Re: UB while dealing with invalid raw pointers, the
std::uninitialized_fill case
On Sep 4, 6:05 pm, Andy Champ <no....@nospam.invalid> wrote:
On 04/09/2010 13:01, Bo Persson wrote:
[...]
C++ wants to be *natively* implementable on a wider range of
platforms. Therefore it cannot specify copying a pointer as an int,
because that might hurt badly on hardware with special address
registers, or CPUs without special integer registers.
http://en.wikipedia.org/wiki/CDC_6600#The_Central_Processor_.28CP.29
http://en.wikipedia.org/wiki/Motorola_68000#Internal_registers
It is true that most current hardware doesn't show these
peculiarities, but why limit a programming language to current
hardware?
I suspect the language is already limited.
Take this nice simple bit of C:
char* p = malloc(10);
How is the compiler supposed to know that the pointer returned from
malloc is supposed to be a pointer to chars?
At that level, it doesn't. All it can do is memorize the upper
and lower bounds.
Some architectures have different pointer types depending on
the target data. That'll give me 10 bytes back, and I suppose
the compiler could work out that the cast from void* to char*
should do a pointer conversion to a char type pointer with
a bound of 10.
Then I write
float* fp = (void*)p;
Floats are bigger. Let's say that they have a size of 8 bytes
on this architecture - so is it going to put a range check on
the memory pointer of 1 float?
No, but it can easily generate code which will detect that fp+1
will access bytes beyond the address p+10 (if it is
dereferenced).
And if it does, and I cast it back to char*, will it remember
the size of 10?
It doesn't have to. All it has to do is remember the upper and
lower bounds.
What do you do on those TI graphics processors, where the
native address is a bit address not a byte address? Is
sizeof(char) == 8?
Or bigger. That's a requirement of the standard.
(We just wrote _c_a_r_e_f_u_l_l_y_, especially when we were
dealing with the arrays of 3-bit items which were the graphics
memory, whose sizeof was not a good thing to ask!)
Most likely, if you were using C, the compiler had some
extensions to support the extra addressing possibilities.
--
James Kanze