detect if a type has a size_t member

[hm567]

Hello

template <typename T>
struct S {
   // how to detect T has a member static const size_t value ?
   // if T has value, define member of S,  m1 of type T1, otherwise m2 of
type T2
};

Regards,

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[Joel Falcou-3]

Hicham Mouline wrote:

> Hello
>
> template <typename T>
> struct S {
>    // how to detect T has a member static const size_t value ?
>    // if T has value, define member of S,  m1 of type T1, otherwise m2 of
> type T2
> };
>
> Regards,
>
> _______________________________________________
> Boost-users mailing list
> Boost-users@...
> http://lists.boost.org/mailman/listinfo.cgi/boost-users
>  

Check the Boost.Vault for th eintrospection library. It has a
HAS_STATIC_MEMBER_NAMED meta-function builder

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[hm567]

> -----Original Message-----
> From: boost-users-bounces@... [mailto:boost-users-
> bounces@...] On Behalf Of joel
> Sent: 03 July 2009 17:03
> To: boost-users@...
> Subject: Re: [Boost-users] detect if a type has a size_t member
>
> Hicham Mouline wrote:
> > Hello
> >
> > template <typename T>
> > struct S {
> >    // how to detect T has a member static const size_t value ?
> >    // if T has value, define member of S,  m1 of type T1, otherwise
> m2 of
> > type T2
> > };
> >
> > Regards,
>
> Check the Boost.Vault for th eintrospection library. It has a
> HAS_STATIC_MEMBER_NAMED meta-function builder

Can I put the macro inside the struct S ?  I guess not. I'd write:

HAS_STATIC_MEMBER_NAMED(T, value)

which would return a integral bool or a bool type?

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[Joel Falcou-3]

Hicham Mouline wrote:
> Can I put the macro inside the struct S ? I guess not. I'd write:
> HAS_STATIC_MEMBER_NAMED(T, value)
>
> which would return a integral bool or a bool type
it works liek BOOST_MPL_HAS_XXXX so put it where you want to define
a boolean meta-function. It supports ::value and ::type return mpl boolean.

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[hm567]

> -----Original Message-----
> From: Hicham Mouline [mailto:hicham@...]
> Sent: 05 July 2009 15:36
> To: Hicham Mouline
> Subject: FW: [Boost-users] detect if a type has a size_t member
>
>
>
> > -----Original Message-----
> > From: boost-users-bounces@... [mailto:boost-users-
> > bounces@...] On Behalf Of joel
> > Sent: 03 July 2009 19:04
> > To: boost-users@...
> > Subject: Re: [Boost-users] detect if a type has a size_t member
> >
> > Hicham Mouline wrote:
> > > Can I put the macro inside the struct S ? I guess not. I'd write:
> > > HAS_STATIC_MEMBER_NAMED(T, value)
> > >
> > > which would return a integral bool or a bool type
> > it works liek BOOST_MPL_HAS_XXXX so put it where you want to define
> > a boolean meta-function. It supports ::value and ::type return mpl
> > boolean.

I'm sorry I can quite get how to use it. I have this:

#include <iostream>
#include <boost/introspection/has_member_data.hpp>

struct S {
  static const size_t maxsize =5;
};

struct T {
};

BOOST_HAS_MEMBER_DATA(S, maxsize)
BOOST_HAS_MEMBER_DATA(T, maxsize)

int main()
{
    if ( boost::introspection::has_member_data_maxsize<T>::value )
    {
        std::cout<<" T has max_size"<<std::endl;
    }
    return 0;
}

which doesn't build because

error C2953: 'boost::introspection::has_member_data_maxsize' : class
template has already been defined

Hicham

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[Joel Falcou-3]

Hicham Mouline wrote:
> which doesn't build because
>
> error C2953: 'boost::introspection::has_member_data_maxsize' : class
> template has already been defined
>  
The macro build a template meta-function that you use on your type :

BOOST_HAS_MEMBER_DATA(size_t, maxsize);


int main()
{
    if ( boost::introspection::has_member_data_maxsize<T>::value )
    {
        std::cout<<" T has max_size"<<std::endl;
    }
    return 0;
}


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[hm567]

> -----Original Message-----
> From: boost-users-bounces@... [mailto:boost-users-
> bounces@...] On Behalf Of joel
> Sent: 06 July 2009 14:26
> To: boost-users@...
> Subject: Re: [Boost-users] detect if a type has a size_t member
>
> Hicham Mouline wrote:
> > which doesn't build because
> >
> > error C2953: 'boost::introspection::has_member_data_maxsize' : class
> > template has already been defined
> >
> The macro build a template meta-function that you use on your type :
>
> BOOST_HAS_MEMBER_DATA(size_t, maxsize);
>
>
> int main()
> {
>     if ( boost::introspection::has_member_data_maxsize<T>::value )
>     {
>         std::cout<<" T has max_size"<<std::endl;
>     }
>     return 0;
> }
>

Thank you. May I suggest to replace "Type" in

#define BOOST_HAS_MEMBER_DATA(Type,Name)  

in the header file by MemberType for e.g.

