Обсуждение: storing "small binary objects"

hi,

I am relatively new to postgresql. Sorry if this is a rather naive
question.

I am trying to store a fixed sized c++ class into the database and
retreive it. What is the simplest (dirtiest?!) way to do it? I couldnt
find any example on the web or in the pgsql/src directories that I could
get to work.

Here is what i did.

I tried defining a column as char(sizeof class) using...
create table firstTable(id integer,className char(sizeofclass));


Then in my c++ program, I used....

char* someVariable=(char*)(&classInstance)
//character handle to the class

exec sql insert into firstTable(id,className)
 values(:temp,:someVariable);
//save "someVariable" 'asis' into the column

Then I tried reading the value back in...

 someVariable=(char*)malloc(sizeof(class));
 memset(someVariable,0,sizeof(class));
 exec sql select id, className
    into :id,:someVariable
    from firstTable
    where id = 5;
 //read "someVariable" back in

While I got the value of id (and there is definitely a valid record), the
"someVariable" memory location is blank. What am I doing wrong?

In addition to examples available with the distribution, pointers to some
more sample code would help.

Thanks,
Swami.

On Wed, Mar 28, 2001 at 07:41:36PM -0500, Swaminathan Natarajan wrote:
> hi,
>
> I am relatively new to postgresql. Sorry if this is a rather naive
> question.
>
> I am trying to store a fixed sized c++ class into the database and
> retreive it. What is the simplest (dirtiest?!) way to do it? I couldnt
> find any example on the web or in the pgsql/src directories that I could
> get to work.

Map your public instance data to the fields of one or more tables, then
when you reread the tuple(s) map them back to a "new" instance of the
class using its "set" methods.  Look at the libpq++ methods rather than
embedded C.  It may not be the simplest/dirtiest method, but then for
what you're doing, why not just use the filesystem?  Or DBM embedded
database file?

>
> Here is what i did.
>
> I tried defining a column as char(sizeof class) using...
> create table firstTable(id integer,className char(sizeofclass));
>
>
> Then in my c++ program, I used....
>
> char* someVariable=(char*)(&classInstance)
> //character handle to the class
>
> exec sql insert into firstTable(id,className)
>  values(:temp,:someVariable);
> //save "someVariable" 'asis' into the column
>
> Then I tried reading the value back in...
>
>  someVariable=(char*)malloc(sizeof(class));
>  memset(someVariable,0,sizeof(class));
>  exec sql select id, className
>     into :id,:someVariable
>     from firstTable
>     where id = 5;
>  //read "someVariable" back in
>
> While I got the value of id (and there is definitely a valid record), the
> "someVariable" memory location is blank. What am I doing wrong?
>
> In addition to examples available with the distribution, pointers to some
> more sample code would help.

--
Eric G. Miller <egm2@jps.net>