[C++0x] Question on list initialization of references

[Jonathan Wakely]

2011/7/5 Magnus Müller:
> Hi,
>
>
> I recently stumbled across the list initialization feature of C++0x and
> tried to use it to initialize references. The
> following compiles nicely using gcc (gcc 4.6.1, see the end of the
> email for gcc -v):
>
> --------------------
> int main()
> {
>    int y{};
>    int &ref{y};
> }
> --------------------
>
> If I try to use the same style of initialization for a reference to a
> user-defined type, gcc reports an error:
>
> --------------------
> class Y
> { };
>
> int main()
> {
>    Y y{};
>    Y &ref{y};
> }
> --------------------
>
> % LANG=C make initialization CXXFLAGS="-std=c++0x -Wall -Wextra"
> g++ -std=c++0x -Wall -Wextra    initialization.cpp   -o initialization
> initialization.cpp: In function `int main()':
> initialization.cpp:9:13: error: invalid initialization of non-const
> reference of type `Y&' from an rvalue of type `<brace-enclosed
> initializer list>' initialization.cpp:9:8: warning: unused variable
> `ref' [-Wunused-variable]
>
>
> This error message makes sense with a certain interpretation of the
> quoted paragraph from §8.5.4 (draft N3126):

The most recent publicly available draft is N3242
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf
That has been superseded by the FDIS.

> [...] if T is a reference to class type or if T is any reference
> type and the initializer list has no elements, a prvalue temporary of
> the type referenced by T is list-initialized, and the reference is
> bound to that temporary. [ Note: As usual, the binding will fail and
> the program is ill-formed if the reference type is an lvalue reference
> to a non-const type. — end note ]
>
> I think this applies here for my user-defined type Y. But I am wondering
> why the first example compiles. Even if there exists a "constructor" for
> integers using references that could produce a temporary, the integer
> reference should still produce an error as it would bind to an rvalue.
>
>
> So my question is: Where am I wrong?

An int is not a class type and your initializer list does not have no
elements, so that paragraph doesn't apply.

However, that paragraph has changed in the FDIS:

Otherwise, if T is a reference type, a prvalue temporary of the type
referenced by T is list-initialized, and the reference is bound to
that temporary. [ Note: As usual, the binding will fail and the
program is ill-formed if the reference type is an lvalue reference to
a non-const type. —end note ]

That would mean both initializations should be rejected, but it seems
that GCC doesn't implement that yet.

I think that contradicts the resolution of DR 1095 which was voted
into the FDIS:
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1095

Hmm, I think there might be a defect in the FDIS ...