A naive user (myself) thought "Type" was his type, not the member data type.

Regards,

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[Joel Falcou-3]

Hicham Mouline wrote:
> Thank you. May I suggest to replace "Type" in
> #define BOOST_HAS_MEMBER_DATA(Type,Name)  
>
> in the header file by MemberType for e.g.
>
> A naive user (myself) thought "Type" was his type, not the member data type
Will do :)

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[Mathias Gaunard-2]

Hicham Mouline wrote:

> #include <boost/introspection/has_member_data.hpp>
>
> struct S {
>   static const size_t maxsize =5;
> };
>
> struct T {
> };
>
> BOOST_HAS_MEMBER_DATA(S, maxsize)
> BOOST_HAS_MEMBER_DATA(T, maxsize)

It's not BOOST_HAS_MEMBER_DATA you need, but BOOST_HAS_STATIC_MEMBER_NAMED.

Indeed, BOOST_HAS_MEMBER_DATA is for non-static members.

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[hm567]

> -----Original Message-----
> From: boost-users-bounces@... [mailto:boost-users-
> bounces@...] On Behalf Of joel
> Sent: 06 July 2009 14:26
> To: boost-users@...
> Subject: Re: [Boost-users] detect if a type has a size_t member
>
> Hicham Mouline wrote:
> > which doesn't build because
> >
> > error C2953: 'boost::introspection::has_member_data_maxsize' : class
> > template has already been defined
> >
> The macro build a template meta-function that you use on your type :
>
> BOOST_HAS_MEMBER_DATA(size_t, maxsize);
>
>
> int main()
> {
>     if ( boost::introspection::has_member_data_maxsize<T>::value )
>     {
>         std::cout<<" T has max_size"<<std::endl;
>     }
>     return 0;
> }

Thank you,

1) struct S {
  static const size_t maxsize =5;
};

2) struct S {
  static size_t maxsize;
};

3) struct S {
  size_t maxsize;
};


Applied to S, the code doesn't compile in 1 and returns false in 2.

Is there a fix that can make it work for 1, 2 and 3 ?
Regards,

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[Mathias Gaunard-2]

Hicham Mouline wrote:

>> BOOST_HAS_MEMBER_DATA(size_t, maxsize);
>>
>>
>> int main()
>> {
>>     if ( boost::introspection::has_member_data_maxsize<T>::value )
>>     {
>>         std::cout<<" T has max_size"<<std::endl;
>>     }
>>     return 0;
>> }
> Thank you,
>
> 1) struct S {
>   static const size_t maxsize =5;
> };
>
> 2) struct S {
>   static size_t maxsize;
> };
>
> 3) struct S {
>   size_t maxsize;
> };
>
>
> Applied to S, the code doesn't compile in 1 and returns false in 2.
>
> Is there a fix that can make it work for 1, 2 and 3 ?

No.
Static members and non-static ones are very different things, and each
requires usage of a different macro.
BOOST_HAS_STATIC_MEMBER_DATA and BOOST_HAS_MEMBER_DATA.


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[Roman Perepelitsa-3]

2009/7/29 Mathias Gaunard <mathias.gaunard@...>

1) struct S {
 static const size_t maxsize =5;
};

2) struct S {
 static size_t maxsize;
};

3) struct S {
 size_t maxsize;
};


Applied to S, the code doesn't compile in 1 and returns false in 2.

Is there a fix that can make it work for 1, 2 and 3 ?

No.
Static members and non-static ones are very different things, and each requires usage of a different macro.
BOOST_HAS_STATIC_MEMBER_DATA and BOOST_HAS_MEMBER_DATA.

It explains why 2 returns false, but does not explain why 1 does not compile.

Roman Perepelitsa. 

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[Mathias Gaunard-2]

Roman Perepelitsa wrote:
> 2009/7/29 Mathias Gaunard <mathias.gaunard@...
> <mailto:mathias.gaunard@...>>
>
>         1) struct S {
>          static const size_t maxsize =5;
>         };

> It explains why 2 returns false, but does not explain why 1 does not
> compile.

Because static const <integral type> = <some value> members are quite
special beasts.
Indeed, unlike other static members, you can't take their address, since
they don't exist in memory but only as compile-time constants.

Testing whether the type is integral is not enough either, since if
there is no definition they behave like normal static members.

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[Joel Falcou-3]

Mathias Gaunard wrote:
> It explains why 2 returns false, but does not explain why 1 does not
> compile.
Introspection is also in the vault as a WIP. Straneg behavior under soem
compilers have to be expected.
I'm working on this quite extensively.


