Casting struct to char array (was: Linked List problem)

[Alex Vinokur]

"Richard Bos" <rlb@hoekstra-uitgeverij.nl> wrote in message news:4180f756.1 97032434@news.i ndividual.net.. . to news:comp.lang. c[color=blue]
> ben19777@hotmai l.com (Ben) wrote:[/color]
[snip][color=blue][color=green]
> > 2) Structure casted into an array of char
> > typedef struct {
> > char name[20];
> > int age;
> > int id;
> > } person;
> >
> > person p = (person *) malloc(sizeof(p erson));
> > p.name="hello";
> > p.age=2;
> > p.id=2;
> >
> > char *a;
> > a=(char *)p;[/color]
>
> No. You cannot cast a struct into a pointer. And what would you do this
> for, anyway? Do you suppose that the bytes represented by "hello"
> somehow form a valid pointer?
> I suspect you meant to access the struct itself _through_, not _as_, a
> pointer to char. In that case, you need to do
>
> a = (char *)&p;
>
> which is perfectly legal, and can be useful - though rarely. Beware the
> padding bytes!
>[/color]
[snip]

Is it possible to write 'operator char*()' inside the C++-structure that enables to avoid the padding problem?


--
Alex Vinokur
email: alex DOT vinokur AT gmail DOT com

http://mathforum.org/library/view/10978.html

http://sourceforge.net/users/alexvn

[Victor Bazarov]

Re: Casting struct to char array

Alex Vinokur wrote:[color=blue]
> "Richard Bos" <rlb@hoekstra-uitgeverij.nl> wrote in message news:4180f756.1 97032434@news.i ndividual.net.. . to news:comp.lang. c
>[color=green]
>>ben19777@hotm ail.com (Ben) wrote:[/color]
>
> [snip]
>[color=green][color=darkred]
>>>2) Structure casted into an array of char
>>>typedef struct {
>>>char name[20];
>>>int age;
>>>int id;
>>>} person;
>>>
>>>person p = (person *) malloc(sizeof(p erson));
>>>p.name="hell o";
>>>p.age=2;
>>>p.id=2;
>>>
>>>char *a;
>>>a=(char *)p;[/color]
>>
>>No. You cannot cast a struct into a pointer. And what would you do this
>>for, anyway? Do you suppose that the bytes represented by "hello"
>>somehow form a valid pointer?
>>I suspect you meant to access the struct itself _through_, not _as_, a
>>pointer to char. In that case, you need to do
>>
>> a = (char *)&p;
>>
>>which is perfectly legal, and can be useful - though rarely. Beware the
>>padding bytes!
>>[/color]
>
> [snip]
>
> Is it possible to write 'operator char*()' inside the C++-structure that enables to avoid the padding problem?[/color]

Which "padding problem" is it?

V

[Alex Vinokur]

Re: Casting struct to char array


"Victor Bazarov" <v.Abazarov@com Acast.net> wrote in message news:Bqygd.7722 $Ae.5313@newsre ad1.dllstx09.us .to.verio.net.. .[color=blue]
> Alex Vinokur wrote:[color=green]
> > "Richard Bos" <rlb@hoekstra-uitgeverij.nl> wrote in message news:4180f756.1 97032434@news.i ndividual.net.. . to news:comp.lang. c
> >[color=darkred]
> >>ben19777@hotm ail.com (Ben) wrote:[/color]
> >
> > [snip]
> >[color=darkred]
> >>>2) Structure casted into an array of char
> >>>typedef struct {
> >>>char name[20];
> >>>int age;
> >>>int id;
> >>>} person;
> >>>
> >>>person p = (person *) malloc(sizeof(p erson));
> >>>p.name="hell o";
> >>>p.age=2;
> >>>p.id=2;
> >>>
> >>>char *a;
> >>>a=(char *)p;
> >>
> >>No. You cannot cast a struct into a pointer. And what would you do this
> >>for, anyway? Do you suppose that the bytes represented by "hello"
> >>somehow form a valid pointer?
> >>I suspect you meant to access the struct itself _through_, not _as_, a
> >>pointer to char. In that case, you need to do
> >>
> >> a = (char *)&p;
> >>
> >>which is perfectly legal, and can be useful - though rarely. Beware the
> >>padding bytes!
> >>[/color]
> >
> > [snip]
> >
> > Is it possible to write 'operator char*()' inside the C++-structure that enables to avoid the padding problem?[/color]
>
> Which "padding problem" is it?
>
> V[/color]


--------- C++ code : BEGIN ---------
// File foo.cpp
#include <iostream>
using namespace std;

struct Foo1
{
char ch1;
char ch2;
short sh1;
Foo1 () : ch1 ('a'), ch2('b'), sh1 (0x6364) {};

