Re: Designated Initializers in C++

[David Brown <david at westcontrol dot com>]

On 16/09/14 13:20, Jakub Jelinek wrote:
> On Tue, Sep 16, 2014 at 01:14:35PM +0200, David Brown wrote:
>> After a recent discussion about designated initializers in C++, I
>> noticed that they are accepted by modern gcc (when gcc extensions are
>> enabled).
>>
>> On <https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html>, the
>> documentation specifically says "This extension is not implemented in
>> GNU C++".  That certainly used to be the case - as far as I have tested,
>> it was implemented in gcc 4.6 or 4.7.
>>
>> Could someone with commit access to the documentation fix that sentence?
> 
> ???  Designated initializers definitely are not implemented in G++,
> what there is is just very limited parsing, but the C++ FE requires that
> the designators are just useless annotations, you can't initialize even
> a POD out of order with the designators, skip some field, initialize
> something twice etc.
> 
> 	Jakub
> 

The discussion I had been involved in was in the context of initialising
unions - I hadn't investigated further regarding designated initialisers
in structs.

My example code was this:

struct SE {
    int x;
    union {
       	int i;
       	float f;
       	double d;
    };
};

struct SE a = { 0, {.f = 1 }};
struct SE b = { 1, {.d = 1 }};
struct SE c = { 2, {.i = 1 }};


Compiling and examining the object code gave identical results with C++
(--std=g++11) and C (--std=gnu11) with a recently built gcc 4.9.1 on
64-bit Linux.  The code initialises the three structs/unions as desired.

With g++ and "-ansi -Wpedantic", I got a warning "ISO C++ does not allow
C99 designated initializers" on the three appropriate lines.


However, I have now tried a little with structs and designed
initialisers - and I see what you mean.  I am getting the errors and
warnings you mention.  So it works for unions - at least in the simple
case I tried - but not for structs.


I had thought I had found an undocumented feature - one that I know many
people would like to see in C++ (of preference, it should be in the
standards - but failing that as a gcc extension).

Thanks anyway,

David