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[hm567]

> -----Original Message-----
> From: boost-users-bounces@... [mailto:boost-users-
> bounces@...] On Behalf Of Mathias Gaunard
> Sent: 29 July 2009 13:12
> To: boost-users@...
> Subject: Re: [Boost-users] detect if a type has a size_t member
>
> Hicham Mouline wrote:
>
> >> BOOST_HAS_MEMBER_DATA(size_t, maxsize);
> >>
> >>
> >> int main()
> >> {
> >>     if ( boost::introspection::has_member_data_maxsize<T>::value )
> >>     {
> >>         std::cout<<" T has max_size"<<std::endl;
> >>     }
> >>     return 0;
> >> }
> > Thank you,
> >
> > 1) struct S {
> >   static const size_t maxsize =5;
> > };
> >
> > 2) struct S {
> >   static size_t maxsize;
> > };
> >
> > 3) struct S {
> >   size_t maxsize;
> > };
> >
> >
> > Applied to S, the code doesn't compile in 1 and returns false in 2.
> >
> > Is there a fix that can make it work for 1, 2 and 3 ?
>
> No.
> Static members and non-static ones are very different things, and each
> requires usage of a different macro.
> BOOST_HAS_STATIC_MEMBER_DATA and BOOST_HAS_MEMBER_DATA.

I can't find BOOST_HAS_STATIC_MEMBER_DATA in introspection in the sandbox.
Were you referring to some header files elsewhere?

Joel, re compiler, I am using vs2005.

The case I really need is 1... so I need to detect the compile-time static
constant,
and not a variable member... so is there a way to do it?

Rds,

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[hm567]

> -----Original Message-----
> From: boost-users-bounces@... [mailto:boost-users-
> bounces@...] On Behalf Of Mathias Gaunard
> Sent: 29 July 2009 13:12
> To: boost-users@...
> Subject: Re: [Boost-users] detect if a type has a size_t member

>I can't find BOOST_HAS_STATIC_MEMBER_DATA in introspection in the sandbox.
>Were you referring to some header files elsewhere?

>Joel, re compiler, I am using vs2005.

The case I really need is 1... so I need to detect the compile-time static
constant,
and not a variable member... so is there a way to do it?

Rds,

----------------
I just realized the current Boost.Introspection in sandbox is different from
the version I downloaded weeks ago.
I had  (has_member_data.hpp  has_member_function.hpp  traits.hpp).

Now I see there are different files (introspection.hpp detail.hpp detail/*)
from
https://svn.boost.org/svn/boost/sandbox/introspection/boost/introspection/

Nevertheless, I still couldn't figure out how to detect the static const
size_t member.

Which version to use?

Rds,

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[Mathias Gaunard-2]

Hicham Mouline wrote:

> Nevertheless, I still couldn't figure out how to detect the static const
> size_t member.
>
> Which version to use?

No version allows this at the moment.
You may, however, try to simply duplicate HAS_STATIC_MEMBER (or whatever
it's called), change &X::Name to X::Name, and Type* to Type.

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[hm567]

> -----Original Message-----
> From: boost-users-bounces@... [mailto:boost-users-
> bounces@...] On Behalf Of Mathias Gaunard
> Sent: 29 July 2009 15:48
> To: boost-users@...
> Subject: Re: [Boost-users] detect if a type has a size_t member
>
> Hicham Mouline wrote:
>
> > Nevertheless, I still couldn't figure out how to detect the static const
> > size_t member.
> >
> > Which version to use?
>
> No version allows this at the moment.
> You may, however, try to simply duplicate HAS_STATIC_MEMBER (or whatever
> it's called), change &X::Name to X::Name, and Type* to Type.

Thanks very much,
It worked for me on vs2005 and g++4.3.3
http://codepad.org/wsNi21Pr

at least for the cases I wanted, that is

struct S1 {
   static const size_t maxsize= 5;
};

struct S5 {
};


Nevertheless, it failed to compile on g++4.3.3 for
struct S2 {
   static size_t maxsize;
};

because of this error:
error: 'S2::maxsize' is not a valid template argument for type 'long
unsigned int' because it is a non-constant expression

but compiled on vs2005 and failed ( which I don't care about )

thanks again,

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[Steven Watanabe-4]

AMDG

Mathias Gaunard wrote:
> Because static const <integral type> = <some value> members are quite
> special beasts.
> Indeed, unlike other static members, you can't take their address,
> since they don't exist in memory but only as compile-time constants.

Yes you can take the address of a static const integral type.
If you do, you are required to have a namespace scope
definition, just like an ordinary member, but failure to
do so, can only cause a linker error.  It won't affect the
compiler.

In Christ,
Steven Watanabe

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[Asif Lodhi]

On 7/29/09, Mathias Gaunard <mathias.gaunard@...> wrote:
> Because static const <integral type> = <some value> members are quite
> special beasts.

May be that's why in "The C++ Programming Language, 3ed", Bjarne says
about them, "I consider this a misfeature. When you need a symbolic
constant within a class declaration, use an enumerator."

Regards,
-Asif
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