Dbstl miscellaneous notes

Special notes about trivial methods
Using correct container and iterator public types

Special notes about trivial methods

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_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:


Using correct container and iterator public types

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


check that the return type for this function is


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

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