Re: Behaviour of istream_iterator on closed stream
On Jul 29, 9:48 am, "Alf P. Steinbach" <al...@start.no> wrote:
* Ian Collins:
Old Wolf wrote:
Code snippet:
std::ifstream file( "does_not_exist" );
You should have tested the state of file before going any
further. It's a pity this std::ifstream constructor doesn't
throw an exception.
std::vector<char> file_vec;
std::copy( std::istream_iterator<char>(file),
std::istream_iterator<char>(), std::back_inserter(file_vec) );
if ( file_vec.empty() )
// file did not exist or had no contents..
I expected this to work, and have the istream_iterator for
closed file return a blank iterator, so the copy would not
copy anything. But it segfaults. Is the behaviour
undefined or is my compiler broken?
I think all bets are off, the std::ifstream object isn't
fully initialised.
Hm. It is perhaps well known to you that I'm not very
enthusiastic about iostreams, but I find that hard to believe.
Is it really true?
No. I don't know where Ian got that (he's usually pretty
reliable about such things). The object is, in fact, fully
constructed before it tries to open the file; if the open fails,
failbit is set. There's also a separate flag is_open, which
would allow testing after the copy.
Yeah, I remember something about internal two-phase
construction with an Init_ call or something, very bad.
There's no two-phase construction for the complete objects.
There is a slight issue concerning construction of the lowest
base class; because inheritance is virtual (since iostream must
derive from istream and ostream, which both share a common base
class), the lowest class must be initialized by the most derived
class. Which potentially doesn't have all of the necessary
information to do so. So the default constructor of the base
class doesn't actually initialize; one of the derived classes is
required to call an init() function later. (And I agree that
this is a bad solution---the default constructor could have
initialized to an error state, which could be overridden by the
init function.)
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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