is_pod failing

[msr...@gmail.com]

std::tr1::is_pod<>. is returning false for trivial user-defined PODs.

struct emptystruct
{
};

struct structWithInt
{
int a;
};

class classWithPrivateMember
{
int a;
};

class emptyClass
{
};

class classWithPtr
{
public:
void* ptr;
int a;
};

class classWithStaticPtr
{
public:
static void* ptr;
int a;
};

class classContainingClassWithPtr
{
public:
classWithPtr c;
};

int main()
{
std::cout << "std::tr1::is_pod<int>::value = " << std::boolalpha
<< std::tr1::is_pod<int>::value << std::endl;
std::cout << "std::tr1::is_pod<emptystruct>::value = " <<
std::boolalpha << std::tr1::is_pod<emptystruct>::value << std::endl;
std::cout << "std::tr1::is_pod<structWithInt>::value = " <<
std::boolalpha << std::tr1::is_pod<structWithInt>::value << std::endl;
std::cout << "std::tr1::is_pod<emptyClass>::value = " <<
std::boolalpha << std::tr1::is_pod<emptyClass>::value << std::endl;
std::cout << "std::tr1::is_pod<classWithPrivateMember>::value = " <<
std::boolalpha << std::tr1::is_pod<classWithPrivateMember>::value <<
std::endl;
std::cout << "std::tr1::is_pod<classWithPtr>::value = " <<
std::boolalpha << std::tr1::is_pod<classWithPtr>::value << std::endl;
std::cout << "std::tr1::is_pod<classWithStaticPtr>::value = " <<
std::boolalpha << std::tr1::is_pod<classWithStaticPtr>::value <<
std::endl;
std::cout << "std::tr1::is_pod<classContainingClassWithPtr>::value =
" << std::boolalpha <<
std::tr1::is_pod<classContainingClassWithPtr>::value << std::endl;
}

All the above except is_pod<int> return false? Is is_pod guarantee to
return true for PODs? I am using gcc 4.2.4

Thanks,
MSR

--
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[Mathias Gaunard]

On 15 mai, 14:45, msr9...@gmail.com wrote:

> All the above except is_pod<int> return false? Is is_pod guarantee to
> return true for PODs? I am using gcc 4.2.4

I think is_pod is allowed to return false negatives to accommodate for
the difficulties of implementing it.
GCC doesn't provide the necessary mechanisms for a library to check
whether a type is a POD, so the library does what it can.

[Pavel Minaev]

There's no such guarantee in TR1. It is required that is_pod to be
true when is_scalar or is_void are true, but otherwise it's a quality
of implementation issue.

[Jiang]

On May 15, 9:45 pm, msr9...@gmail.com wrote:

[...]

> All the above except is_pod<int> return false?

Which is maybe quite reasonable for your gcc 4.2.4.
Did you check gcc 4.2.4's tr1 status before writing
the test code?

Also please note for tr1's is_pod, implementation
can choose true or false freely, except that the
following should be true:

is_pod <Ty>::value >=
(is_scalar <Ty>:: value || is_void <Ty>::value)

is_pod <Ty>::value == is_pod <const Ty>::value

is_pod <Ty>::value == is_pod <volatile Ty>::value

is_pod <Ty>::value ==
is_pod <remove_extent <Ty>::type>::value

Check Pete's book for further details please.

> Is is_pod guarantee to return true for PODs?

No. As explained above, if even is_pod reports
false, the type maybe is a POD.

BTW, if is_pod returns true, then type should
be a POD.

> I am using gcc 4.2.4

Try a new one please.

My gcc 4.4.0 said:

$ ./pod
std::tr1::is_pod<int>::value = true
std::tr1::is_pod<emptystruct>::value = true
std::tr1::is_pod<structWithInt>::value = true
std::tr1::is_pod<emptyClass>::value = true
std::tr1::is_pod<classWithPrivateMember>::value = false
std::tr1::is_pod<classWithPtr>::value = true
std::tr1::is_pod<classWithStaticPtr>::value = true
std::tr1::is_pod<classContainingClassWithPtr>::value = true

Regards,

Jiang

[Edward Rosten]

On 15 May, 20:49, Mathias Gaunard <loufo...@gmail.com> wrote:
> On 15 mai, 14:45, msr9...@gmail.com wrote:
>
> > All the above except is_pod<int> return false? Is is_pod guarantee to
> > return true for PODs? I am using gcc 4.2.4
>
> I think is_pod is allowed to return false negatives to accommodate for
> the difficulties of implementing it.
> GCC doesn't provide the necessary mechanisms for a library to check
> whether a type is a POD, so the library does what it can.

GCC 4.3.1 provides the __is_pod extension:

#include <iostream>
using namespace std;

struct a{};
class b{};
struct c{ int i; };
struct d{ private: int j; };
struct e: public c{ int j;};
struct f { c i;};

int main()
{
cout << __is_pod(a) << endl;
cout << __is_pod(b) << endl;
cout << __is_pod(c) << endl;
cout << __is_pod(d) << endl;
cout << __is_pod(e) << endl;
cout << __is_pod(f) << endl;
}

prints:

1
1
1
0
0
1

tr1/type_traits is implemented using __is_pod. For 4.2.1, at any rate,
the implementation doesn't use __is_pod.

-Ed
--
(You can't go wrong with psycho-rats.)(http://mi.eng.cam.ac.uk/~er258)

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