Delete a line in an ofstream in C++

I want to erase lines within a file. I know you can store the content of the file (in a vector for example), erase the line and write again. However, it feels very cum开发者_如何学JAVAbersome, and not very efficient if the file gets bigger.

Anyone knows of a better, more efficient, more elegant way of doing it?

On most file-systems, this is the only option you have, short of switching to an actual database.

However, if you find yourself in this situation (i.e. very large files, with inserts/deletes in the middle), consider whether you can do something like maintaining a bitmap at the top of the file, where each bit represents one line of your file. To "delete" a line, simply flip the corresponding bit value.

There's nothing particularly magical about disk files. They still like to store their data in contiguous areas (typically called something like "blocks"). They don't have ways of leaving data-free holes in the middle of these areas. So if you want to "remove" three bytes from the middle of one of these areas, something somewhere is going to have to accomplish this by moving everything else in that area back by three bytes. No, it is not efficient.

This is why text editors (which have to do this kind of thing a lot) tend to load as much of the file as possible (if not all of it) into RAM, where moving data around is much faster. They typically only write changes back to disk when requested (or periodically). If you are going to have to make lots of changes like this, I'd suggest taking a page from their book and doing something similar.

The BerkeleyDB (dbopen(3)) has an access method called DB_RECNO. This allows one to manipulate files with arbitrary lengths using any sort of record delimiter. The default uses variable-length records with unix newlines as delimiters. You then access each "record" using an integer index. Using this, you can delete arbitrary lines from your text file. This isn't specific to C++, but if you are on most Unix/Linux systems, this API is already available to you.