[LWG #198] std::reverse_iterator::operator*() invalidates cached values

• Key: STDCXX-690
• Type: Bug
• Status: Open
• Priority: Minor
• Assignee: Unassigned
• Reporter: Martin Sebor
• Component/s: 24. Iterators
• Affects Version/s: 4.1.2, 4.1.3, 4.1.4, 4.2.0, 4.2.1
• Fix Version/s: 4.3.0

Description

According to LWG issue 198 (http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#198), the implementation of reverse_iterator::operator*() should cache the "base" iterator to prevent dangling references to values cached by it. The test case below demonstrates the problem caused by not doing so:

$ cat t.cpp && make t && ./t
#include <cassert>
#include <iterator>

struct Iterator: std::iterator<std::random_access_iterator_tag, int>
{
int *cur;
int cache;

Iterator (int *p = 0): cur (p) { }
~Iterator () { cache = ~cache; }

reference operator*() { return cache; }

Iterator& operator++() { cache = *++cur; return *this; }
Iterator& operator--() { cache = *--cur; return *this; }
};

int main ()
{
int a[] = { 1, 2, 3 };
Iterator it (a + sizeof a / sizeof *a);
std::reverse_iterator<Iterator> rit (it);

const int &ref = *rit;
const int val = ref;

++rit;
assert (val == ref);
}
gcc -c -I/home/sebor/stdcxx/include/ansi -D_RWSTDDEBUG -pthread -I/home/sebor/stdcxx/include -I/build/sebor/stdcxx-gcc-4.1.2-15D/include -I/home/sebor/stdcxx/examples/include -pedantic -nostdinc++ -g -W -Wall -Wcast-qual -Winline -Wshadow -Wwrite-strings -Wno-long-long -Wcast-align t.cpp
gcc t.o -o t -pthread -L/build/sebor/stdcxx-gcc-4.1.2-15D/lib -Wl,-R/build/sebor/stdcxx-gcc-4.1.2-15D/lib -lstd15D -lsupc++ -lm
t: t.cpp:29: int main(): Assertion `val == ref' failed.
Aborted