[Bug c++/55805] New: Empty brace-init-list causes warning "missing initializer for member" in C++11

[Bug c++/55805] New: Empty brace-init-list causes warning "missing initializer for member" in C++11

[moritz at bunkus dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55805

             Bug #: 55805
           Summary: Empty brace-init-list causes warning "missing
                    initializer for member" in C++11
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: moritz@bunkus.org


The C++11 standard has this to say on empty brace-init-lists in 8.5.4.3:

  " If the initializer list has no elements and T is a class type with a
default constructor, the object is value-initialized."

However, g++ issues a warning with "-Wmissing-field-initializers" (which is
included in "-Wextra"). Example:

class A {
public:
  int f;
};

int
main() {
  A a3{};
  return 0;
}

Compile with "g++ -Wextra -std=c++11 -o test1 test1.cpp"

clang++ 3.1 does not emit that warning. g++ 4.6, 4.7 and 4.7.2 do (those are
the ones I have available here).

[Bug c++/55805] Empty brace-init-list causes warning "missing initializer for member" in C++11

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55805

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |jason at gcc dot gnu.org
         Resolution|                            |INVALID

--- Comment #1 from Jason Merrill <jason at gcc dot gnu.org> 2013-02-13 05:06:52 UTC ---
This is the purpose of the warning; to indicate when some fields do not have an
explicit initializer during aggregate initialization and are therefore
value-initialized.  If you don't want this warning, you can turn it off with
-Wno-missing-field-initializers.

[Bug c++/55805] Empty brace-init-list causes warning "missing initializer for member" in C++11

[dave.gittins at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55805

Gubbins <dave.gittins at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dave.gittins at gmail dot com

--- Comment #2 from Gubbins <dave.gittins at gmail dot com> ---
The original bug report points that in C++11 this is *not* aggregate
initialization, but is in fact value initialization (because this is a class
type with a default constructor).

Therefore no field initializers are involved. The warning in this situation is
surely incorrect? I think the original bug report was correct and the problem
should be fixed.

[Bug c++/55805] Empty brace-init-list causes warning "missing initializer for member" in C++11

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55805

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Gubbins from comment #2)
> The original bug report points that in C++11 this is *not* aggregate
> initialization, but is in fact value initialization (because this is a class
> type with a default constructor).

No it isn't, it's an aggregate.

[Bug c++/55805] Empty brace-init-list causes warning "missing initializer for member" in C++11

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55805

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Gubbins from comment #2)
> Therefore no field initializers are involved. The warning in this situation
> is surely incorrect? I think the original bug report was correct and the
> problem should be fixed.

In any case, there is no warning for any currently supported release, and we
are not making any more releases from the gcc 4.7 branch, so there's nothing to
fix.

[Bug c++/55805] Empty brace-init-list causes warning "missing initializer for member" in C++11

[dave.gittins at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55805

--- Comment #5 from Gubbins <dave.gittins at gmail dot com> ---
(In reply to Jonathan Wakely from comment #4)
> (In reply to Gubbins from comment #2)
> > Therefore no field initializers are involved. The warning in this situation
> > is surely incorrect? I think the original bug report was correct and the
> > problem should be fixed.
> 
> In any case, there is no warning for any currently supported release, and we
> are not making any more releases from the gcc 4.7 branch, so there's nothing
> to fix.

The warning is still produced with gcc 4.9.2 (surely that's supported?)

[Bug c++/55805] Empty brace-init-list causes warning "missing initializer for member" in C++11

[dave.gittins at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55805

--- Comment #6 from Gubbins <dave.gittins at gmail dot com> ---
(In reply to Jonathan Wakely from comment #3)
> (In reply to Gubbins from comment #2)
> > The original bug report points that in C++11 this is *not* aggregate
> > initialization, but is in fact value initialization (because this is a class
> > type with a default constructor).
> 
> No it isn't, it's an aggregate.

It's clear that A is an aggregate type, but it is also a class type with a
default constructor. Doesn't the quoted section of the C++11 specification
therefore state that the object should be value-initialized?

[Bug c++/55805] Empty brace-init-list causes warning "missing initializer for member" in C++11

[dave.gittins at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55805

--- Comment #7 from Gubbins <dave.gittins at gmail dot com> ---
(In reply to Gubbins from comment #6)
> (In reply to Jonathan Wakely from comment #3)
> > (In reply to Gubbins from comment #2)
> > > The original bug report points that in C++11 this is *not* aggregate
> > > initialization, but is in fact value initialization (because this is a class
> > > type with a default constructor).
> > 
> > No it isn't, it's an aggregate.
> 
> It's clear that A is an aggregate type, but it is also a class type with a
> default constructor. Doesn't the quoted section of the C++11 specification
> therefore state that the object should be value-initialized?

Anyway, even if this is classified as aggregate initialization (which I believe
it is in C++14?), I believe the behaviour should be changed. The distinction
between value-initializing a class and aggregate-initializing it with zero
field initializers is not a useful one here, IMHO - it's more of a
technicality. The result is the same either way (if we, say, add a base class
to A in the example then A is no longer aggregate and we get the same
initialization outcome with no warning). The current warning behaviour is an
annoyance that means I either need to disable -Wmissing-field-initializers (I
prefer to keep it enabled) or initialize my object in a more long-winded way.

