C++: casting to void* and back

Question

* ---Edit - now the whole sourse*

When I debug it on the end, "get" and "value" have different values! Probably, I convert to void* and back to User the wrong way?

#include <db_cxx.h>
#include <stdio.h>

struct User{
User(){}
int name;
int town;
User(int a){};
inline int get_index(int a){
    return town;
} //for another stuff
};
int main(){ 
try {
DbEnv* env = new DbEnv(NULL);
env->open("./", 
    DB_CREATE | DB_INIT_MPOOL | DB_THREAD | 
DB_INIT_LOCK | DB_INIT_TXN | DB_RECOVER | DB_INIT_LOG, 0);
Db* datab = new Db(env, 0);
datab->open(NULL, "db.dbf", NULL, DB_BTREE, DB_CREATE | DB_AUTO_COMMIT, 0);

Dbt key, value, get;
char a[10] = "bbaaccd";
User u;
u.name = 1;
u.town = 34;
key.set_data(a);
key.set_size(strlen(a) + 1 );
value.set_data((void*)&u);
value.set_size(sizeof(u));
get.set_flags(DB_DBT_MALLOC);

DbTxn* txn;
env->txn_begin(NULL, &txn, 0);
datab->put(txn, &key, &value, 0);
datab->get(txn, &key, &get, 0);
txn->commit(0);
User g;
g = *((User*)&get);
printf("%d", g.town);
getchar();
return 0;
}catch (DbException &e){
    printf("%s", e.what());
    getchar();
}

solution

create a kind of "serializator" what would convert all POD's into void* and then will unite these pieces

PS Or I'd rewrite User into POD type and everything will be all right, I hope.

Add

It's strange, but... I cast a defenetly non-pod object to void* and back (it has std::string inside) and it's all right (without sending it to the db and back). How could it be? And after I cast and send 'trough' db defenetly pod object (no extra methods, all members are pod, it's a simple struct {int a; int b; ...}) I get back dirted one. What's wrong with my approach?

Add about week after first 'add'

Damn... I've compiled it ones, just for have a look at which kind of dirt it returnes, and oh! it's okay!... I can't ! ... AAh!.. Lord... A reasonable question (in 99.999 percent of situations right answer is 'my', but... here...) - whos is this fault? My or VSs?

Answer 1

Unless User is a POD this is undefined in C++.

Edit:

Looking at db_cxx.h, aren't you supposed to do call get_doff(), get_dlen(), and get_data() or something on Dbt instead of just casting (and assigning) it to the user type?

Answer 2

Since there is no check of the return value of put(), it could well be there is an error which prevented updating. The documentation indicates quite a few error condtions:

Answer 3

You are almost definitely NOT supposed to cast 'get' directly to a User. Instead, extract the data you stored and then cast that. We can't know for sure unless you share with us the definition of Dbt. Making a guess, based on what we can see:

datab->get(txn, &key, &get, 0);
void* data = get.get_data();
User g = *((User*)data);

Tell us more about Dbt and we can probably help you out more.

Answer 4

I would do something like this:

User               user;
std::stringstream  dbStotrStream;

dbStoreStream << user;  // Serialize user
std::string        dbStore(bdStoreStream.str());
value.set_data(dbStore.c_str());  
value.set_size(dbStore.lenght());

////  Put in DB

Then extracting it would look like this:

//// Get from DB

std::string        dbStore(get.get_data(),get.get_date() + get.get_size());
std::stringstream  dbStoreStream(dbStore);
User              outUser;

 dbStoreStream >> outUSer;