内容简介:Take the following code:All three functions
Take the following code:
#include <string_view> int * f1() { int x = 5; return &x; } struct V { int * p; }; V f2() { int x = 5; return { &x }; } std::string_view f3() { char tmp[] = "tmp"; return tmp; }
All three functions obviously return dangling pointers to local stack variables. Let’s see what a few major compilers have to say on the matter.
g++ 10.1 -O2 -std=c++2a -fanalyzer -Wall -Wextra
( link
):
f1(): xor eax, eax ret f2(): lea rax, [rsp-4] ret f3(): mov eax, 3 lea rdx, [rsp-4] ret
<source>: In function 'int* f1()': <source>:6:12: warning: address of local variable 'x' returned [-Wreturn-local-addr] 6 | return &x; | ^~
In addition to the warning in f1
, it even zapped the pointer to nullptr
. An interesting
choice, with which not everyone agrees, but in my opinion returning a null pointer is
much better than returning a dangling pointer to just-deallocated stack memory… which is
exactly what happens in f2
and f3
.
Let’s try Microsoft cl.exe 19.24 /O2 /std:c++latest /W4 /analyze
( link
):
<source>(6) : warning C4172: returning address of local variable or temporary: x <source>(17) : warning C4172: returning address of local variable or temporary: x
That’s better, but not better enough. std::string_view
is a rather important type, and
a potential rich source of lifetime mistakes.
Maybe Intel icc 19.0.1 -O2 -std=c++17 -Wall -Wextra
( link
) will fare better?
<source>(6): warning #1251: returning pointer to local variable return &x; ^
Sadly, not really. clang++ 10.0.0 -O2 -std=c++2a -Wall -Wextra
( link
) is our last hope.
<source>:6:13: warning: address of stack memory associated with local variable 'x' returned [-Wreturn-stack-address] return &x; ^ <source>:17:15: warning: address of stack memory associated with local variable 'x' returned [-Wreturn-stack-address] return { &x }; ^
Good but not still good enough.
Everything is lost, then? We’ll never have compilers that catch obvious lifetime mistakes?
Maybe not. Let’s try our real last hope, the experimental -Wlifetime
build of clang
( link
):
<source>:6:13: warning: address of stack memory associated with local variable 'x' returned [-Wreturn-stack-address] return &x; ^ <source>:6:5: warning: returning a dangling pointer [-Wlifetime] return &x; ^~~~~~~~~ <source>:6:5: note: pointee 'x' left the scope here return &x; ^~~~~~~~~ <source>:17:15: warning: address of stack memory associated with local variable 'x' returned [-Wreturn-stack-address] return { &x }; ^ <source>:23:12: warning: address of stack memory associated with local variable 'tmp' returned [-Wreturn-stack-address] return tmp; ^~~ <source>:23:5: warning: returning a dangling pointer [-Wlifetime] return tmp; ^~~~~~~~~~ <source>:23:5: note: pointee 'tmp' left the scope here return tmp; ^~~~~~~~~~
Interesting. Not only did -Wlifetime
catch f1
and f3
(but not f2
for some reason!), the
normal -Wreturn-stack-address
warning caught f3
this time as well, in addition to f1
and f2
.
(Herb Sutter has
an interesting post about the experimental -Wlifetime
compiler
. It can’t arrive soon enough
if you ask me.)
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