};

struct Foo2
{
char ch1;
short sh1;
char ch2;
Foo2 () : ch1 ('a'), sh1 (0x6364), ch2('b') {};

};


#define SHOW(x,i) cout << #x << "[" << i << "] = " << x[i] << endl

int main ()
{
Foo1 foo1;
Foo2 foo2;
char* charray1 = (char*)&foo1;
char* charray2 = (char*)&foo2;

cout << "Foo1" << endl;
SHOW (charray1, 0);
SHOW (charray1, 1);
SHOW (charray1, 2);
SHOW (charray1, 3);

cout << endl;
cout << "Foo2" << endl;
SHOW (charray2, 0);
SHOW (charray2, 1);
SHOW (charray2, 2);
SHOW (charray2, 3);
SHOW (charray2, 4);

return 0;
}
--------- C++ code : END -----------



--------- Compilation & Run : BEGIN ---------

$ g++ -v
[omitted]
gcc version 3.3.3 (cygwin special)


$ g++ -W -Wall foo.cpp
// No errors/warnings

$ a

Foo1
charray1[0] = a
charray1[1] = b
charray1[2] = d
charray1[3] = c

Foo2
charray2[0] = a
charray2[1] = ~
charray2[2] = d
charray2[3] = c
charray2[4] = b

--------- Compilation & Run : END -----------

We can see that ch2 from Foo1 and Foo2 is in the different places of charray1 and charray2
because charray2[1] is a hole.


--
Alex Vinokur
email: alex DOT vinokur AT gmail DOT com

http://mathforum.org/library/view/10978.html

http://sourceforge.net/users/alexvn

[Victor Bazarov]

Re: Casting struct to char array

Alex Vinokur wrote:[color=blue]
> "Victor Bazarov" <v.Abazarov@com Acast.net> wrote in message news:Bqygd.7722 $Ae.5313@newsre ad1.dllstx09.us .to.verio.net.. .
>[color=green]
>>Alex Vinokur wrote:
>>[color=darkred]
>>>"Richard Bos" <rlb@hoekstra-uitgeverij.nl> wrote in message news:4180f756.1 97032434@news.i ndividual.net.. . to news:comp.lang. c
>>>
>>>
>>>>ben19777@ho tmail.com (Ben) wrote:
>>>
>>>[snip]
>>>
>>>
>>>>>2) Structure casted into an array of char
>>>>>typedef struct {
>>>>>char name[20];
>>>>>int age;
>>>>>int id;
>>>>>} person;
>>>>>
>>>>>person p = (person *) malloc(sizeof(p erson));
>>>>>p.name="he llo";
>>>>>p.age=2;
>>>>>p.id=2;
>>>>>
>>>>>char *a;
>>>>>a=(char *)p;
>>>>
>>>>No. You cannot cast a struct into a pointer. And what would you do this
>>>>for, anyway? Do you suppose that the bytes represented by "hello"
>>>>somehow form a valid pointer?
>>>>I suspect you meant to access the struct itself _through_, not _as_, a
>>>>pointer to char. In that case, you need to do
>>>>
>>>> a = (char *)&p;
>>>>
>>>>which is perfectly legal, and can be useful - though rarely. Beware the
>>>>padding bytes!
>>>>
>>>
>>>[snip]
>>>
>>>Is it possible to write 'operator char*()' inside the C++-structure that enables to avoid the padding problem?[/color]
>>
>>Which "padding problem" is it?
>>
>>V[/color]
>
>
>
> --------- C++ code : BEGIN ---------
> // File foo.cpp
> #include <iostream>
> using namespace std;
>
> struct Foo1
> {
> char ch1;
> char ch2;
> short sh1;
> Foo1 () : ch1 ('a'), ch2('b'), sh1 (0x6364) {};
>
> };
>
> struct Foo2
> {
> char ch1;
> short sh1;
> char ch2;
> Foo2 () : ch1 ('a'), sh1 (0x6364), ch2('b') {};
>
> };
>
> [...]
>
> --------- Compilation & Run : END -----------
>
> We can see that ch2 from Foo1 and Foo2 is in the different places of charray1 and charray2
> because charray2[1] is a hole.[/color]

OK, I'll bite. How is that a problem? If your struct is a POD, you may
use 'offsetof' to figure out where exactly a member is, but if it's not
POD, what use do you have for the "position"?

