[Mesa-dev] [PATCH] nir: Make C++ more happy with NIR_SRC_INIT and NIR_DEST_INIT

[Jason Ekstrand]

On Fri, Jun 26, 2015 at 3:03 PM, Francisco Jerez <currojerez at riseup.net> wrote:
> Jason Ekstrand <jason at jlekstrand.net> writes:
>
>> On Fri, Jun 26, 2015 at 12:08 PM, Francisco Jerez <currojerez at riseup.net> wrote:
>>> Jason Ekstrand <jason at jlekstrand.net> writes:
>>>
>>>> In C, if you partially initialize a structure, the rest of the struct gets
>>>> set to 0.  C++, however, does not have this rule so GCC throws warnings
>>>> whenver NIR_SRC_INIT or NIR_DEST_INIT is used in C++.
>>>
>>> I don't think that's right, in C++ initializers missing from an
>>> aggregate initializer list are also defined to be initialized
>>> (value-initialized to be more precise, what would set them to zero in
>>> this case just like in C).
>>
>> Yes, that is correct.  I just did a second attempt that, instead,
>> defines a static const variable named NIR_SRC_INIT with a partial
>> initializer.  C++ still gets grumpy and gives me a pile of "missing
>> initializer" warnings.
>>
> That's likely related to the warning flags you have enabled in CXXFLAGS,
> not to C++ itself.  Maybe you have -Wmissing-field-initializers enabled
> for C++ only?
>
>>>> Since nir.h contains a static inline that uses NIR_SRC_INIT, every C++
>>>> file that includes nir.h complains about this.
>>>>
>>> I suspect the reason why this causes a warning may be that you're using
>>> compound literals? (which are a C99-specific feature and not part of C++)
>>>
>>>> This patch adds a small static inline function that makes a struct,
>>>> memsets it to 0, and returns it.  NIR_SRC_INIT and NIR_DEST_INIT are then
>>>> wrappers around this function.
>>>
>>> In C++ you could just call the implicitly defined default constructor
>>> for nir_src or nir_dest, like 'nir_src()'.
>>
>> The implicitly defined default constructor does nothing to POD types,
>> so doing so would explicitly *not* perform the desired action of
>> zeroing out the data.
>>
>
> Indeed, but 'nir_src()' doesn't only call the implicitly-defined trivial
> default constructor, it value-initializes the object (See section 8.5/8
> of the C++14 spec) what for POD types causes all members to be
> zero-initialized.

It looks like this greatly depends on your C++ version.  If it's C++11
or above, I believe it does get zero-initialized.  If it's earlier
than C++11, it doesn't.  At least that's the way I read this:

http://en.cppreference.com/w/cpp/language/value_initialization