public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
* [Bug c++/114220] New: False positive warning: possibly dangling reference to a temporary [-Wdangling-reference]
@ 2024-03-03 8:26 dilyan.palauzov at aegee dot org
2024-03-03 8:33 ` [Bug c++/114220] " pinskia at gcc dot gnu.org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: dilyan.palauzov at aegee dot org @ 2024-03-03 8:26 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114220
Bug ID: 114220
Summary: False positive warning: possibly dangling reference to
a temporary [-Wdangling-reference]
Product: gcc
Version: 13.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: dilyan.palauzov at aegee dot org
Target Milestone: ---
Created attachment 57598
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57598&action=edit
The inlined programme
This code
#include <string>
#include <unordered_map>
static const std::unordered_map<int, std::string> hashTable1 = {{2, "u"}};
static const std::unordered_map<std::string, std::string> hashTable2 = {{"u",
"u"}};
const std::string& z1(int m);
const std::string& z1(int m) { return hashTable1.at(m); }
const std::string& z2(const std::string& m);
const std::string& z2(const std::string& m) { return hashTable2.at(m); }
const std::string& z3();
const std::string& z3() { return hashTable2.at("u"); }
int main() {
const std::string& b1 { z1(2) };
const std::string& b2 { z2("u") };
const std::string& b3 { z3() };
const std::string& f1 = hashTable1.at(2);
const std::string& f2 = hashTable2.at("u");
printf("%s %s %s %s %s\n", b1.c_str(), b2.c_str(), f1.c_str(), f2.c_str(),
b3.c_str());
}
produces with g++ (GCC) 13.2.1 20240302:
$ g++ -Wall -Wextra -o a a.cpp
a.c: In function ‘int main()’:
a.c:16:22: warning: possibly dangling reference to a temporary
[-Wdangling-reference]
16 | const std::string& b2 { z2("u") };
| ^~
a.c:16:29: note: the temporary was destroyed at the end of the full expression
‘z2(std::__cxx11::basic_string<char>(((const char*)"u"),
std::allocator<char>()))’
16 | const std::string& b2 { z2("u") };
| ~~^~~~~
So std::string as first template parameter is problematic (b2), when only the
second template parameter is relevant (b3), but int as first template parameter
is fine (b1).
I think this is a bug.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/114220] False positive warning: possibly dangling reference to a temporary [-Wdangling-reference]
2024-03-03 8:26 [Bug c++/114220] New: False positive warning: possibly dangling reference to a temporary [-Wdangling-reference] dilyan.palauzov at aegee dot org
@ 2024-03-03 8:33 ` pinskia at gcc dot gnu.org
2024-03-03 10:23 ` dilyan.palauzov at aegee dot org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-03-03 8:33 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114220
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution|--- |INVALID
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
const std::string& z2(const std::string& m);
const std::string& z2(const std::string& m) { return hashTable2.at(m); }
const std::string& b2 { z2("u") };
The warning is designed this way explictly because you are returning a
reference and taking a reference as an argument and in the case of b2, the
tempory is `std::string("u")` .
In GCC 14+ (since r14-9263-gc7607c4cf18986), you can add [[gnu::no_dangling]]
to the z2 function definition to mark it as not returning a dangling reference
(from the arguments). and the warning goes away.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/114220] False positive warning: possibly dangling reference to a temporary [-Wdangling-reference]
2024-03-03 8:26 [Bug c++/114220] New: False positive warning: possibly dangling reference to a temporary [-Wdangling-reference] dilyan.palauzov at aegee dot org
2024-03-03 8:33 ` [Bug c++/114220] " pinskia at gcc dot gnu.org
@ 2024-03-03 10:23 ` dilyan.palauzov at aegee dot org
2024-03-03 10:30 ` dilyan.palauzov at aegee dot org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: dilyan.palauzov at aegee dot org @ 2024-03-03 10:23 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114220
--- Comment #2 from Дилян Палаузов <dilyan.palauzov at aegee dot org> ---
Is my reading correct, that the warning is triggered, when a function receives
as parameter a reference to a temporary and returns a reference? If this is
the only criterion, then it is a wrong assumption, that both references are
somehow related.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/114220] False positive warning: possibly dangling reference to a temporary [-Wdangling-reference]
2024-03-03 8:26 [Bug c++/114220] New: False positive warning: possibly dangling reference to a temporary [-Wdangling-reference] dilyan.palauzov at aegee dot org
2024-03-03 8:33 ` [Bug c++/114220] " pinskia at gcc dot gnu.org
2024-03-03 10:23 ` dilyan.palauzov at aegee dot org
@ 2024-03-03 10:30 ` dilyan.palauzov at aegee dot org
2024-03-03 18:37 ` pinskia at gcc dot gnu.org
2024-03-04 9:07 ` xry111 at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: dilyan.palauzov at aegee dot org @ 2024-03-03 10:30 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114220
--- Comment #3 from Дилян Палаузов <dilyan.palauzov at aegee dot org> ---
> The warning is designed this way explictly because you are returning a reference and taking a reference as an argument and in the case of b2, the tempory is `std::string("u")` .
