计算机科学与技术专业外文翻译--对象的创建和存在时间（可编辑）.doc

外文原文 Object landscapes and lifetimes Technically, OOP is just about abstract data typing, inheritance, and polymorphism, but other issues can be at least as important. The remainder of this section will cover these issuesOne of the most important factors is the way objects are created and destroyed. Where is the data for an object and how is the lifetime of the object controlled? There are different philosophies at work here. C++ takes the approach that control of efficiency is the most important issue, so it gives the programmer a choice. For imum run-time speed, the storage and lifetime can be determined while the program is being written, by placing the objects on the stack these are sometimes called automatic or scoped variables or in the static storage area. This places a priority on the speed of storage allocation and release, and control of these can be very valuable in some situations. However, you sacrifice flexibility because you must know the exact quantity, lifetime, and type of objects while youre writing the program. If you are trying to solve a more general problem such as computer-aided design, warehouse management, or air-traffic control, this is too restrictiveThe second approach is to create objects dynamically in a pool of memory called the heap. In this approach, you dont know until run-time how many objects you need, what their lifetime is, or what their exact type is. Those are determined at the spur of the moment while the program is running. If you need a new object, you simply make it on the heap at the point that you need it. Because the storage is managed dynamically, at run-time, the amount of time required to allocate storage on the heap is significantly longer than the time to create storage on the stack. Creating storage on the stack is often a single assembly instruction to move the stack pointer down, and another to move it back up. The dynamic approach makes the generally logical assumption that objects