The following sections provide information on how to achieve persistence using dbstl.
    Each container has a begin() method
    which produces an iterator.  These 
    begin methods take a boolean parameter,
    directdb_get, which controls the
    caching behavior of the iterator.  The default value of this parameter
    is true.
    If directdb_get is
    true, then the persistent object is fetched anew
    from the database each time the iterator is dereferenced as a pointer
    by use of the star-operator 
    (*iterator) or by use of the arrow-operator
    (iterator->member).  If 
    directdb_get is false, then
    the first dereferencing of the iterator fetches the object from the
    database, but later dereferences can return cached data.
    With directdb_get set to true, if you call: 
(*iterator).datamember1=new-value1; (*iterator).datamember2=new-value2;
    then the assignment to datamember1 will be lost,
    because the second dereferencing of the iterator would cause the cached
    copy of the object to be overwritten by the object's persistent data
    from the database.
You also can use the arrow operator like this:
iterator->datamember1=new-value1; iterator->datamember2=new-value2;
This works exactly the same way as iterator::operator*. For this reason, the same caching rules apply to arrow operators as they do for star operators.
One way to avoid this problem is to create a reference to the object, and use it to access the object:
container::value_type &ref = *iterator; ref.datamember1=new-value1; ref.datamember2=new-value2; ...// more member function calls and datamember assignments ref._DB_STL_StoreElement();
    The above code will not lose the newly assigned value of ref.datamember1 
    in the way that the previous example did.
In order to avoid these complications, you can assign to the object referenced by an iterator with another object of the same type like this:
container::value_type obj2; obj2.datamember1 = new-value1; obj2.datamember2 = new-value2; *itr = obj2;
    This code snippet causes the new values in obj2 to
    be stored into the underlying database.
If you have two iterators going through the same container like this:
for (iterator1 = v.begin(), iterator2 = v.begin();
     iterator1 != v.end();
     ++iterator1, ++iterator2) {
        *iterator1 = new_value;
        print(*iterator2);
}  
        
    then the printed value will depend on the value of 
    directdb_get with which the
    iterator had been created.  If directdb_get 
    is false, then the original, persistent value is
    printed; otherwise the newly assigned value is returned from the
    cache when iterator2 is dereferenced.  This
    happens because each iterator has its own cached copy of the
    persistent object, and the dereferencing of
    iterator2 refreshes
    iterator2's copy from the database, retrieving
    the value stored by the assignment to
    *iterator1.
    Alternatively, you can set directdb_get
     to false and call
    iterator2->refresh() immediately before
    the dereferencing of iterator2, so that
    iterator2's cached value is refreshed.
    If directdb_get is
    false, a few of the tests in dbstl's test kit
    will fail. This is because the above contrived case appears in
    several of C++ STL tests. Consequently, the default value of the
    directdb_get parameter in the
    container::begin() methods is
    true. If your use cases avoid such bizarre usage
    of iterators, you can set it to false, which
    makes the iterator read operation faster.
If you modify the object to which an iterator refers by using one of the following:
(*iterator).member_function_call()
or
(*iterator).data_member = new_value
    then you should call
    iterator->_DB_STL_StoreElement() to
    store the change. Otherwise the change is lost after the
    iterator moves on to other elements.
    If you are storing a sequence, and you modified some part of it, you
    should also call 
    iterator->_DB_STL_StoreElement()
    before moving the iterator.
    And in both cases, if directdb_get
    is true (this is the default value), you should
    call _DB_STL_StoreElement() after the
    change and before the next iterator movement OR the next
    dereferencing of the iterator by the star or arrow operators
    (iterator::operator* or
    iterator::operator->).  Otherwise, you will
    lose the change.
If you update the element by assigning to a dereferenced iterator like this:
*iterator = new_element;
    then you never have to call
    _DB_STL_StoreElement() because the change
    is stored in the database automatically.
Dbstl is an interface to Berkeley DB, so it is used to store data persistently. This is really a different purpose from that of regular C++ STL. This difference in their goals has implications on expected object lifetime: In standard STL, when you store an object A of type ID into C++ stl vector V using V.push_back(A), if a proper copy constructor is provided in A's class type, then the copy of A (call it B) and everything in B, such as another object C pointed to by B's data member B.c_ptr, will be stored in V and will live as long as B is still in V and V is alive. B will be destroyed when V is destroyed or B is erased from V.
    This is not true for dbstl, which will copy A's data and store it
    in the underlying database. The copy is by default a shallow copy,
    but users can register their object marshalling and unmarshalling
    functions using the DbstlElemTraits class
    template. So if A is passed to a db_vector
    container, dv, by using
    dv.push_back(A), then dbstl copies A's data
    using the registered functions, and stores data into the underlying
    database. Consequently, A will be valid, even if the container is
    destroyed, because it is stored into the database.
    If the copy is simply a shallow copy, and A is later destroyed, then
    the pointer stored in the database will become invalid. The next time
    we use the retrieved object, we will be using an invalid pointer, which
    probably will result in errors. To avoid this, store the referred
    object C rather than the pointer member A.c_ptr itself, by registering
    the right marshalling/unmarshalling function with
    DbstlElemTraits.
For example, consider the following example class declaration:
class ID
{
public:
    string Name;
    int Score;
};  
        
    Here, the class ID has a data member Name, which refers to a memory address of
    the actual characters in the string. If we simply shallow copy an
    object, id,  of class ID to store it, then the
    stored data, idd, is invalid when
    id is destroyed. This is because
    idd and id refer to a common
    memory address which is the base address of the memory space storing
    all characters in the string, and this memory space is released when
    id is destroyed.  So idd will be
    referring to an invalid address. The next time we retrieve
    idd and use it, there will probably be memory
    corruption.
    The way to store id is to write a marshal/unmarshal
    function pair like this:
void copy_id(void *dest, const ID&elem)
{
	memcpy(dest, &elem.Score, sizeof(elem.Score));
	char *p = ((char *)dest) + sizeof(elem.Score);
	strcpy(p, elem.Name.c_str());
}
void restore_id(ID& dest, const void *srcdata)
{
	memcpy(&dest.Score, srcdata, sizeof(dest.Score));
	const char *p = ((char *)srcdata) + sizeof(dest.Score);
	dest.Name = p;
}
size_t size_id(const ID& elem)
{
	return sizeof(elem.Score) + elem.Name.size() + 
	    1;// store the '\0' char.
}  
        
        Then register the above functions before storing any instance of 
        ID:
	
DbstlElemTraits<ID>::instance()->set_copy_function(copy_id); DbstlElemTraits<ID>::instance()->set_size_function(size_id); DbstlElemTraits<ID>::instance()->set_restore_function(restore_id);
This way, the actual data of instances of ID are stored, and so the data will persist even if the container itself is destroyed.