Template aliases and attributes

Template aliases and attributes

[Martin Reinecke]

Hi,

g++ is unhappy about the statement:

template<typename T, int len> using vtype = T __attribute__
((vector_size (len*sizeof(T))));

and reports:

vectest.cc:1:91: warning: ignoring attributes applied to dependent type
‘T’ without an associated declaration [-Wattributes]
    1 |  using vtype = T __attribute__ ((vector_size (len*sizeof(T))));
      |                                                              ^

clang++ on the other hand seems to be OK with this.

Is there some subtlety I'm missing?
If so, is there analternative way to express what I have in mind?

Thanks in advance,
  Martin

Re: Template aliases and attributes

[Alexander Monakov]

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On Fri, 29 Nov 2019, Martin Reinecke wrote:

> Hi,
> 
> g++ is unhappy about the statement:
> 
> template<typename T, int len> using vtype = T __attribute__
> ((vector_size (len*sizeof(T))));
> 
> and reports:
> 
> vectest.cc:1:91: warning: ignoring attributes applied to dependent type
> ‘T’ without an associated declaration [-Wattributes]
>     1 |  using vtype = T __attribute__ ((vector_size (len*sizeof(T))));
>       |                                                              ^
> 
> clang++ on the other hand seems to be OK with this.
> 
> Is there some subtlety I'm missing?
> If so, is there analternative way to express what I have in mind?

gcc expects the attribute on the type being declared, not the base type,
so you need to place the attribute on vtype:

template<typename T, int len>
using vtype __attribute__ ((vector_size (len*sizeof(T)))) = T;

This matches how [[attributes]] are meant to be placed in C++11 alias templates.

Alexander

Re: Template aliases and attributes

[Martin Reinecke]

On 11/29/19 4:44 PM, Alexander Monakov wrote:

> gcc expects the attribute on the type being declared, not the base type,
> so you need to place the attribute on vtype:
> 
> template<typename T, int len>
> using vtype __attribute__ ((vector_size (len*sizeof(T)))) = T;
> 
> This matches how [[attributes]] are meant to be placed in C++11 alias templates.

Oh, I see! Thank you very much for the explanation!

Cheers,
  Martin