Coming at it the other way, the point of the warning is presumably to catch
errors in which the user accidentally supplies only N initializers for M
fields, which is good. I'd suggest that when N=0, it's very unlikely to be an
accident!

I propose that the missing-field-initializers warning should not apply to empty
aggregate initialization lists, but a separate (new) warning
-Wempty-field-initializers could be added which warns for such empty lists if
your class happens to be aggregate.

[Bug c++/55805] Empty brace-init-list causes warning "missing initializer for member" in C++11

[redi at gcc dot gnu.org]

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55805

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Gubbins from comment #5)
> The warning is still produced with gcc 4.9.2 (surely that's supported?)

The warning isn't given for 5.1 and trunk (not sure what I tested before).

(In reply to Gubbins from comment #6)
> It's clear that A is an aggregate type, but it is also a class type with a
> default constructor. Doesn't the quoted section of the C++11 specification
> therefore state that the object should be value-initialized?

Nope.

8.5.4 says:

— Otherwise, if T is an aggregate, aggregate initialization is performed
(8.5.1).
— Otherwise, if the initializer list has no elements and T is a class type with
a default constructor, the object is value-initialized. The C++11 wording was
fixed by a defect report, see
http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1301


(In reply to Gubbins from comment #7)
> Anyway, even if this is classified as aggregate initialization (which I
> believe it is in C++14?), I believe the behaviour should be changed. 

It already has been for gcc-5.
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Subject: [Bug c++/55805] Empty brace-init-list causes warning "missing initializer for member" in C++11
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55805

--- Comment #9 from Gubbins <dave.gittins at gmail dot com> ---
(In reply to Jonathan Wakely from comment #8)
> (In reply to Gubbins from comment #5)
> > The warning is still produced with gcc 4.9.2 (surely that's supported?)
> 
> The warning isn't given for 5.1 and trunk (not sure what I tested before).
> 
> (In reply to Gubbins from comment #6)
> > It's clear that A is an aggregate type, but it is also a class type with a
> > default constructor. Doesn't the quoted section of the C++11 specification
> > therefore state that the object should be value-initialized?
> 
> Nope.
> 
> 8.5.4 says:
> 
> — Otherwise, if T is an aggregate, aggregate initialization is performed
> (8.5.1).
> — Otherwise, if the initializer list has no elements and T is a class type
> with a default constructor, the object is value-initialized. The C++11
> wording was fixed by a defect report, see
> http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1301

Oh I see... sorry, I was working off the text from before the defect report.

> (In reply to Gubbins from comment #7)
> > Anyway, even if this is classified as aggregate initialization (which I
> > believe it is in C++14?), I believe the behaviour should be changed. 
> 
> It already has been for gcc-5.

I see, thanks. Will there be no more releases on the 4.9 branch?
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idf551

--- Comment #2 from John Paul Adrian Glaubitz <glaubitz at physik dot fu-berlin.de> ---
(In reply to Eric Botcazou from comment #1)
> So this is a full 64-bit compiler and not the 32-bit sparc64 compiler
> usually built on Debian?  If so, what base compiler are you using (gcc -v)?

Full 64-bit compiler:

(unstable-sparc64-sbuild)root@raverin:/# gcc-4.9 -v
Using built-in specs.
COLLECT_GCC=gcc-4.9
COLLECT_LTO_WRAPPER=/usr/lib/gcc/sparc64-linux-gnu/4.9/lto-wrapper
Target: sparc64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.9.0-7'
--with-bugurl=file:///usr/share/doc/gcc-4.9/README.Bugs
--enable-languages=c,c++,java,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.9 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.9 --libdir=/usr/lib --enable-nls
--with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-libmudflap
--disable-libitm --disable-libquadmath --enable-plugin --with-system-zlib
--disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.9-sparc64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.9-sparc64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.9-sparc64
--with-arch-directory=sparc64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --enable-targets=all --with-long-double-128
--enable-multilib --enable-checking=release --build=sparc64-linux-gnu
--host=sparc64-linux-gnu --target=sparc64-linux-gnu
Thread model: posix
gcc version 4.9.0 (Debian 4.9.0-7)
(unstable-sparc64-sbuild)root@raverin:/# file `which gcc-4.9`
/usr/bin/gcc-4.9: ELF 64-bit MSB executable, SPARC V9, Sun UltraSPARC1
Extensions Required, relaxed memory ordering, version 1 (SYSV), dynamically
linked, interpreter /lib64/ld-linux.so.2, for GNU/Linux 2.6.32,
BuildID[sha1]½7aefb97bd2c824a8ef7277b3ab90495e5efaa1, stripped
(unstable-sparc64-sbuild)root@raverin:/# ls -l `which gcc`
lrwxrwxrwx 1 root root 7 Jun 28  2014 /usr/bin/gcc -> gcc-4.9
(unstable-sparc64-sbuild)root@raverin:/#

Adrian

[Bug c++/55805] Empty brace-init-list causes warning "missing initializer for member" in C++11

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55805

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Gubbins from comment #9)
> I see, thanks. Will there be no more releases on the 4.9 branch?

There will be more. https://gcc.gnu.org/ml/gcc/2015-06/msg00163.html