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C++ Coding Standards Item 5 : Give One entity one cohesive responsibility
Summary
Focus on one thing at a time: Prefer to give each entity (variable, class, function, namespace, module, library) one well-defined responsibility. As an entity grows, its scope of responsibility naturally increases, but its responsibility should not diverge.
同一时间只关注一件事情：最好给每一个实体（variable, class, function, namespace, module, library）一个明确的职责。当实体增长（扩展，怎么翻译好一点？）的时候，它的职责范围自然的扩大了，但是它的职责不应该有不同的发展方向。
Discussion
A good business idea, they say, can be explained in one sentence. Similarly, each program entity should have one clear purpose.
一个好的商业计划可以用一句话说明。类似的，一个好的程序实体应该有一个清楚地目的。
这条规则主要讲有很多功能（职责）的实体，它们通常很难设计，用起来也很困难，所以，还是让我们的实体（class，function……）目的单一些，单纯一点好。
Prefer to build higher-level abstractions from smaller lower-level abstractions. Avoid collecting several low-level abstractions into a larger low-level conglomerate. Implementing a complex behavior out of several simple ones is easier than the reverse.
最好在比较小的低级别的抽象基础上建立高级别的抽象。避免把许多小的低级别抽象聚合到一个大的低级别的抽象中。相对而言，通过几个简单的实体实现一个复杂的行为，要比通过一个实体来实现简单的多。（这句话好难讲！！）
Examples
Example 1: realloc. In Standard C, realloc is an infamous example of bad design. It has to do too many things: allocate memory if passed NULL, free it if passed a zero size, reallocate it in place if it can, or move memory around if it cannot. It is not easily extensible. It is widely viewed as a shortsighted design failure.
Example 1 讲的是标准C语言中的realloc函数，realloc必须要做太多的事情了：传入NULL的话就分配内存，传入0值得话就释放内存，如果在那个地方（程序执行点）可以realloc的话，就正常的工作，否则就要做内存移动……。而且不能很容易的扩展。基本上都认为它(realloc)的设计是很失败的。
Example 2: basic_string. In Standard C++, std::basic_string is an equally infamous example of monolithic class design. Too many "nice-to-have" features were added to a bloated class that tries to be a container but isn't quite, is undecided on iteration vs. indexing, and gratuitously duplicates many standard algorithms while leaving little space for extensibility. (See Item 44's Example.)
Example 2：basic_string.又一个在单一功能的类的设计中的反面例子，它的“罪过”我就不逐句翻译了，大家读原文也能清楚。
这章讲述的内容比较简单，以前我总是注意不到，或者说在刻意的违反，总是希望一个类可以做这个也可以做那个，呵呵，记得我曾经把几个功能函数全部写在一个 CPP文件中，无论从程序设计角度还是文学欣赏的角度看，那个文件都极度的不具备可读性，所以它也从我的历史中永久的消失了……
Copy Left (C) Scorpio Auding