> In GCC 14+ (since r14-9263-gc7607c4cf18986), you can add [[gnu::no_dangling]] to the z2 function definition to mark it as not returning a dangling reference (from the arguments). and the warning goes away.
If the declaration and definitions are in different files, do I have to add
[[gnu::no_dangling]] only to the function declaration ?
That said, is the warning triggered only based on the function declaration
(accepting as parameter a reference to temporary and returning a reference),
when on the function invocation the parameter is indeed a reference to a
temporary?
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/114220] False positive warning: possibly dangling reference to a temporary [-Wdangling-reference]
2024-03-03 8:26 [Bug c++/114220] New: False positive warning: possibly dangling reference to a temporary [-Wdangling-reference] dilyan.palauzov at aegee dot org
` (2 preceding siblings ...)
2024-03-03 10:30 ` dilyan.palauzov at aegee dot org
@ 2024-03-03 18:37 ` pinskia at gcc dot gnu.org
2024-03-04 9:07 ` xry111 at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-03-03 18:37 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114220
--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Дилян Палаузов from comment #3)
> > The warning is designed this way explictly because you are returning a reference and taking a reference as an argument and in the case of b2, the tempory is `std::string("u")` .
>
> > In GCC 14+ (since r14-9263-gc7607c4cf18986), you can add [[gnu::no_dangling]] to the z2 function definition to mark it as not returning a dangling reference (from the arguments). and the warning goes away.
>
> If the declaration and definitions are in different files, do I have to add
> [[gnu::no_dangling]] only to the function declaration ?
Yes only on the declaration is needed.
>
> That said, is the warning triggered only based on the function declaration
> (accepting as parameter a reference to temporary and returning a reference),
> when on the function invocation the parameter is indeed a reference to a
> temporary?
Yes that is correct, it is based on the function declaration only and the
warning does not look into the function to see it does not return the a
referenced based on the reference being passed.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/114220] False positive warning: possibly dangling reference to a temporary [-Wdangling-reference]
2024-03-03 8:26 [Bug c++/114220] New: False positive warning: possibly dangling reference to a temporary [-Wdangling-reference] dilyan.palauzov at aegee dot org
` (3 preceding siblings ...)
2024-03-03 18:37 ` pinskia at gcc dot gnu.org
@ 2024-03-04 9:07 ` xry111 at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: xry111 at gcc dot gnu.org @ 2024-03-04 9:07 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114220
Xi Ruoyao <xry111 at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |asharafutdinov at adalisk dot com
--- Comment #5 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
*** Bug 114225 has been marked as a duplicate of this bug. ***
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2024-03-04 9:07 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2024-03-03 8:26 [Bug c++/114220] New: False positive warning: possibly dangling reference to a temporary [-Wdangling-reference] dilyan.palauzov at aegee dot org
2024-03-03 8:33 ` [Bug c++/114220] " pinskia at gcc dot gnu.org
2024-03-03 10:23 ` dilyan.palauzov at aegee dot org
2024-03-03 10:30 ` dilyan.palauzov at aegee dot org
2024-03-03 18:37 ` pinskia at gcc dot gnu.org
2024-03-04 9:07 ` xry111 at gcc dot gnu.org
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).