[Bug c++/91742] User defined conversion references
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91742
--- Comment #9 from Diego Franco ---
Correction/editing of last section of former comment:
...
- init brace does not work for user defined conversion reference WITHOUT static
cast:
A a {};
const std::vector& b {a}; // changed from "auto" to "std::vector"
assert( == ); // does not work
...
...
[Bug c++/91742] User defined conversion references
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91742
--- Comment #8 from Diego Franco ---
So to summarize, these are the main reason why I believe this should be
addressed:
- init brace works for references of any type:
std::vector a {};
std::vector& b {a};
assert( == ); // works
int c {};
int& d {c};
assert( == ); // works
- init brace works for user defined conversion reference when using static
cast:
class A {
operator const std::vector&() const {return a_;}
std::vector a_;
};
A a {};
const auto& b {static_cast&>(a)};
assert( == ); // works
- init brace does not work for user defined conversion reference WITHOUT static
cast:
A a {};
const auto& b {a};
assert( == ); // does not work
I think the above behavior is quite unexpected, and does not follow any logic
of other behaviors in the language. I found this unexpected behavior by running
unit tests. Also, semantic wise is correct to use user-defined-conversion
without static cast (above example).
[Bug c++/91742] User defined conversion references
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91742
--- Comment #7 from Diego Franco ---
This works:
#include
#include
struct A
{
operator const int&() const
{
return a_;
}
int a_;
};
int main()
{
A a {};
const auto& b1 {static_cast(a)};
const int& b2 {a};
assert(_ == );
assert( == ); // works
[Bug c++/91742] User defined conversion references
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91742 --- Comment #6 from Diego Franco --- Also the brace initialization works with primitive types for the code I posted in the first place. That's definitely a code smell.
[Bug c++/91742] User defined conversion references
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91742
--- Comment #4 from Diego Franco ---
> It's not implementation defined, but I think all implementations are required
> to do what GCC 8 does (and other compilers agree).
It worked on linaro 7.4.1 gcc with c++17, gcc 7.1.0 with c++17 only.
> If "const X& b2 (a)" is used instead of list-init, the second assertion
> passes.
That seems like an issue no working with {}.
[Bug c++/91742] User defined conversion references
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91742 --- Comment #1 from Diego --- The code does work when changing std::vector for any primitive types, i.e. int.
[Bug c++/91742] New: User defined conversion references
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91742
Bug ID: 91742
Summary: User defined conversion references
Product: gcc
Version: 8.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: di...@franco-technologies.com
Target Milestone: ---
The following code does not work as intended:
//Example-
#include
#include
struct A
{
operator const std::vector&() const
{
return a_;
}
std::vector a_;
};
int main()
{
A a {};
const auto& b1 {static_cast&>(a)};
const std::vector& b2 {a};
assert(_ == );
assert( == ); // does not work with gcc 8.3.0 any standard
// works with gcc 7.1.0 with c++17 only
}
//End of example-
I read through the c++17 standard and did not find any information about this
behavior being implementation defined.
Diego
