work, use

ifs.clear();

before seeking.