On 03/10/2017 22:39, Petr Ovtchenkov wrote:
On Thu, 28 Sep 2017 13:38:06 +0100
Jonathan Wakely<jwak...@redhat.com> wrote:
On 28/09/17 15:06 +0300, Petr Ovtchenkov wrote:
On Thu, 28 Sep 2017 11:34:25 +0100
Jonathan Wakely<jwak...@redhat.com> wrote:
+ VERIFY(i == std::istreambuf_iterator<char>());
+
+ VERIFY(memcmp(b, r, 36) == 0);
+
+ s << q;
+ VERIFY(!s.fail());
+
+ copy_n(i, 36, r);
This is undefined behaviour. The end-of-stream iterator value cannot
be dereferenced.
Within this test istreambuf_iterator in eof state never dereferenced.
That is quite implementation dependent.
The libc++ and VC++ implementations fail this test, because once an
istreambuf_iterator has been detected to reach end-of-stream it
doesn't get "reset" by changes to the streambuf.
If we will keep even "unspecified" behaviour same, then bug fix/drawback
removing become extremely hard: it should be identified as drawback
in all libs almost simultaneously.
The libc++ implementation crashes, because operator== on an
end-of-stream iterator sets its streambuf* to null, and any further
increment or dereference will segfault.
So this is testing something that other implementations don't support,
and isn't justifiable from the standard.
I will use N4687 as reference.
27.2.3 par.2 Table 95:
++r
Requires: r is dereferenceable. Postconditions: r is dereferenceable or r is
past-the-end; any copies of the previous value of r are no longer required
either to be dereferenceable or to be in the domain of ==.
(void)r++ equivalent to (void)++r
*r++
{ T tmp = *r;
++r;
return tmp; }
[BTW, you see that r++ without dereference has no sense, and even more,
copies of the previous
value of r are no longer
required either to be
dereferenceable or to be in
the domain of ==.
>From this follow, that postfix increment operator shouldn't return
istreambuf_iterator.
]
The test itself simulate "stop and go" istream usage.
stringstream is convenient for behaviuor illustration, but in "real life"
I can assume socket or tty on this place.
At the very minimum we should have a comment in the test explaining
how it relies on non-standard, non-portable behaviour.
But I'd prefer to avoid introducing more divergence from other
implementations.
Standard itself say nothting about "stop and go" scenario.
At least I don't see any words pro or contra.
But current implementation block usage of istreambuf_iterator
with underlying streams like socket or tty, so istreambuf_iterator become
almost useless structure for practice.
Why not creating a new istreambuf_iterator each time you need to check
that streambuf is not in eof anymore ?
We have three issues with istreambuf_iterator:
- debug-dependent behaviour
Fixed.
- EOL of istreambuf_iterator when it reach EOF (but this not mean
EOL of associated streambuf)
Controversial.
- postfix increment operator return istreambuf_iterator, but here
expected restricted type, that accept only dereference, if it possible.
I agree that we need to fix this last point too.
Consider this code:
std::istringstream inf("abc");
std::istreambuf_iterator<char> j(inf), eof;
std::istreambuf_iterator<char> i = j++;
assert( *i == 'a' );
At this point it looks like i is pointing to 'a' but then when you do:
std::string str(i, eof);
you have:
assert( str == "ac" );
We jump other the 'b'.
We could improve the situation by adding a debug assertion that _M_c is
eof when pre-increment is being used or by changing semantic of
pre-increment to only call sbumpc if _M_c is eof. But then we would need
to consider _M_c in find overload and in some other places in the lib I
think.
Rather than going through this complicated path I agree with Petr that
we need to simply implement the solution advised by the Standard with
the nested proxy type.
This is what I have done in the attached patch in a naive way. Do we
need to have abi compatibility here ? If so I'll rework it.
This patch will make libstdc++ pass the llvm test. I even duplicate it
on our side with a small refinement to check for the return value of the
proxy::operator*().
François
diff --git a/libstdc++-v3/include/bits/streambuf_iterator.h b/libstdc++-v3/include/bits/streambuf_iterator.h
index 64b8cfd..a556fce 100644
--- a/libstdc++-v3/include/bits/streambuf_iterator.h
+++ b/libstdc++-v3/include/bits/streambuf_iterator.h
@@ -95,12 +95,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
// NB: This implementation assumes the "end of stream" value
// is EOF, or -1.
mutable streambuf_type* _M_sbuf;
- int_type _M_c;
public:
/// Construct end of input stream iterator.
