Re: is std::ifstream buffered or not?

From:

Carl Barron <cbarron413@adelphia.net>

Newsgroups:

comp.lang.c++.moderated

Date:

30 Jun 2006 07:53:18 -0400

Message-ID:

<290620060641566804%cbarron413@adelphia.net>

In article <1151479715.696126.38520@75g2000cwc.googlegroups.com>, Maxim
Yegorushkin <maxim.yegorushkin@gmail.com> wrote:

kanze wrote:

[]

I understand std::cout and std::cin are buffered, and it makes
sense that they are. However, I do not see why std::ifstream
and std::ofstream would be buffered because the filesystem and
even the harddrive does some buffering. Wouldn't that be
meaningless?

No. System requests are (or were) expensive. In fact, I would
generally say that the reverse is true: cout and cin are often
connected to interactive devices, where you don't want
buffering; ifstream and ofstream rarely are.

Then again, this could depend on the implementation. I'm not
familiar enough with the C++ STL specification.

There is a function in streambuf: setbuf, which can be used to
set a user defined buffer. But an implementation is only
required to respect it if it is used to request unbuffered IO
(which means in practice a buffer size of 1).

Is it true that there is no way to make std::streambuf unbuffered? The
standard requires underflow() to either fill the pending sequence or
return EOF, which means that the pending sequence (buffer) has to be
present. IOW, std::streambuf input buffer has to be at least one
character long.

  see also streambuf::uflow() if the derived streambuf is unbuffered
it must provide a uflow() and a underflow() function, Example:

class Example:public std::streambuf
{
    std::streambuf *sb;
    int convert(int c) {return c=='\r'?'\n':c;}
    int underflow() {return convert(sb->sgetc());}
    int uflow() {return convert(sb->sbumpc());}
public:
    Example(std::streambuf *p):sb(p){}
};

is a simple example that merely converts '\r' to '\n'. it is not
buffered,but *sp may be buffered or not buffered.

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