software Architecture(2）
过程调用体系结构风格
1.软件体系结构风格：描述特定领域中软件体系家族的组织方式的惯用模式，反映了领域中众多系统所共有的结构和语义特性，并指导如何将各个模块和子系统有效地组织成一个完整的系统。
2软件体系结构风格的组成：a set of component types,a set of connector types/interation mechanisms,a topological layout of these components,a set of constrains on topology an d behavior,an informal description of the costs and benefits of the style.
经典体系结构风格的分类：
2、Unstructured programming
all the program code written in a single continuous main program.
disadvantages for large programs:
1) difficult to follow logic;
2)if something needs to be done more than once must be-typed;
3)hard to incorpirated other code;
4)not easily modified;
5)difficult to test praticular portions of the code.
3、Hierarchical decomposition:
the system is designed from a functional viewpoint,starting with a high-level view an d progressively refining this into a more detailed design.
the methodology is exemplified by structured design and stepwise refinement.(结构化设计与逐步细化是这种风格的典型实例)
hierarchical decomposition(逐步分解)：
based on definition-use relationship;uses procedure call as interaction mechanism;correctness of a subroutine depend on the correctness of the subroutines it calls(主程序的正确性依赖于它所调用的子程序的正确性).
component(构件：主程序、子程序)
connector(连接器：调用-返回机制)
topology(拓扑结构：层次化结构)
本质：将大系统分解为若干模块（模块化），主程序调用这些模块实现完整的系统功能。
过程调用机制硬件基础（寄存器、跳转指令、栈操作指令）
实现机制：栈。函数调用实际上是进行程序的跳转，在转去执行函数之前，应把现场保护起来（栈、寄存器），以备函数执行完毕还回到刚才跳转的地方，接着执行后继程序。
主程序-子过程风格有点与缺点
advantage：
this has proved to be a highly sucessful design methodology.It has allowed the development of large programs.
disadvantages:
however,as program size increases beyond this point(10W行),the approach performs poorly;
we obeserve that code development becomes too slow and that it becomes increamsingly difficult to test the software and guarance its reliablity.
4、Object-Oriented Style
object is also called abstract data type;
the system is viewed as a collection of objects rather than as functions with messages passed from object to obeject.
eace objects has its own set of associated oprations.(每个对象都有一个它自己的功能集合。数据及作用在数据上的操作被封装成抽象数据类型——对象)
information hidding
encapsulation：
an object is responsible for preserving the integrity of its presentation ,and the presentation is hidden from other objects.
model the real world:
program with nouns not verbs, entities defined by actions it suffers and requires(每个对象也定义了它所能执行的动作，以及可以作用在它上面的动作)
component:classes and objects
connectors:objects interact and procedure invocations.
属性(Attribute)：描述对象静态特性的数据项；
操作(Operation)：描述对象动态特性的一个动作；
advantages of oo:
reuse and maintenance : Exploit encapsulation and locality to increase productivity (复用和维护：利用封装和聚合提高生产力)
Characteristics of OO:
Encapsulation: Restrict access to certain information (封装：限制对某些信息的访问)
Interaction: Via procedure calls or similar protocol (交互：通过过程调用或类似的协议)
Polymorphism: Choose the method at run-time (多态：在运行时选择具体的操作)
Inheritance: Keep one definition of shared functionality (继承：对共享的功能保持唯一的接口)
Dynamic binding: Determine actual operation to call at runtime (动态绑定：运行时决定实际调用的操作)
advantages:
Reuse and maintenance: Exploit capsulation and locality to increase prodycrivity(复用和维护);real world mapping;easy decomposition of a system.
disadvantages of oo:
managements of many objects:Need structures on large set of definitions
in order for one boject to interact with another(via procedure call) it must know
the identity of that object.(必须知道对象的身份)