_GLIBCXX_CONSTEXPR istreambuf_iterator() _GLIBCXX_USE_NOEXCEPT
- : _M_sbuf(0), _M_c(traits_type::eof()) { }
+ : _M_sbuf() { }
#if __cplusplus >= 201103L
istreambuf_iterator(const istreambuf_iterator&) noexcept = default;
@@ -110,11 +109,33 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
/// Construct start of input stream iterator.
istreambuf_iterator(istream_type& __s) _GLIBCXX_USE_NOEXCEPT
- : _M_sbuf(__s.rdbuf()), _M_c(traits_type::eof()) { }
+ : _M_sbuf(__s.rdbuf()) { }
/// Construct start of streambuf iterator.
istreambuf_iterator(streambuf_type* __s) _GLIBCXX_USE_NOEXCEPT
- : _M_sbuf(__s), _M_c(traits_type::eof()) { }
+ : _M_sbuf(__s) { }
+
+ class proxy
+ {
+ friend class istreambuf_iterator;
+
+ proxy(streambuf_type* __buf, int_type __c)
+ : _M_sbuf(__buf), _M_c(__c)
+ { }
+
+ streambuf_type* _M_sbuf;
+ int_type _M_c;
+
+ public:
+ char_type
+ operator*() const
+ { return traits_type::to_char_type(_M_c); }
+ };
+
+ /// Construct start of streambuf iterator.
+ istreambuf_iterator(const proxy& __prx) _GLIBCXX_USE_NOEXCEPT
+ : _M_sbuf(__prx._M_sbuf)
+ { }
/// Return the current character pointed to by iterator. This returns
/// streambuf.sgetc(). It cannot be assigned. NB: The result of
@@ -138,29 +159,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
istreambuf_iterator&
operator++()
{
- __glibcxx_requires_cond(_M_sbuf &&
- (!_S_is_eof(_M_c) || !_S_is_eof(_M_sbuf->sgetc())),
+ __glibcxx_requires_cond(_M_sbuf && !_S_is_eof(_M_sbuf->sgetc()),
_M_message(__gnu_debug::__msg_inc_istreambuf)
._M_iterator(*this));
_M_sbuf->sbumpc();
- _M_c = traits_type::eof();
return *this;
}
/// Advance the iterator. Calls streambuf.sbumpc().
- istreambuf_iterator
+ proxy
operator++(int)
{
- __glibcxx_requires_cond(_M_sbuf &&
- (!_S_is_eof(_M_c) || !_S_is_eof(_M_sbuf->sgetc())),
+ __glibcxx_requires_cond(_M_sbuf && !_S_is_eof(_M_sbuf->sgetc()),
_M_message(__gnu_debug::__msg_inc_istreambuf)
._M_iterator(*this));
- istreambuf_iterator __old = *this;
- __old._M_c = _M_sbuf->sbumpc();
- _M_c = traits_type::eof();
- return __old;
+ return proxy(_M_sbuf, _M_sbuf->sbumpc());
}
// _GLIBCXX_RESOLVE_LIB_DEFECTS
@@ -175,8 +190,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
int_type
_M_get() const
{
- int_type __ret = _M_c;
- if (_M_sbuf && _S_is_eof(__ret) && _S_is_eof(__ret = _M_sbuf->sgetc()))
+ int_type __ret = traits_type::eof();
+ if (_M_sbuf && _S_is_eof(__ret = _M_sbuf->sgetc()))
_M_sbuf = 0;
return __ret;
}
@@ -390,8 +405,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
else
__c = __sb->snextc();
}
-
- __first._M_c = __eof;
}
return __first;
diff --git a/libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/2.cc b/libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/2.cc
index 572ea9f..de4b8e4 100644
--- a/libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/2.cc
+++ b/libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/2.cc
@@ -23,7 +23,18 @@
#include <iterator>
#include <testsuite_hooks.h>
-void test02(void)
+void test01()
+{
+ std::istringstream inf("abc");
+ std::istreambuf_iterator<char> j(inf);
+ std::istreambuf_iterator<char> i = j++;
+
+ VERIFY( i != std::istreambuf_iterator<char>() );
+ VERIFY( *i == 'b' );
+ VERIFY( *j++ == 'b' );
+}
+
+void test02()
{
typedef std::istreambuf_iterator<char> cistreambuf_iter;
const char slit01[] = "playa hermosa, liberia, guanacaste";
@@ -111,6 +122,7 @@ void test02(void)
int main()
{
+ test01();
test02();
return 0;
}
