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Alf P. Steinbach
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 Posted: Tue Sep 13, 2005 8:23 pm Post subject: Why is tellg not const? |
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Why is tellg not const?
(This is the third attempted posting of this question: it can't be rejected
for ever, can it?)
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
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Howard Hinnant
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| Posted: Tue Sep 13, 2005 9:45 pm Post subject: Re: Why is tellg not const? |
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In article <432665de.186071859 (AT) news (DOT) individual.net>,
[email]alfps (AT) start (DOT) no[/email] (Alf P. Steinbach) wrote:
| Quote: | Why is tellg not const?
|
Consider:
#include <streambuf>
#include <istream>
#include <cassert>
struct my_stream_buf
: public std::streambuf
{
typedef std::streambuf base;
protected:
virtual base::pos_type seekoff(base::off_type off,
std::ios_base::seekdir way,
std::ios_base::openmode which =
std::ios_base::in |
std::ios_base::out)
{throw 0; return base::pos_type(-1);}
};
int main()
{
my_stream_buf sb;
std::istream in(&sb);
assert(in.good());
in.tellg();
assert(in.good());
}
The second assert fires. tellg() has changed the observable state of
the istream in this example.
That being said, I can sympathize with your intent. tellg does seem
like a non-modifying function and a seekoff that throws while seeking to
the current position is pathological. If we were redesigning the I/O
classes today I would look for a way to avoid this contradiction
(perhaps a const interface in the streambuf to report current position
which could not throw).
-Howard
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