C++ :: Sequential Reading Blocks Of Lines From A File

I have a file that I need to read in blocks. I need to read in n lines at a time, do some processing, and then read in the next block of n lines until the file is done. I know the size of the block, but not the number of lines in the file unless I check that first. Normally I would read in like,

Code:
// declarations
string new_input_line, input_file;
// create an input stream and open the input_file
ifstream input_file_istream;
input_file_istream.open( input_file.c_str() );

[Code] .....

// process through data_block

With this approach, I'm not sure how I would keep looping to read the next block until I hit the end of the file without knowing how many lines are in the input file. I could process the file to find that out, or get that number from bash and pass it it as an argument, but it seems like that shouldn't be necessary.

I could also read in and store the entire file as a vector of string and then process through it afterwords, but that would not be a very efficient use of memory.