structure and constant memebers

[ravinderthakur]

hi all experts,
i have a structure with the constant memebers such as one given below:

typedef struct {
const int cbcode;
int cberror;
} xtsetplatestate rec;

now in my function i want to create a stack based object for the same:

void CreateObjectAnd UseIt{
xtsetplatestate rec plate;
xtsetplatestate rec plate2 = {0 , 0};
}
now this function is giving the compilation errors while compiling on
vc++ 6.0 since the
cbcode memeber is a constant. the error is
error C2512: 'xtsetbleedvalu esrec' : no appropriate default constructor
available
error C2552: 'tmpsetplatesta terec' : non-aggregates cannot be
initialized with initializer list

can anybody plz explain me what could be done to create instances of
these objects on stack ???
thanks
rt

[usr.root]

ra************@ gmail.com 写�：

hi all experts,
i have a structure with the constant memebers such as one given below:

typedef struct {
const int cbcode;
int cberror;
} xtsetplatestate rec;

now in my function i want to create a stack based object for the same:

void CreateObjectAnd UseIt{
xtsetplatestate rec plate;
xtsetplatestate rec plate2 = {0 , 0};
}
now this function is giving the compilation errors while compiling on
vc++ 6.0 since the
cbcode memeber is a constant. the error is
error C2512: 'xtsetbleedvalu esrec' : no appropriate default constructor
available
error C2552: 'tmpsetplatesta terec' : non-aggregates cannot be
initialized with initializer list

can anybody plz explain me what could be done to create instances of
these objects on stack ???
thanks
rt

why you use const there,maybe you can write like this :

typedef struct {
const int cbcode=0;
int cberror;
} xtsetplatestate rec;

[Martin Ambuhl]

ra************@ gmail.com wrote:

hi all experts,
i have a structure with the constant memebers such as one given below:

typedef struct {
const int cbcode;
int cberror;
} xtsetplatestate rec;

now in my function i want to create a stack based object for the same:

void CreateObjectAnd UseIt{
xtsetplatestate rec plate;
xtsetplatestate rec plate2 = {0 , 0};
}
The above has nothing to do with "stack based object"s. You are (quite
legally) using local variables. There is not a sign of your use of a
stack in sight.

now this function is giving the compilation errors while compiling on
vc++ 6.0 since the
cbcode memeber is a constant. the error is
error C2512: 'xtsetbleedvalu esrec' : no appropriate default constructor
available
The above error message suggests you are using a C++ compiler.
error C2552: 'tmpsetplatesta terec' : non-aggregates cannot be
initialized with initializer list
Since we can't see tmpsetplatestat erec or what you are trying to
initialize, it is impossible to tell what you are doing wrong.
can anybody plz explain me what could be done to create instances of
these objects on stack ???

If the code you are using is giving a problem (and you are confusing
local variables with "objects on stack", which means nothing in C unless
you create and maintain a stack), then stop using a C++ compiler. C and
C++ are different languages; compiling C code as if it were C++ is
asking for trouble.

If you are actually trying to modify (rather than to initialize) a const
member of a structure, then don't. Either don't modify the member or
remove the 'const' which, after all, is a lie.

[Martin Ambuhl]

us******@gmail. com wrote:

why you use const there,maybe you can write like this :

typedef struct {
const int cbcode=0;
int cberror;
} xtsetplatestate rec;

Why would you think such a foolish thing?

[Richard Tobin]

In article <11************ **********@g49g 2000cwa.googleg roups.com>,
<ra************ @gmail.com> wrote:

error C2512: 'xtsetbleedvalu esrec' : no appropriate default constructor
available

This appears to be a C++ error message. Probably there is some way to
persuade your compiler to compile your program as C, perhaps by giving
the file a name with a suitable extension.

-- Richard

[Keith Thompson]

Martin Ambuhl <ma*****@earthl ink.net> writes:

ra************@ gmail.com wrote:

hi all experts,
i have a structure with the constant memebers such as one given
below:
typedef struct {
const int cbcode;
int cberror;
} xtsetplatestate rec;
now in my function i want to create a stack based object for the
same:
void CreateObjectAnd UseIt{
xtsetplatestate rec plate;
xtsetplatestate rec plate2 = {0 , 0};
}

The above has nothing to do with "stack based object"s. You are
(quite legally) using local variables. There is not a sign of your
use of a stack in sight.

