Reading part of a binary file into an pre-existing basic_string object

Question

I have an large string buffer, and a input stream

basic_string<uint8_t> *buf = ......;
istream in = ......;

What is the most efficient way to read a part of the file into the string? Say, the 0xE3CC'th to 0x1A481'th bytes from the file.

Here istream::read seems not an answer since it reads to a raw char[]. Since the data is quite large, having a temporary variable is [in]efficient.

And sadly, I don't have C++0x, so copy_n can't be used. What would you suggest? Thanks.

*"stream::read seems not an answer since it reads to a raw char[]."* Make the string big enough then pass `&buf[0]` and you're good to go. — jrok, Oct 22 '13 at 10:34
But is a string in C++ guaranteed to be stored contiguously? — , Oct 22 '13 at 10:38
@user2139538: technically - yes, please read here: http://stackoverflow.com/questions/1986966/does-s0-point-to-contiguous-characters-in-a-stdstring — Andriy Tylychko, Oct 22 '13 at 10:41
Since C++11, it is. Before that, it wasnt' officialy but IIRC, all implementations did it anyway. — jrok, Oct 22 '13 at 10:41
`since it reads to a raw char[]. Since the data is quite large, having a temporary variable is [in]efficient.` You're supposed to read it in chunks, into a buffer that you don't create and destroy over and over again. This is the efficient approach. — Lightness Races in Orbit, Oct 22 '13 at 11:10
Actually, slightly more efficient way (depending on how you access the data in the chunk) would be to memory map the chunk of the file as required... — Nim, Oct 22 '13 at 12:59

score 1 · Accepted Answer · answered Oct 22 '13 at 10:39

1

buf->resize(size);
in.read(&((*buf)[0], size);

BTW, do you really need buf to be a pointer?

answered Oct 22 '13 at 10:39

Andriy Tylychko

1 Answers1