Initialize pointer-to-struct declaration to an unnamed struct?

[Ehud Shapira]

Is it possible to have a declaration of a struct pointer initialized
to an unnamed struct?
(I'm only concerned with static/global variables, if it matters.)

I'm trying to do something like:

struct st_a {
int i, j;
};

struct st_b {
int k, l;
st_a *m;
} b[] = {
1, 2, {0, 1},
3, 4, {0, 2}
};

It could be done by declaring each "sub-struct" with a name, then
pointing to these,
but besides being more work it's also more difficult to read when the
idea is a hierarchy.

[Rolf Magnus]

Ehud Shapira wrote:

Is it possible to have a declaration of a struct pointer initialized
to an unnamed struct?
(I'm only concerned with static/global variables, if it matters.)

I'm trying to do something like:

struct st_a {
int i, j;
};

struct st_b {
int k, l;
st_a *m;
} b[] = {
1, 2, {0, 1},
3, 4, {0, 2}

{0, 1} is not a valid initializer for a pointer.

};

It could be done by declaring each "sub-struct" with a name, then
pointing to these,
but besides being more work it's also more difficult to read when the
idea is a hierarchy.

It's not quite clear to me what you actually want.

[Ehud Shapira]

{0, 1} is not a valid initializer for a pointer.
I know. What *can* be valid? You can see what sort of initialization
I'm trying to do.

[Rolf Magnus]

Ehud Shapira wrote:

>{0, 1} is not a valid initializer for a pointer.

I know. What *can* be valid? You can see what sort of initialization
I'm trying to do.

Actually, I can't. Why does it have to be a pointer? Can't you just store a
struct directly?

[Ehud Shapira]

On Jun 26, 11:17 pm, Rolf Magnus <[e-mail]:

Actually, I can't. Why does it have to be a pointer? Can't you just store a
struct directly?

What I want to do involves a hierarchy of variable length struct
arrays. But regardless of the why, the question is whether it's
possible at all to do something like ( char *pc = "something" ) for a
struct and not char.

[Victor Bazarov]

Ehud Shapira wrote:

On Jun 26, 11:17 pm, Rolf Magnus <[e-mail]:

>Actually, I can't. Why does it have to be a pointer? Can't you just
store a struct directly?

What I want to do involves a hierarchy of variable length struct
arrays. But regardless of the why, the question is whether it's
possible at all to do something like ( char *pc = "something" ) for a
struct and not char.

Usually not. There is a huge difference between a string literal (that
has a very very very long lifetime) and a struct temporary. You could
create a static object of the struct type, and then declare a pointer
and initialise it with the address of the static object:

static somestruct something = { blah };
somestruct *ps = &something;

V
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[Ehud Shapira]

>There is a huge difference between a string literal and a struct temporary.
The struct is just as static -- it's a global definition.

You could create a static object of the struct type, and then declare a pointer
and initialise it with the address of the static object:

That's what I'm trying to avoid; naming every linked array of structs.

[Victor Bazarov]

Ehud Shapira wrote:

>There is a huge difference between a string literal and a struct
temporary.

The struct is just as static -- it's a global definition.

No, a temporary created for the purposes of initialising something is
definitely not static. It's temporary.

>You could create a static object of the struct type, and then
declare a pointer and initialise it with the address of the static
object:

That's what I'm trying to avoid; naming every linked array of structs.

<shrug Don't name them. Have an array and get pointers to its
elements:

struct st_a {
int i, j;
} a[] = { { 0, 1 }, {0, 2 } };

struct st_b {
int k, l;
st_a *m;
} b[] = {
{ 1, 2, a+0 },
{ 3, 4, a+1 }
};

You won't have "every" element named, only one.

Another solution is to create those dynamically:

struct st_a {
int i, j;
};

st_a* make_st_a(int i, j) {
st_a a = {i, j};
return new st_a(a);
}

struct st_b {
int k, l;
st_a *m;
} b[] = {
1, 2, make_st_a(0, 1),
3, 4, make_st_a(0, 2)
};

If 'b' is static, you don't have to care about disposing of
those dynamic objects, a small leak like that isn't going to
hurt much. Just be aware of it.

V
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[James Kanze]

On Jun 26, 8:52 pm, Ehud Shapira <ehudshap...@ho tmail.comwrote:

Is it possible to have a declaration of a struct pointer initialized
to an unnamed struct?
(I'm only concerned with static/global variables, if it matters.)

I'm trying to do something like:

struct st_a {
int i, j;
};

struct st_b {
int k, l;
st_a *m;} b[] = {
1, 2, {0, 1},
3, 4, {0, 2}
};

It can't be done in C++. In C you can write something like:

typedef struct
{
int i, j ;
} st_a ;

typedef struct
{
int k, l ;
st_a* m ;
} st_b ;

st_b b[] = {
{ 1, 2, &(st_a){ 0, 1 }, },
{ 3, 4, &(st_a){ 0, 2 }, },
} ;

but this was added to C after C++ was standardized. (Possibly
some C++ compilers support it anyway. I don't know.)

It could be done by declaring each "sub-struct" with a name, then
pointing to these,
but besides being more work it's also more difficult to read when the
idea is a hierarchy.

In such cases, I usually define the data in a simple text file,
and then write a small program to convert it into legal C++.

--
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[Victor Bazarov]

eh*********@gma il.com wrote:

On Jun 27, 2:33 am, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:

>>The struct is just as static -- it's a global definition.

No, a temporary created for the purposes of initialising something is
definitely not static. It's temporary.

I don't understand that. Maybe we are talking about different things?
The struct will be initialized at compile-time, and so will live
"forever". How is that temporary?

In the statement

T obj = someexpression;

(provided it appears at the namespace scope) 'obj' has static storage
duration, but whatever 'someexpression ' returns does NOT have static
storage duration, it's a temporary. That's significantly different
from

char const* p = "blah";

where the string literal ("blah") has static storage duration.

>

><shrug Don't name them. Have an array and get pointers to its
elements:

Bookkeeping array indexes isn't much better. And dynamically creating
them would add overhead, etc.

On Jun 27, 12:03 pm, James Kanze <james.ka...@gm ail.comwrote:

>In such cases, I usually define the data in a simple text file,
and then write a small program to convert it into legal C++.

Yeah, if that's the case, for big definitions I'll probably go with a
custom preprocessor, possibly outputting binary directly (which would
also rid me of unclear handling of union initialization by VC6... but
maybe that's the problem; using an old compiler.)

Too bad C/++ doesn't offer (in many cases) such terseness as that
possible by, say, JavaScript.

Huh? Terseness?

V
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