To be fair, local variables are allocated on the system stack in most
C implementations , just as malloc()-allocated objects are allocated on
the "heap". Even though the C standard doesn't use the word "stack"
or "heap" (I just checked), both terms can be convenient shorthands
for automatic and allocated storage duration, respectively.

It's worth pointing out that the standard doesn't use the term
"stack", and that not all C implementations *necessarily* use a system
stack in the commonly understood sense, but I don't see the need to
pretend we don't know what it means.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Martin Ambuhl]

Keith Thompson wrote:

It's worth pointing out that the standard doesn't use the term
"stack", and that not all C implementations *necessarily* use a system
stack in the commonly understood sense, but I don't see the need to
pretend we don't know what it means.

Since I *do* know what it means to maintain a stack in program, and I
*do* know how to store objects in such a stack, it would be wrong to
pretend that I know that the OP's use of "stack based objects" is a
mistake for "local variables." Since I more than once allowed for the
possibility that he suffered from such confusion, it would be dishonest
for someone to claim that I pretended not know what that -- of more than
one possibility -- might be what the OP poster meant. Do you purposely
misread my posts so you can play the mis-directed anti-pedant?

[Keith Thompson]

Martin Ambuhl <ma*****@earthl ink.net> writes:
[snip]

Do you purposely misread my posts so you can play the mis-directed
anti-pedant?

No. If I've misread your post (which is entirely possible), it's
unintentional.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Keith Thompson]

Martin Ambuhl <ma*****@earthl ink.net> writes:

Keith Thompson wrote:

It's worth pointing out that the standard doesn't use the term
"stack", and that not all C implementations *necessarily* use a system
stack in the commonly understood sense, but I don't see the need to
pretend we don't know what it means.

Since I *do* know what it means to maintain a stack in program, and I
*do* know how to store objects in such a stack, it would be wrong to
pretend that I know that the OP's use of "stack based objects" is a
mistake for "local variables." Since I more than once allowed for the
possibility that he suffered from such confusion, it would be
dishonest for someone to claim that I pretended not know what that --
of more than one possibility -- might be what the OP poster meant. Do
you purposely misread my posts so you can play the mis-directed
anti-pedant?

I should expand on what I wrote a moment ago.

I assumed that the OP was using the term "stack-based objects" to
refer to objects allocated locally to a function. I still think
that's likely, but I made that assumption without carefully reading
the previous articles. I also assumed that it should have been
obvious to you, and that you were being overly pedantic, an incorrect
assumption for which I apologize.

Now that I've re-read the previous article, it's less clear to me just
what the OP is asking -- especially since he seems to be using C++
rather than C.

Going off on a bit of a tangent, I realize I've never used, and rarely
seen, structs with const members. It looks like this is allowed, but
you can't provide an initial value within the struct declaration. If
I understand this correctly, a const member can be initialized by the
initialization of the enclosing object, but can't be re-assigned later
without invoking undefined behavior -- and there doesn't seem to be
any way to initialize a const member of a malloc()ed structure without
invoking UB. This would seem to make const members not very useful.
Perhaps they're allowed because it was easier than adding a rule to
forbid them. Am I missing something?

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Tim Rentsch]

Keith Thompson <ks***@mib.or g> writes:

[snip]

Going off on a bit of a tangent, I realize I've never used, and rarely
seen, structs with const members. It looks like this is allowed, but
you can't provide an initial value within the struct declaration. If
I understand this correctly, a const member can be initialized by the
initialization of the enclosing object, but can't be re-assigned later
without invoking undefined behavior -- and there doesn't seem to be
any way to initialize a const member of a malloc()ed structure without
invoking UB. This would seem to make const members not very useful.
Perhaps they're allowed because it was easier than adding a rule to
forbid them. Am I missing something?

It is possible to construct a struct with const members in a
malloc'ed space, without UB. (Disclaimer: code has not been
compiled.)

... #includes as needed ...

struct thing {
int a;
const int b;
int c;
};

struct thing *
malloc_thing( int a, int b, int c ){
void *v;
struct thing *r;

r = v = malloc( sizeof *r );
if( !r ) exit( EXIT_FAILURE ); /* whatever... */

r->a = a;
*(int*) ((char*)v + offsetof(struct thing, b)) = b;
r->c = c;
return r;
}

Some people might squawk about the '(int*)' cast; they might want to
use

memcpy( (char*)v + offsetof(struct thing, b), &b, sizeof r->b );

instead.