Re: [Boost-users] [serialize] struct save_array_type in oserializer.hpp

From: Oleg V. Zhylin (ovz_at_[hidden])
Date: 2007-04-12 10:26:55

 Thanks for the detailed response, Ovanes.

 As C4267 MSVC warning tells us, problem is that size_t could be larger
than int, particularly for 64 bit projects, and thus the definition of
ptrdiff_t in MSVC header files.

> typedef _W64 int    ptrdiff_t;

 I agree that ptrdiff_t might be a better option as a type of count
variable in the expression discussed.

--- Ovanes Markarian <om_boost_at_[hidden]> wrote:

> Well,
> 
> the standart states in [expr.mul] §5.6 section 4:
> The binary / operator yields the quotient, and the binary % operator
> yields the remainder from the
> division of the first expression by the second. If the second operand
> of / or % is zero the
> behavior is undefined; otherwise (a/b)*b + a%b is equal to a. If both
> operands are nonnegative
> then the remainder is nonnegative; if not, the sign of the remainder
> is implementation-defined...
> 
> 
> and later in [expr.add] §5.7 secion 6:
> When two pointers to elements of the same array object are
> subtracted, the result is the
> difference of the subscripts of the two array elements. The type of
> the result is an
> implementation-defined signed integral type; this type shall be the
> same type that is defined as
> ptrdiff_t in the <cstddef> header (18.1)...
> 
> 
> In your second example you have no pointer arithmetics (subtraction,
> which you had in the first
> example). You have only cast to the pointer type, which is in fact
> unsigned and the produced
> warning is correct.
> 
> Using ptrdiff_t instead of int results in no warning at MSVC 8.0,
> where ptrdiff_t is defined as:
> 
> typedef _W64 int    ptrdiff_t;
> 
> Which is signed ;)
> 
> The following test code in MSVC:
> 
> ptrdiff_t result =
>
sizeof(int)/(reinterpret_cast<int*>(&arr[0])-reinterpret_cast<int*>(&arr[1]));
> 
> std::cout << result << '\n';
> std::cout <<
>
typeid(reinterpret_cast<int*>(&arr[0])-reinterpret_cast<int*>(&arr[1])).name()
> <<'\n';
> std::cout <<
>
typeid(sizeof(int)/(reinterpret_cast<int*>(&arr[0])-reinterpret_cast<int*>(&arr[1]))).name()
> <<'\n';
> 
> Produces the output:
> 0
> int
> unsigned int
> 
> The first output is implementation defined (size_t/negative number)
> but you get no warning.
> 
> 
> With Kind Regards,
> 
> Ovanes Markarian
> 
> 
> According to the standart IMO this line is correct.
> 
> On Thu, April 12, 2007 15:08, Oleg V. Zhylin wrote:
> > At least for VC7.1 compiler
> >
> >   int a, b;
> >
> >   cout<<typeid(sizeof(a)/(a - b)).name()<<endl;
> >
> >  prints "unsingned int" and issues no warnings about int -> size_t
> > conversion. Thus I still wonder if usage of int type in
> >
> >> >         int count = sizeof(t) / (
> >> >             static_cast<const char *>(static_cast<const void
> >> *>(&t[1]))
> >
> > dictated by consideration for some other C++ compiler. VC7.1 does
> issue
> > a warning
> >
> > warning C4267: 'initializing' : conversion from 'size_t' to 'int',
> > possible loss of data
> >
> > for that code. I would suggest to consider either change type of
> > variable count from int to size_t or disable C4267 for this piece
> of
> > code for MS compilers.
> >
> > --- Ovanes Markarian <om_boost_at_[hidden]> wrote:
> >
> >>
> >> On Wed, April 11, 2007 17:51, Oleg V. Zhylin wrote:
> >> > Hi,
> >> >
> >> >   Is there a reason to use type int in the code at line 487 in
> >> > oserializer.hpp?
> >> >
> >> >         int count = sizeof(t) / (
> >> >             static_cast<const char *>(static_cast<const void
> >> *>(&t[1]))
> >> >
> >> >             - static_cast<const char *>(static_cast<const void
> >> > *>(&t[0]))
> >> >
> >> >   C++ standard draft states in section 5.3.3 [expr.sizeof]
> >> >
> >> > The result is a constant of an implementation-defined
> >> > type which is the same type as that which is named
> >> > size_t in the standard header <cstddef>(18.1).
> >>
> >>
> >> I assume otherwise you will get a warning, that a sign can be lost
> >> through the assignment. The
> >> problem here is that expression (type* - type*) results not in an
> >> unsigned type (size_t) but a
> >> signed difference type. If you divide unsigned value through a
> signed
> >> value you get a signed
> >> value. And then you would (according to your suggestion) assign it
> to
> >> an unsigned value =>
> >> warning.
> >>
> >>
> >>
> >> With Kind Regards,
> >>
> >> Ovanes Markarian
> >>
> >> _______________________________________________
> >> Boost-users mailing list
> >> Boost-users_at_[hidden]
> >> http://lists.boost.org/mailman/listinfo.cgi/boost-users
> >>
> >
> >
> > WBR Oleg V. Zhylin   ovz_at_[hidden]
> >
> >
> >
> >
>
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WBR Oleg V. Zhylin   ovz_at_[hidden]

       
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