V

[Alex Vinokur]

Re: Casting struct to char array


"Victor Bazarov" <v.Abazarov@com Acast.net> wrote in message news:t8zgd.7727 $Ae.1489@newsre ad1.dllstx09.us .to.verio.net.. .[color=blue]
> Alex Vinokur wrote:[color=green]
> > "Victor Bazarov" <v.Abazarov@com Acast.net> wrote in message news:Bqygd.7722 $Ae.5313@newsre ad1.dllstx09.us .to.verio.net.. .
> >[color=darkred]
> >>Alex Vinokur wrote:
> >>
> >>>"Richard Bos" <rlb@hoekstra-uitgeverij.nl> wrote in message news:4180f756.1 97032434@news.i ndividual.net.. . to news:comp.lang. c
> >>>
> >>>
> >>>>ben19777@ho tmail.com (Ben) wrote:
> >>>
> >>>[snip]
> >>>
> >>>
> >>>>>2) Structure casted into an array of char
> >>>>>typedef struct {
> >>>>>char name[20];
> >>>>>int age;
> >>>>>int id;
> >>>>>} person;
> >>>>>
> >>>>>person p = (person *) malloc(sizeof(p erson));
> >>>>>p.name="he llo";
> >>>>>p.age=2;
> >>>>>p.id=2;
> >>>>>
> >>>>>char *a;
> >>>>>a=(char *)p;
> >>>>
> >>>>No. You cannot cast a struct into a pointer. And what would you do this
> >>>>for, anyway? Do you suppose that the bytes represented by "hello"
> >>>>somehow form a valid pointer?
> >>>>I suspect you meant to access the struct itself _through_, not _as_, a
> >>>>pointer to char. In that case, you need to do
> >>>>
> >>>> a = (char *)&p;
> >>>>
> >>>>which is perfectly legal, and can be useful - though rarely. Beware the
> >>>>padding bytes!
> >>>>
> >>>
> >>>[snip]
> >>>
> >>>Is it possible to write 'operator char*()' inside the C++-structure that enables to avoid the padding problem?
> >>
> >>Which "padding problem" is it?
> >>
> >>V[/color]
> >
> >
> >
> > --------- C++ code : BEGIN ---------
> > // File foo.cpp
> > #include <iostream>
> > using namespace std;
> >
> > struct Foo1
> > {
> > char ch1;
> > char ch2;
> > short sh1;
> > Foo1 () : ch1 ('a'), ch2('b'), sh1 (0x6364) {};
> >
> > };
> >
> > struct Foo2
> > {
> > char ch1;
> > short sh1;
> > char ch2;
> > Foo2 () : ch1 ('a'), sh1 (0x6364), ch2('b') {};
> >
> > };
> >
> > [...]
> >
> > --------- Compilation & Run : END -----------
> >
> > We can see that ch2 from Foo1 and Foo2 is in the different places of charray1 and charray2
> > because charray2[1] is a hole.[/color]
>
> OK, I'll bite. How is that a problem? If your struct is a POD, you may
> use 'offsetof' to figure out where exactly a member is, but if it's not
> POD, what use do you have for the "position"?
>
> V[/color]

Something like ?


--------- C++ code : BEGIN ---------

#include <iostream>
#include <cassert>
using namespace std;

struct Foo
{
char ch1;
short sh;

char ch2;
char cstr[3];

char ch3;

Foo () : ch1 ('a'), sh (0x6667), ch2('b'), ch3('c') { strcpy (cstr, "de");};

operator char *()
{
const int pure_size (sizeof(ch1) + sizeof(sh) + sizeof(ch2) + strlen(cstr) + sizeof(ch3) + 1);
char* ret_ptr = new char [pure_size];

int i = 0;
ret_ptr[i++] = ch1;

char* tmp_ptr = (char*)&sh;
for (int j = 0; j < sizeof(sh); j++) ret_ptr[i++] = *tmp_ptr++;
ret_ptr[i++] = ch2;

for (int j = 0; j < strlen(cstr); j++) ret_ptr[i++] = cstr[j];

ret_ptr[i++] = ch3;
ret_ptr[i] = 0;
assert (i < pure_size);

return ret_ptr;

};

};


#define SHOW(x,i) cout << #x << "[" << i << "] : ch = " << x[i] << "; hex = " << hex << uint (x[i]) << endl

int main ()
{
Foo foo;
char* charray1 = (char*)&foo;
char* charray2 = (char*)foo;

cout << "Regular (char*)" << endl;
for (int i = 0; i < sizeof (foo); i++) SHOW (charray1, i);

cout << endl;
cout << "Overloaded (char*)" << endl;
for (int i = 0; i < sizeof (foo); i++) SHOW (charray2, i);

return 0;
}

--------- C++ code : END -----------



--------- Compilation & Run : BEGIN ---------

$ g++ -v
[omitted]
gcc version 3.3.3 (cygwin special)


