On 11/1/22 18:06, Marek Polacek wrote:
-Wdangling-reference complains here:
std::vector<int> v = ...;
std::vector<int>::const_iterator it = v.begin();
while (it != v.end()) {
const int &r = *it++; // warning
}
because it sees a call to
__gnu_cxx::__normal_iterator<const int*, std::vector<int> >::operator*
which returns a reference and its argument is a TARGET_EXPR representing
the result of
__gnu_cxx::__normal_iterator<const int*, std::vector<int> >::operator++
But 'r' above refers to one of the int elements of the vector 'v', not
to a temporary object. Therefore the warning is a false positive.
I suppose code like the above is relatively common (the warning broke
cppunit-1.15.1 and a few other projects), so presumably it makes sense
to suppress the warning when it comes to member operator*. In this case
it's defined as
reference
operator*() const _GLIBCXX_NOEXCEPT
{ return *_M_current; }
and I'm guessing a lot of member operator* are like that, at least when
it comes to iterators. I've looked at _Fwd_list_iterator,
_Fwd_list_const_iterator, __shared_ptr_access, _Deque_iterator,
istream_iterator, etc, and they're all like that, so adding #pragmas
would be quite tedious. :/
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
OK.
It also occurred to me that we should avoid warning if the reference
we're initializing is a non-const lvalue reference, which can't bind to
a temporary.
Maybe also if the function returns a non-const lvalue reference.
PR c++/107488
gcc/cp/ChangeLog:
* call.cc (do_warn_dangling_reference): Quash -Wdangling-reference
for member operator*.
gcc/testsuite/ChangeLog:
* g++.dg/warn/Wdangling-reference5.C: New test.
---
gcc/cp/call.cc | 12 +++++++++-
.../g++.dg/warn/Wdangling-reference5.C | 22 +++++++++++++++++++
2 files changed, 33 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference5.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index c7c7a122045..2c0fa37f53a 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13467,7 +13467,17 @@ do_warn_dangling_reference (tree expr)
can be e.g.
const int& z = std::min({1, 2, 3, 4, 5, 6, 7});
which doesn't dangle: std::min here returns an int. */
- || !TYPE_REF_OBJ_P (TREE_TYPE (TREE_TYPE (fndecl))))
+ || !TYPE_REF_OBJ_P (TREE_TYPE (TREE_TYPE (fndecl)))
+ /* Don't emit a false positive for:
+ std::vector<int> v = ...;
+ std::vector<int>::const_iterator it = v.begin();
+ const int &r = *it++;
+ because R refers to one of the int elements of V, not to
+ a temporary object. Member operator* may return a reference
+ but probably not to one of its arguments. */
+ || (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
+ && DECL_OVERLOADED_OPERATOR_P (fndecl)
+ && DECL_OVERLOADED_OPERATOR_IS (fndecl, INDIRECT_REF)))
return NULL_TREE;
/* Here we're looking to see if any of the arguments is a temporary
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference5.C
b/gcc/testsuite/g++.dg/warn/Wdangling-reference5.C
new file mode 100644
index 00000000000..59b5538aee5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference5.C
@@ -0,0 +1,22 @@
+// PR c++/107488
+// { dg-do compile }
+// { dg-options "-Wdangling-reference" }
+
+#include <vector>
+
+int
+do_sum (std::vector<int>& v)
+{
+ int sum = 0;
+
+ std::vector<int>::const_iterator it = v.begin();
+ while (it != v.end())
+ {
+ // R refers to one of the int elements of V, not to a temporary
+ // object, so no dangling reference here.
+ const int &r = *it++; // { dg-bogus "dangling reference" }
+ sum += r;
+ }
+
+ return sum;
+}
base-commit: 2b0e81d5cc2f7e1d773f6c502bd65b097f392675
