There are some standard STL methods which are meaningless in dbstl, but they are kept in dbstl as no-ops so as to stay consistent with the standard. These are:

db_vecter::reserve();

db_vector::max_size();

db_vector::capacity();

db_map::reserve();

db_map::max_size();

db_vector<>::max_size() and db_map<>::max_size() both return 2^30. This does not mean that Berkeley DB can only hold that much data. This value is returned to conform to some compilers' overflow rules — if we set bigger numbers like 2^32 or 2^31, some compilers complain that the number has overflowed.

See the Berkeley DB documentation for information about limitations on how much data a database can store.

There are also some read-only functions. You set the configuration for these using the Berkeley DB API. You access them using the container's methods. Again, this is to keep consistent with C++ standard STL containers, such as:

db_map::key_comp();

db_map::value_comp();

db_map::hash_funct();

db_map::key_eq();

All public types defined by the C++ STL specification are present in dbstl. One thing to note is the value_type. dbstl defines the value_type for each iterator and container class to be the raw type without the ElementRef/ElementHolder wrapper, so this type of variable can not be used to store data in a database. There is a value_type_wrap type for each container and iterator type, with the raw type wrapped by the ElementRef/ElementHolder.

For example, when type int_vector_t is defined as

db_vector<int, ElementHolder<int> >

its value_type is int, its value_type_wrap is ElementHolder<int>, and its reference and pointer types are ElementHolder<int>& and ElementHolder<int>* respectively. If you need to store data, use value_type_wrap to make use of the wrapper to store data into database.

The reason we leave value_type as the raw type is that we want the existing algorithms in the STL library to work with dbstl because we have seen that without doing so, a few tests will fail.

You need to use the same type as the return type of the data element retrieval functions to hold a value in order to properly manipulate the data element. For example, when calling

db_vector<T>::operator[]

check that the return type for this function is

db_vector<T>::datatype_wrap

Then, hold the return value using an object of the same type:

db_vector<T>::datatype_wrap refelem = vctr[3];