$ g++ foo.cpp
// No errors/warnings

$ a

Regular (char*)
charray1[0] : ch = a; hex = 61
charray1[1] : ch = ; hex = 3
charray1[2] : ch = g; hex = 67
charray1[3] : ch = f; hex = 66
charray1[4] : ch = b; hex = 62
charray1[5] : ch = d; hex = 64
charray1[6] : ch = e; hex = 65
charray1[7] : ch = ; hex = 0
charray1[8] : ch = c; hex = 63
charray1[9] : ch = ð; hex = fffffff0

Overloaded (char*)
charray2[0] : ch = a; hex = 61
charray2[1] : ch = g; hex = 67
charray2[2] : ch = f; hex = 66
charray2[3] : ch = b; hex = 62
charray2[4] : ch = d; hex = 64
charray2[5] : ch = e; hex = 65
charray2[6] : ch = c; hex = 63
charray2[7] : ch = ; hex = 0
charray2[8] : ch = ; hex = 0
charray2[9] : ch = ; hex = 0

--------- Compilation & Run : END -----------


--
Alex Vinokur
email: alex DOT vinokur AT gmail DOT com

http://mathforum.org/library/view/10978.html

http://sourceforge.net/users/alexvn

[Alex Vinokur]

Re: Casting struct to char array


"Alex Vinokur" <alexvn@big-foot.com> wrote in message news:2uh2isF2ac qqdU1@uni-berlin.de...
[snip]
-----------------------
For not POD[color=blue]
> Something like ?[/color]
-----------------------
[snip]


For POD

--------- C++ code : BEGIN ---------
#include <iostream>
#include <cassert>
using namespace std;

struct Foo
{
char ch1;
short sh;

char ch2;
char cstr[3];

char ch3;

};

char* get_as_cstr(con st Foo& foo_i)
{
const int pure_size (sizeof(foo_i.c h1) + sizeof(foo_i.sh ) + sizeof(foo_i.ch 2) + strlen(foo_i.cs tr) + sizeof(foo_i.ch 3) + 1);
char* ret_ptr = new char [pure_size];

int i = 0;
ret_ptr[i++] = foo_i.ch1;

char* tmp_ptr = (char*)&foo_i.s h;
for (int j = 0; j < sizeof(foo_i.sh ); j++) ret_ptr[i++] = *tmp_ptr++;
ret_ptr[i++] = foo_i.ch2;

for (int j = 0; j < strlen(foo_i.cs tr); j++) ret_ptr[i++] = foo_i.cstr[j];

ret_ptr[i++] = foo_i.ch3;
ret_ptr[i] = 0;
assert (i < pure_size);

return ret_ptr;
}


#define SHOW(x,i) cout << #x << "[" << i << "] : ch = " << x[i] << "; hex = " << hex << uint (x[i]) << endl

int main ()
{
Foo foo;
foo.ch1 = 'a';
foo.sh = 0x6667;
foo.ch2 = 'b';
strcpy (foo.cstr, "de");
foo.ch3 = 'c';

char* charray1 = (char*)&foo;
char* charray2 = get_as_cstr (foo);

cout << "POD : Regular (char*)" << endl;
for (int i = 0; i < sizeof (foo); i++) SHOW (charray1, i);

cout << endl;
cout << "POD : Function get_as_cstr" << endl;
for (int i = 0; i < sizeof (foo); i++) SHOW (charray2, i);

return 0;
}


--------- C++ code : END -----------



--------- Compilation & Run : BEGIN ---------

$ g++ -v
[omitted]
gcc version 3.3.3 (cygwin special)


$ g++ foo.cpp
// No errors/warnings

$ a

POD : Regular (char*)
charray1[0] : ch = a; hex = 61
charray1[1] : ch = ; hex = 3
charray1[2] : ch = g; hex = 67
charray1[3] : ch = f; hex = 66
charray1[4] : ch = b; hex = 62
charray1[5] : ch = d; hex = 64
charray1[6] : ch = e; hex = 65
charray1[7] : ch = ; hex = 0
charray1[8] : ch = c; hex = 63
charray1[9] : ch = ð; hex = fffffff0

POD : Function get_as_cstr
charray2[0] : ch = a; hex = 61
charray2[1] : ch = g; hex = 67
charray2[2] : ch = f; hex = 66
charray2[3] : ch = b; hex = 62
charray2[4] : ch = d; hex = 64
charray2[5] : ch = e; hex = 65
charray2[6] : ch = c; hex = 63
charray2[7] : ch = ; hex = 0
charray2[8] : ch = ; hex = 0
charray2[9] : ch = ; hex = 0

--------- Compilation & Run : END -----------


--
Alex Vinokur
email: alex DOT vinokur AT gmail DOT com

http://mathforum.org/library/view/10978.html

http://sourceforge.net/users/alexvn