with -std=c++0x typeof is gone

with -std=c++0x typeof is gone

[Patrick Horgan]

In a templated class we define our difference type like this:

        typedef typeof(T() - T()) differenceType;

How simple, how elegant, but typeof is not part of the C++ standard, and 
isn't available when we use -std=c++0x.  Other than using 
specializations for each type that we might use with this class, is 
there some other equivalently simple way to do this?

Patrick

Re: with -std=c++0x typeof is gone

[me22]

On 28 January 2010 01:31, Patrick Horgan <phorgan1@yahoo.com> wrote:
> In a templated class we define our difference type like this:
>
>       typedef typeof(T() - T()) differenceType;
>

That works?  Wouldn't it give unsigned when T is unsigned?

> How simple, how elegant, but typeof is not part of the C++ standard, and
> isn't available when we use -std=c++0x.  Other than using specializations
> for each type that we might use with this class, is there some other
> equivalently simple way to do this?
>

I assume that decltype can do it.

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2115.pdf

~ Scott

Re: with -std=c++0x typeof is gone

[Ian Lance Taylor]

Patrick Horgan <phorgan1@yahoo.com> writes:

> In a templated class we define our difference type like this:
>
>        typedef typeof(T() - T()) differenceType;
>
> How simple, how elegant, but typeof is not part of the C++ standard,
> and isn't available when we use -std=c++0x.  Other than using
> specializations for each type that we might use with this class, is
> there some other equivalently simple way to do this?

g++ should support __typeof__ even with -std=c++0x.

Ian

Re: with -std=c++0x typeof is gone

[Patrick Horgan]

me22 wrote:
> On 28 January 2010 01:31, Patrick Horgan <phorgan1@yahoo.com> wrote:
>   
>> In a templated class we define our difference type like this:
>>
>>       typedef typeof(T() - T()) differenceType;
>>
>>     
>
> That works?  Wouldn't it give unsigned when T is unsigned?
>   
Yeah, that's a problem!  Love the decltype solution, but it does the 
same.  So gcc chooses between typeof and decltype with #ifdef 
__GXX_EXPERIMENTAL_CXX0X__
Anyone know if that's always set for -std=c++0x?

Patrick

Re: with -std=c++0x typeof is gone

[me22]

On 28 January 2010 03:42, Patrick Horgan <phorgan1@yahoo.com> wrote:
> me22 wrote:
>>
>> That works?  Wouldn't it give unsigned when T is unsigned?
>>
>
> Yeah, that's a problem!
>

You probably want type traits instead.  In C++0x, make_signed (see
meta.trans.sign in n3000), or in boost
http://www.boost.org/libs/type_traits/doc/html/boost_typetraits/reference/make_signed.html

Re: with -std=c++0x typeof is gone

[me22]

On 28 January 2010 17:16, Patrick Horgan <phorgan1@yahoo.com> wrote:
>
> So are you saying something like:
>
>     typedef std::make_signed<declspec(T() - T())> difftype;
>

Just this should be sufficient:

     typedef std::make_signed<T> difftype;

No need for extensions or C++0x features.

Re: with -std=c++0x typeof is gone

[me22]

2010/1/28 Patrick Horgan <phorgan1@yahoo.com>:
>
> But that assumes that operator- returns the same type (possibly signed) as
> its arguments.  How could you know that?  The declspec(T()-T()) would take
> the return type of the (possible) operator-, no?
>

You're right; I am assuming that.

I don't know your situation.  I do wonder whether even the declspec
solution would be sufficient if T used expression templates or other
such techniques, though.

~ Scott