The evolution of frameworks

• Complexity. In a complex system, the class hierarchies for related classes can become extremely confusing, with many dozens or even hundreds of classes. Unless the programmer has access to detailed documentation or some other way of organizing the class library, it can be difficult to figure out what was intended by the class library developers--for example, which functions call other functions and when. It can also be difficult to figure out how to derive new classes within the class hierarchy. The programmer has to understand a lot about relationships among classes to make changes and additions.
• Flow of control. A program written with the aid of class libraries is still responsible for the flow of control--that is, it must control the interactions among all the objects created from a particular library. The programmer has to decide which functions to call at what times for which kinds of objects, because the library is basically passive and doesn't do anything unless the programmer figures out how to make it happen. This can become a significant problem in a program that involves multiple derived objects with many slightly different kinds of data and behavior.
• Duplication of effort. Although class libraries allow programmers to use and reuse many small pieces of code, each programmer puts those pieces together in a different way. Two different programmers can use the same set of class libraries to write two programs that do exactly the same thing but whose internal structure (that is, design) may be quite different, depending on hundreds of small decisions each programmer makes along the way. This becomes a maintenance problem, especially for developers working on a series of related programs. Inevitably, similar pieces of code end up doing similar things in slightly different ways and don't work as well together as they should.

Frameworks, which were introduced in Chapter 1, represent the next level of abstraction beyond class libraries. They were first developed to free application programmers from the chores involved in displaying menus, windows, dialog boxes, and other standard user interface elements for personal computers. Frameworks also represent a change in the way programmers think about the interaction between the code they write and code written by others. Figure 12 shows how this interaction has evolved over the years.