C++ allows the programmer to write the interface and implementation separately, to place them in separate files and compile separately, and to hide the implementation details. This book goes a step further: The interface and implementation are discussed in separate parts of the book. Part I (Objects and C++), Part II (Algorithms and Building Blocks), and Part III (Applications) lay the groundwork by discussing basic concepts and tools and providing some practical examples, but implementation of data structures are not shown until Part IV (Implementations). This separation of interface and implementation promotes abstract thinking. Class interfaces are written and used before the implementation is known, forcing the reader to think about the functionality and potential efficiency of the various data structures (e.g. hash tables are written well before the hash table is implemented).