https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117224
Bug ID: 117224
Summary: warning states "this" is explicit by-copy capture but
it's an explicit by-reference capture
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: vries at gcc dot gnu.org
Target Milestone: ---
Consider test-case:
...
$ cat test.cc
#include <stdio.h>
class X
{
public:
X (int v) {
value = v;
}
X (const X& a) {
value = a.value;
printf ("copy constructor\n");
}
int value;
int f (void) {
auto l = [=, THIS] {
return value + 2;
};
return l ();
}
};
int
main (void)
{
X a (3);
return a.f ();
}
...
As documented [1], a "this" capture means a by-reference capture (so, copy
constructor is not called), and a "*this" means a by-copy capture (so, copy
constructor is called):
...
$ g++ test.cc -DTHIS="this"
$ ./a.out
$ g++ test.cc -DTHIS="*this"
$ ./a.out
copy constructor
$
...
Sofar the sanity check.
With -pedantic, we run into this warning:
...
$ g++ test.cc -DTHIS="this" -pedantic
test.cc: In member function ‘int X::f()’:
<command-line>: warning: explicit by-copy capture of ‘this’ with by-copy
capture default only available with ‘-std=c++20’ or ‘-std=gnu++20’
[-Wc++20-extensions]
test.cc:18:18: note: in expansion of macro ‘THIS’
18 | auto l = [=, THIS] {
| ^~~~
...
The text of the warning is incorrect: the warning states that "this" is an
explicit by-copy capture, but it's an explicit by-reference capture.
Clang show this warning text instead:
...
$ clang++ test.cc -DTHIS="this" -pedantic
test.cc:18:18: warning: explicit capture of 'this' with a capture default of
'=' is a C++20 extension [-Wc++20-extensions]
18 | auto l = [=, THIS] {
| ^
<command line>:1:14: note: expanded from macro 'THIS'
1 | #define THIS this
| ^
1 warning generated.
...
[1] https://en.cppreference.com/w/cpp/language/lambda
