How do you track type information when converting int* to void*

[Chad]

Keith made the following comment

" You can convert an int*, or an int**, or an int***, or ... to void*
without loss of information -- except that you lose the type
information, which you have to carefully keep track of yourself. "

How would you track the type infornation?

[Nick Keighley]

On 18 Sep, 12:27, Chad <cdal...@gmail. comwrote:

Keith made the following comment

" You can convert an int*, or an int**, or an int***, or ... to void*
without loss of information -- except that you lose the type
information, which you have to carefully keep track of yourself. "

How would you track the type infornation?

why are you casting a whatever-type to a void*?
If you need the type information why not keep the original type?

I suppose some sort of generic container might be a reason
to do this.

Basically you need some sort of tag

enum Tag {INT, FLOAT, STRING};

struct Node
{
enum Tag tag;
void *data;
};

store data like this

struct Node nod;
int i;
nod.tag = INT;
nod.data = &i;

add (&handle, &nod);

casting it back when you retrieve it.

You might consider a union instead. Or a redesign.
--
Nick Keighley

[Richard Heathfield]

Nick Keighley said:

On 18 Sep, 12:27, Chad <cdal...@gmail. comwrote:

>Keith made the following comment

>" You can convert an int*, or an int**, or an int***, or ... to void*
without loss of information -- except that you lose the type
information, which you have to carefully keep track of yourself. "

How would you track the type infornation?

why are you casting a whatever-type to a void*?

Where did he say he was casting?

If you need the type information why not keep the original type?

Because you're writing generic code?

I suppose some sort of generic container might be a reason
to do this.

Ha - yes, quite so.

Basically you need some sort of tag

enum Tag {INT, FLOAT, STRING};

or a function whose address you hand to the container, and which knows how
to deal with the type.

>
struct Node
{
enum Tag tag;
void *data;
};

store data like this

struct Node nod;
int i;
nod.tag = INT;
nod.data = &i;

add (&handle, &nod);

casting it back when you retrieve it.

Why?

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[blargg]

In article
<83************ *************** *******@l43g200 0hsh.googlegrou ps.com>,
Chad <cd*****@gmail. comwrote:

Keith made the following comment

" You can convert an int*, or an int**, or an int***, or ... to void*
without loss of information -- except that you lose the type
information, which you have to carefully keep track of yourself. "

How would you track the type infornation?

In many cases you know the type that the void* points to, so you can
just cast back to that. Why use void* at all, you ask? Often a function
doesn't need to know anything about the type pointed to, so using void*
allows one function to work on many types. The best example is probably
qsort(), where the comparison function gets a void*.

[Eric Sosman]

blargg wrote:

In article
<83************ *************** *******@l43g200 0hsh.googlegrou ps.com>,
Chad <cd*****@gmail. comwrote:

>Keith made the following comment

" You can convert an int*, or an int**, or an int***, or ... to void*
without loss of information -- except that you lose the type
information, which you have to carefully keep track of yourself. "

How would you track the type infornation?

In many cases you know the type that the void* points to, so you can
just cast back to that. Why use void* at all, you ask? Often a function
doesn't need to know anything about the type pointed to, so using void*
allows one function to work on many types. The best example is probably
qsort(), where the comparison function gets a void*.

... and where the true type of what the void* points to
is "tracked" by the comparison function: The caller supplies
a pointer to a function that "knows" what's on the other end
of the pointer, and thus specifies its type.

One other aspect of the type, its size, is provided to
qsort() as an explicit argument.

--
Eric Sosman
es*****@ieee-dot-org.invalid

[Nick Keighley]

On 18 Sep, 13:01, Richard Heathfield <[e-mail].i nvalidwrote:

Nick Keighleysaid:

On 18 Sep, 12:27, Chad <cdal...@gmail. comwrote:

Keith made the following comment

" You can convert an int*, or an int**, or an int***, or ... to void*
without loss of information -- except that you lose the type
information, which you have to carefully keep track of yourself. "

How would you track the type infornation?

why are you casting a whatever-type to a void*?

Where did he say he was casting?

a moment of insanity on my part

If you need the type information why not keep the original type?

Because you're writing generic code?

ok, but does he need generic code? Sometimes things
get stuffed into void* for no good reason that I can see.
But I should assume Good Faith; that the OP is not in fact
crazy.

I suppose some sort of generic container might be a reason
to do this.

Ha - yes, quite so.

Basically you need some sort of tag
casting it back when you retrieve it.

Why?

a *second* moment of insanity
--
Nick Keighley

"Almost every species in the universe has an irrational fear of the
dark.
But they're wrong- cos it's not irrational. It's Vashta Nerada."
The Doctor