从文件读取或将文件读入缓冲区然后使用缓冲区（在C ++中）？

Question

I am writing a parser wherein, I need to read characters from a file. But I will be reading the file character by character, and may even stop reading in the middle if come conditions do not satisy.

So is it advisable to create an ifstream of the file, and seek to the position everytime and start reading from there, Or should I read the entire file into a stream or buffer, and then use that instead??

Answer 1

If you can, use a memory-mapped file.

Boost offers a cross-platform one: http://www.boost.org/doc/libs/1_35_0/libs/iostreams/doc/classes/mapped_file.html

Answer 2

How big is the file? Do you make more than one pass? Whether you read it into an in-memory buffer or not, reading the file will consume (file size/ BUFSIZ ) reads to go through the whole thing. Reading character by character doesn't matter, because the underlying read still consumes BUFSIZ bytes at a time (unless you take steps to change that behavior) -- it just hands them out character-by-character.

If you're reading it and processing it in one pass anyway, then reading it into memory will mean you always need (file size/ BUFSIZ ) reads, where -- assuming the reason for stopping is distributed equiprobably -- reading it and processing in line will take on average (file size/ BUFSIZ ) * 0.5 reads, which on a big file could be a substantial gain.

An even more important question might be "what are you doing looking for this complicated a solution?" The amount of time it takes to figure out the cute solution probably dominates any gains you'll make from looking for something fancier than the standard "while not end of file, get character and process" solution.

Answer 3

Seeking the position every time and reading wouldn't be a better option for this as it degrades the performance, Try creating a Buffer and read from that that would be a better idea and more efficient

Try to read all the file contents at a stretch to the buffer and then process the subsequent input needs with the buffer and without reading from the file everytime,,

Answer 4

On a full service OS (ie Windows, Mac OS, Linux, BSD...) the operating system will have a caching mechanism that handles this for you to some extent (and assuming your usage patterns meet some definition of "usual").

Unless you are experiencing unacceptable performance you might want to merrily ignore the whole issue (ie just use the naive file access primitives).