In the following C++ code GCC 14 with -Wall -Wextra flags produces a possibly dangling reference warning for function g(), but not f(). Why?
/* getval.h */
#include <string>
std::string const & getVal(std::string const & key);
/* main.cpp */
#include "getval.h"
#include <iostream>
void f()
{
std::string key{"x"};
std::string const &s = getVal(key);
std::cout << "x->" << s << 'n';
}
void g()
{
std::string const &s = getVal("x");
std::cout << "x->" << s << 'n';
}
int main()
{
f();
g();
}
/* getval.cpp */
#include "getval.h"
#include <map>
std::string const & getVal(std::string const & key)
{
static std::map<std::string, std::string> map = [](){
std::map<std::string, std::string> m;
m.emplace("x", "123");
m.emplace("y", "456");
return m;
}();
static std::string empty;
auto it = map.find(key);
return it != map.end() ? it->second : empty;
}
/*
Output:
/app/main.cpp: In function 'void g()':
/app/main.cpp:13:22: warning: possibly dangling reference to a temporary [-Wdangling-reference]
13 | std::string const &s = getVal("x");
| ^
/app/main.cpp:13:32: note: the temporary was destroyed at the end of the full expression 'getVal(std::__cxx11::basic_string<char>(((const char*)"x"), std::allocator<char>()))'
13 | std::string const &s = getVal("x");
| ~~~~~~^~~~~
Program returned: 0
x->123
x->123
I would expect it to produce the warning in both cases or none (earlier gcc versions or clang do not produce a warning).
3
This is known gcc issue.
And false positive.
You can
- Disable warning with
-Wno-dangling-referenceor drop-Wextra. - or modernize your code with use of
std::string_view(this is just like a fancy version of strings const reference):
[[nodiscard]] std::string_view getVal(std::string_view key)
{
using namespace std::literals;
static const std::map<std::string_view, std::string_view> map {
{ "x"sv, "123"sv },
{ "y"sv, "456"sv },
};
auto it = map.find(key);
return it != map.end() ? it->second : ""sv;
}
https://godbolt.org/z/P4nxh8nYa
Edit:
Since you prefer use #pragma GCC diagnostic and you did it incorrectly, here is better way to do it:
// check if problematic warning -Wdangling-reference is available
#if __GNUC__ >= 13
#define GCC_PUSH_DIAGNOSTIC_DISABLE_DANGLING_REF _Pragma("GCC diagnostic push")
_Pragma("GCC diagnostic ignored "-Wdangling-reference"")
#define GCC_DIAGNOSTIC_POP _Pragma("GCC diagnostic pop")
#else
#define GCC_PUSH_DIAGNOSTIC_DISABLE_DANGLING_REF
#define GCC_DIAGNOSTIC_POP
#endif
[[nodiscard]] std::string const& getVal(std::string const& key);
void f()
{
std::string key { "x" };
std::string const& s = getVal(key);
std::cout << "x->" << s << 'n';
}
void g()
{
GCC_PUSH_DIAGNOSTIC_DISABLE_DANGLING_REF
std::string const& s = getVal("x");
GCC_DIAGNOSTIC_POP
std::cout << "x->" << s << 'n';
}
Live demo
2
I’m posting this as a self-answer, thanks to Marek R’s comment and the references therein. GCC seems to try to guess what the implementation of the function might be, and it probably considers something like return key; as a likely one. As only the implementer of the function really knows what is happening, I think a reasonable way to deal with this is to use compiler #pragma around the function declaration:
/* getval.h */
#include <string>
#if defined(__GNUC__)
# pragma GCC diagnostic push
# pragma GCC diagnostic ignored "-Wdangling-reference"
#endif
std::string const & getVal(std::string const & key);
#if defined(__GNUC__)
# pragma GCC diagnostic pop
#endif
3