Why can't I static_cast to a pointer type?

[Jeff Schwab]

int main()
{
char const* const p =
static_cast< char const* const >( 0xffffffff );
}


Fails to compile with g++; message is:

g++ -c -o main.o main.cc
main.cc: In function `int main()':
main.cc:3: error: invalid static_cast from type `unsigned int' to type
`const
char* const'
make: *** [main.o] Error 1


Why doesn't this work? How can one write to specific addresses? For
example, Lupher Cypher posted a C-style cast to "char* far", in an
attempt to write to a text display. I'm willing to accept that "far"
might be a necessary extension for some architectures, e.g. 16-bit chips
in PDA's with 32 MB of RAM. On such an architecture, he may well need
to write data directly to particular addresses in order to communicate
with a memory-mapped device; how is he supposed to do it? Are the old
C-style casts the only way?

Special thanks to anyone with experience on this issue.

-Jeff

[Rob Williscroft]

Re: Why can't I static_cast to a pointer type?

Jeff Schwab wrote in news:o5edncanwd dazXeiRVn-gw@comcast.com:
[color=blue]
> int main()
> {
> char const* const p =
> static_cast< char const* const >( 0xffffffff );
> }
>[/color]

try:

reinterpret_cas t< char const* const >( 0xFFFFFFFF );


Check the spelling though, its not very often I get to write this cast
(making the cast ugly(tm) and thus difficult to spell (for the likes of
me anyway :) was as I understand it intentional.

How (and in what way its meanfull/usefull) is implemenation defined.

But if you know the bit layout of your platform's pointers, it should
do what you want.
[color=blue]
>
> Fails to compile with g++; message is:
>[/color]
[snip][color=blue]
>
> Why doesn't this work? How can one write to specific addresses? For
> example, Lupher Cypher posted a C-style cast to "char* far", in an
> attempt to write to a text display. I'm willing to accept that "far"
> might be a necessary extension for some architectures, e.g. 16-bit
> chips in PDA's with 32 MB of RAM. On such an architecture, he may
> well need to write data directly to particular addresses in order to
> communicate with a memory-mapped device; how is he supposed to do it?
> Are the old C-style casts the only way?
>[/color]

I would hope that the compilers/libraries that come with such systems
would provide a mechanism to do this (worst case a macro maybe).


Rob.
--

http://www.victim-prime.dsl.pipex.com/

[Jeff Schwab]

Re: Why can't I static_cast to a pointer type?

Rob Williscroft wrote:[color=blue]
> Jeff Schwab wrote in news:o5edncanwd dazXeiRVn-gw@comcast.com:
>
>[color=green]
>>int main()
>>{
>> char const* const p =
>> static_cast< char const* const >( 0xffffffff );
>>}
>>[/color]
>
>
> try:
>
> reinterpret_cas t< char const* const >( 0xFFFFFFFF );[/color]


Bingo! Thanks, Rob.
[color=blue]
> Check the spelling though, its not very often I get to write this cast
> (making the cast ugly(tm) and thus difficult to spell (for the likes of
> me anyway :) was as I understand it intentional.
>
> How (and in what way its meanfull/usefull) is implemenation defined.
>
> But if you know the bit layout of your platform's pointers, it should
> do what you want.
>
>[color=green]
>>Fails to compile with g++; message is:
>>[/color]
>
> [snip]
>[color=green]
>>Why doesn't this work? How can one write to specific addresses? For
>>example, Lupher Cypher posted a C-style cast to "char* far", in an
>>attempt to write to a text display. I'm willing to accept that "far"
>>might be a necessary extension for some architectures, e.g. 16-bit
>>chips in PDA's with 32 MB of RAM. On such an architecture, he may
>>well need to write data directly to particular addresses in order to
>>communicate with a memory-mapped device; how is he supposed to do it?
>>Are the old C-style casts the only way?
>>[/color]
>
>
> I would hope that the compilers/libraries that come with such systems
> would provide a mechanism to do this (worst case a macro maybe).
>
>
> Rob.[/color]